Technical Resources
Everything you need to understand what a project costs, what the law requires, and how we keep your build compliant — from material rates and planning duties to party walls, utilities and HSE enforcement.
How an extension is designed
A domestic extension isn't just a bigger box. It's a designed space that must feel comfortable, perform efficiently and meet building regulations. Here's the five-stage process and the design criteria we apply to every building element.
Five Design Stages
Brief & Feasibility
We capture how you live, what you need, your budget and your site's constraints — orientation, boundaries, neighbours, planning history. Output: a written brief and a feasibility sketch with size, cost and planning risk.
Concept Design
Layout options, massing and daylight studies. We test room sizes against ergonomic standards, circulation, and outlook. Output: preferred layout, 3D sketch and a materials direction.
Planning Application
Drawings and a Design & Access Statement submitted to the local authority. We handle listed-building, conservation-area and neighbour consultations. Output: planning approval (8–13 weeks typical).
Technical Design
Building Regulations package: structure, thermal (Part L), ventilation (Part F), drainage, electrics and overheating (Part O). Coordinated with our structural engineer and services. Output: approved drawings and spec.
Construction Drawings
Build-ready details: junctions, materials schedule, setting-out and a priced scope. Output: a construction set the site team can build to, with no guesswork.
Ergonomics: Kitchen Work Triangle
The sink, fridge and hob form a triangle whose total leg length should stay under 6.6m — keeping daily movement efficient without cramped corners or wasted steps.
- Legs between 1.2m and 2.7m each
- No major traffic route crosses the triangle
- Min. 900mm clearance in front of appliances
Lighting: Daylight First
Daylight penetration reaches roughly 2.5× the window head height into a room. We set glazing area to ≥ 15% of floor area for living spaces and layer electric lighting for after dark.
- South-facing glazing for winter gain, shaded for summer
- North light for even, glare-free rooms
- High-level clerestory windows for deep plans
- LED task lighting at 300–500 lux work planes
Ventilation: Cross-Flow & Extract
Fresh air enters through trickle vents or opening windows on the windward side, flows across the room, and moist air is extracted at the kitchen and bathroom — meeting Part F without wasting heat if MVHR is fitted.
- Trickle vents 5000mm² EA equivalent (Part F)
- Mechanical extract: 30 l/s kitchen, 15 l/s bath
- MVHR option for airtight low-carbon builds
- Purge ventilation via opening windows (1/20 floor)
Heating & Cooling Strategy
A low-carbon heat source (heat pump) feeds low-temperature underfloor heating for even comfort. Summer overheating is controlled passively — external shading, openable vents and thermal mass — before mechanical cooling is considered.
- Heat-loss calculation sizes the heat pump (Part L)
- Underfloor heating at 35–45°C flow temp
- External blinds / brise-soleil for Part O
- Night-purge ventilation uses thermal mass
Noise & Sound Insulation
External noise from traffic, aircraft and neighbours is controlled at the facade — glazing, trickle-vent attenuators and sealed junctions. Internal sound is managed with mass and isolation between rooms, especially bedrooms and home offices.
- Acoustic glazing / laminated panes for road & aircraft noise
- Trickle-vent attenuators keep fresh air quiet
- Resilient layers on floors to cut impact sound
- Min. Rw 40 dB internal walls between bedrooms
Indoor Air Quality
Good indoor air quality means diluting pollutants and controlling moisture at source. Building Regulations and related standards set the minimum we design to — ventilation rates, material emissions, radon protection and condensation control.
- Part F: whole-house ventilation ≥ 0.3 l/s·m² floor area, plus extract in wet rooms
- Part L: airtight build demands designed ventilation — uncontrolled leaks are not ventilation
- Radon protective measures where maps require (BRE BR 211)
- Low-VOC / low-formaldehyde materials to limit indoor emissions (EN 16516 / LEED limits)
- Moisture & condensation control to BS 5250 — interstitial and surface
- Combustion appliances: CO alarms and dedicated ventilation (Part J)
EMF & Mobile Signals
Electromagnetic fields (EMF) and radio-frequency (RF) signals surround the modern home — mobile masts, Wi-Fi, smart meters, power lines and appliances. UK Health Security Agency guidance (based on ICNIRP) confirms no conclusive evidence of harm at everyday exposure levels, but a precautionary approach means reducing unnecessary exposure where simple, low-cost measures are available.
- Sources: mobile base stations, Wi-Fi, smart meters, DECT phones, 5G, high-voltage lines, household appliances
- UKHSA / ICNIRP: public exposure limits — no established health effects below these limits
- Reported effects: headaches & sleep disturbance are common complaints, though causation is not established; thermal effects are controlled by limits
- Measure: low-cost EMF/Gauss meters (e.g. EMFields, Cornet) for ELF; RF power-density meters (e.g. Acoustimeter, RF Explorer) for mobile/Wi-Fi
- Distance & time: field strength drops rapidly with distance; sleeping areas away from fuse boards & wireless hubs
- Shielding: low-e metallic window film, RF-shielding paint (carbon/nickel), shielded fly mesh, CAT6 wired data instead of Wi-Fi
- Reduce sources: switch Wi-Fi off at night, place smart meters at distance, hardwire desk devices, keep bedrooms device-free
Building-Element Design Criteria
Beyond layout, every element of the extension is designed to a recognised standard. These are the criteria we apply — and check — at technical design stage.
Ergonomics & Space Planning
Rooms are sized against established human-scale standards — clearances, furniture fits and circulation routes — so the space actually works, not just looks right on paper.
- Min. 900mm clear circulation between furniture
- Kitchen work triangle ≤ 6.6m total
- Min. 2.0m ceiling, 2.4m preferred in living areas
- Doorways 760mm clear (Part M accessible)
Daylight & Electric Lighting
We design natural daylight first — window-to-floor ratios and room depth — then layer task, ambient and accent electric lighting to suit each activity.
- Window area ≥ 15–20% of floor area (living rooms)
- Daylight factor ≥ 1.5% average (kitchens)
- Glare & solar gain controlled by orientation
- Three-layer lighting: task / ambient / accent
Ventilation & Air Quality
Part F requires controlled fresh-air supply and moisture extraction. We choose between trickle vents + extract fans or a whole-house MVHR system based on airtightness.
- Extract: 30 l/s kitchen, 15 l/s bathroom (Part F)
- Background ventilation via trickle vents or MVHR
- Cross-ventilation where plan allows
- MVHR recovers up to 90% of heat
Heating, Cooling & Overheating
Part L sets the heat-loss and CO₂ limits; Part O addresses summer overheating. We size low-carbon heat sources and passive cooling to keep the space comfortable year-round.
- Heat loss calculated to size heat emitters
- Low-carbon: heat pump, underfloor or radiators
- Part O: limit summer overheating (g-value, shading)
- Buffer / thermal mass for stable temperatures
Thermal & Moisture Control
Insulation U-values, airtightness and condensation risk are designed together to keep the fabric warm, dry and efficient over the building's life.
- Target U-values per Part L (walls 0.18, roof 0.11)
- Airtightness ≤ 5 m³/h·m² at 50 Pa (Part L)
- Interstitial condensation risk assessed (BS 5250)
- Continuous insulation, minimised thermal bridging
Acoustics & Sound Insulation
Sound insulation between rooms and from outside is designed in — not added later. Particularly important for home offices, bedrooms and open-plan living.
- Internal walls: min. Rw 40 dB between bedrooms
- Impact sound: resilient layers on floors
- External glazing for traffic / aircraft noise
- Soft finishes absorb reverberation in open-plan
Accessibility & Future-Proofing
Even when not required, we design for step-free access, wider doors and adaptable layouts so the extension stays usable as needs change.
- Step-free entry and level thresholds
- 760mm+ door clearances for mobility aids
- Downstairs WC / wet-room option
- Adaptable layout — walls removable, not load-bearing
Design-Led From Day OneEvery extension we build starts here — a measured brief, a tested layout and a technical design that meets every relevant regulation. Bring us your ideas and we'll turn them into a buildable plan.
Materials, Labour & Cost Calculator
Indicative Hampshire rates to help you budget. Use the calculator for a rough build cost — choose your finish level, size and opening counts.
Typical Labour Rates
Per day, Hampshire
Typical Material Rates
Supply only, indicative
Build Cost Calculator
Rough estimate only
Disclaimer
These figures are for general information only and do not constitute an estimate, a quotation, or construction advice. Rates vary widely with site conditions, specification, location, groundworks, design complexity and market fluctuations. No liability is accepted for any reliance on this information for construction, budgeting or any other purpose. For a binding price, request a fixed written quotation following a site visit.
Free materials quantity calculators
Estimate quantities and costs for foundations, floors, walls and roofing before you order. Calculators are provided by Source4me and open in a new tab — figures are for guidance only.
These free online calculators cover the common materials for a domestic build — from excavated spoil and concrete through to brickwork, plaster and roofing. Use them to sanity-check an order or brief us with your own figures.
Foundations & Substructure
Excavated Material (Bulking)
Length, width & depth of excavation → spoil volume in m³ and tonnes.
Open calculator →Sub Base
Area & compacted depth → crusher-run tonnage for a consolidated sub base.
Open calculator →Concrete (Readymix)
Area & depth → sharp sand, aggregate, cement, water or all-in ballast.
Open calculator →Concrete (Mix on Site)
Area & depth → batch quantities for site-mixed concrete.
Open calculator →Paving Bed
Area & depth → sand/cement per kg/25kg and sand-only tonnes.
Open calculator →Patio Paving
Area & slab dimensions → materials to lay a specified patio.
Open calculator →Paving Cost
Slab size & unit cost → overall cost per m²/yd.
Open calculator →Screed Quantities
Area & thickness → sand, cement, lime & plasticiser.
Open calculator →Ready Mixed Screed
Area & depth → volume of ready-mix screed in tonnes/m³.
Open calculator →Internal & Floors
Floorboards
Room size & board dimensions → board count and linear metreage.
Open calculator →Wall & Floor Tiles
Area, tile size & wastage → number of tiles required.
Open calculator →Plaster Estimator
Wall/ceiling dimensions → bags of bonding, browning, hardwall & finish.
Open calculator →Plaster Stud Wall
Wall dimensions → stud, insulation, board, tape, screws & plaster.
Open calculator →Walls & Mortar
Mortar
Wall length & height → sand & cement for a 4:1 mix in kg/25kg.
Open calculator →Brick, Block & Mortar
Wall dimensions → brick, block & mortar for 4", 6" and 9" walls.
Open calculator →Cavity Wall
Wall dimensions → bricks, blocks, mortar, insulation, ties & render.
Open calculator →Render / Stucco Quantities
Area & thickness → sand, lime & cement for external rendering.
Open calculator →Calculators provided by Source4me — Standard Calculators. Figures are for guidance only; Source4me accepts no liability for the quantities given.
Skips & Materials on the Road
Putting a skip, building materials, scaffolding or plant on a public road or pavement needs a permit. Different councils administer these — here's who to contact and where to apply across our area.
Skip licence
A skip on a public road, verge or pavement needs a licence — usually arranged by your skip hire company. Private land is exempt.
Materials consent
A separate consent is required to store building materials, spoil, plant or containers on the highway. A skip licence does NOT cover loose materials.
Scaffolding & hoarding
Erecting scaffolding, hoarding or a temp structure over the public highway needs its own licence — applied for before works start.
Visibility & safety
Skips must be coned and lamp-lit at night, kept clear of junctions, drains and access points, and leave a 1 m pedestrian walkway where a footway is obstructed.
Local Authority Permit Links
Gosport
Hampshire County Council (HCC) is the highway authority. Skip licence £78.80 first week + £26/week after; allow 5 working days. £10m public liability required.
Fareham
Hampshire County Council (HCC) is the highway authority. Skip licence £78.80 first week + £26/week after; allow 5 working days. £10m public liability required.
Havant
Hampshire County Council (HCC) is the highway authority. Skip licence £78.80 first week + £26/week after; allow 5 working days. £10m public liability required.
Eastleigh
Hampshire County Council (HCC) is the highway authority. Skip licence £78.80 first week + £26/week after; allow 5 working days. £10m public liability required.
Portsmouth
Unitary authority — issues its own highway permits. Contact the Licensing Team: 023 9283 4073 · licensing@portsmouthcc.gov.uk
Southampton
Unitary authority. Standard skip licence £37/week; Roll-on/Roll-off £142 first week. Allow 7 working days.
Good to know
Permit fees, lead times and conditions change — always confirm with the issuing authority before booking. We can arrange skips, materials consent and scaffolding licences on your behalf as part of the build. Links are provided for information only; no liability is accepted for their accuracy.
We'll handle the paperworkTell us your postcode and we'll secure the right permits for your project.
Get Permits SortedElectrical, Gas & Refrigerant
Domestic electrical and gas work is highly regulated — and if your project includes air conditioning or heat pumps, the F-Gas regime adds another layer. Here's what governs each, who can do it, and what you receive.
Electrical Safety in Dwellings
All electrical work in a home must comply with Building Regulations Part P and the national wiring standard.
- Building Regs Part PAll electrical work in dwellings must be safe and comply. Notifiable work — new circuits and work in bathrooms, kitchens or outdoors — needs Building Control sign-off or self-certification by a competent person.
- BS 7671 (IET Wiring Regs)The technical standard — currently the 18th Edition (Amendment 2, 2022). All installations and alterations must meet it.
- Electricity at Work Regs 1989Applies to temporary site supplies and any work activity involving electricity during the build.
- Landlord electrical safety (2020)Rental homes in England need an Electrical Installation Condition Report (EICR) at least every 5 years.
Work signed off by an NICEIC, NAPIT or equivalent competent person; you receive an Electrical Installation Certificate (EIC) or Minor Works Certificate.
Gas Installation & Use
Any gas work — boiler, fire, hob or pipework — is strictly controlled to protect against carbon monoxide and explosion.
- Gas Safety (Installation & Use) Regs 1998All gas work must be carried out by a Gas Safe registered engineer. DIY gas work is illegal.
- Building Regs Part JCovers flues, combustion air ventilation, hearths and fuel storage for gas, oil and solid-fuel appliances.
- Landlord Gas Safety RegsRented homes need a Gas Safety Record (CP12) every 12 months, with a copy given to the tenant.
- Gas Safety (Rights to Entry) 2015Gives landlords a defined route to access rental properties to carry out mandatory gas safety checks.
A Gas Safe engineer self-certifies the work and notifies it; you receive a Gas Safety Certificate and boiler commissioning records.
Air Conditioning & Heat Pumps (F-Gas)
Air conditioning and heat-pump systems containing fluorinated greenhouse gases are governed by F-Gas rules to limit leaks and emissions.
- The F-Gas Regulations 2015 (GB)Retained EU Reg 517/2014 — covers stationary refrigeration, A/C and heat pumps. Operators must prevent leakage, use certified technicians and keep system records.
- HFC phase-down & bansHigh-GWP refrigerants (e.g. R410A, R404A) are being phased out; new systems increasingly use lower-GWP alternatives such as R32.
- Leak-checking thresholdsSystems ≥ 5 t CO₂e (hermetically sealed ≥ 10 t) need periodic leak checks; ≥ 500 t CO₂e require fixed leak-detection systems.
- Recovery & record-keepingRefrigerant must be recovered by certified personnel at end of life. Operators maintain logs of refrigerant, servicing and any top-ups for each system.
Only engineers holding a valid F-Gas Company Certificate and individual qualification may install, service or recover refrigerant. Your system must be registered and logged on the F-Gas register.
Why it matters at handoverEvery certificate you receive — EIC, Gas Safe Record, F-Gas log — is required by law and by your insurer, and must be kept with your property records for resale. We hand over a complete compliance pack on every project.
Get Certified WorkHot & Cold Water Systems
Three arrangements dominate English homes. The choice affects pressure, cupboard space, energy use and — crucially — the Building Regs paperwork. Here's how each works, with a plain-English comparison.
Gravity / Indirect
Mains / Unvented
Combi Boiler
Gravity / Indirect System
The traditional English arrangement. Cold water enters a loft cistern, feeding taps cold and gravity-filling a vented hot-water cylinder. Reliable, cheap, and still common in older homes — but low pressure on upstairs outlets and a cold tank that can stagnate.
- Loft cold cistern
- Vented hot cylinder
- Gravity-fed outlets
- Low installation cost
- + Simple & well understood
- + Works during mains outages
- + Cheap parts
- – Low upper-floor pressure
- – Loft tank takes space
- – Cistern hygiene risk
Mains-Fed / Unvented System
Cold water comes straight from the mains — no loft tank. Hot water is stored in a sealed unvented cylinder (often pressurised by the mains) delivering balanced high pressure to every tap and shower. The modern standard for extensions and renovations.
- No loft tank
- Sealed unvented cylinder
- Balanced high pressure
- G3 notification required
- + Excellent hot pressure
- + Balanced mixer showers
- + No stored cold water
- – Needs G3 Building Regs
- – Pressure relief valve + tundish
- – Higher install cost
Combination Boiler (Combi)
A single wall-mounted boiler heats mains cold water on demand — no hot cylinder, no cold tank, instant hot water. Compact and ideal for smaller homes and extensions, though flow drops if several taps run at once.
- Instant hot water
- No cylinder or tank
- Mains cold throughout
- Single appliance
- + Frees up cupboard space
- + No stored water losses
- + Fast to install
- – Limited simultaneous flow
- – Cold-water mains dependent
- – No power-shower boost
Building Regulations — Part G & G3
All hot and cold water installations must meet Approved Document G (sanitation, hot water safety and water efficiency). Unvented hot-water systems over 15 litres additionally require G3 notification to the local authority, with safety discharge via a tundish and temperature/pressure relief valves. We handle the notification and certification as part of the build.
Planning new plumbing?
We'll specify the right system for your pressure, demand and space — and sort the Building Regs notification.
Air-Source & Solar Hot Water
Beyond boilers, low-carbon heat sources can produce your domestic hot water — air-source heat pumps drawing heat from outside air, and rooftop solar thermal collectors harvesting sunlight. Both feed a hot-water cylinder and can work alongside a conventional boiler.
Air Source Heat Pump
Flat-Plate Solar Thermal
Evacuated-Tube Collector
Air Source Heat Pump (DHW)
An air-to-water heat pump extracts heat from outside air (even down to -20°C) and transfers it to a hot-water cylinder. The most common low-carbon hot water route for retrofits and extensions — typically 3 units of heat for every 1 unit of electricity (COP ≈ 3).
- COP around 3.0–3.5
- Works to -20°C
- Needs a DHW cylinder
- MCS grant eligible
- + Big carbon reduction
- + Single unit for heat + hot water
- + Low running cost vs direct electric
- – Needs a well-insulated cylinder
- – Slower reheat than gas
- – Slight outdoor fan noise
Flat-Plate Solar Thermal
Roof-mounted glazed collectors absorb sunlight and heat a fluid that runs through a coil in the hot-water cylinder. The simplest solar hot water technology — robust, lower cost, but lower efficiency in winter and diffuse light than evacuated tubes.
- Glazed flat absorber
- Glycol loop + cylinder coil
- No moving parts on roof
- Lower install cost
- + Free summer hot water
- + Low maintenance
- + Long collector life
- – Lower winter yield
- – Needs a twin-coil cylinder
- – Roof orientation matters
Evacuated-Tube Solar Thermal
Rows of vacuum-insulated glass tubes that capture heat far more efficiently, especially in cold or overcast conditions. Higher output per m² and better year-round performance, at a higher upfront cost than flat-plate.
- Vacuum insulation
- High efficiency / m²
- Tubes individually replaceable
- Best winter yield
- + Works in low light
- + Smaller roof footprint
- + Modular tube replacement
- – Higher cost
- – More fragile in transit
- – Twin-coil cylinder required
Sizing a Cylinder
Allow 40–50 litres per person per day of hot water. ASHPs reheat slower than gas, so size the cylinder generously (200–300 L for a family). Solar thermal needs a twin-coil cylinder — one coil for the solar loop, one for boiler or ASHP backup.
Grants & Compliance
MCS-certified ASHP installations qualify for the Boiler Upgrade Scheme (BUS) grant. All work must meet Building Regs Part G (sanitation), Part L (conservation of fuel), and Part P where electrical work is involved. We handle certification and notification.
Switch to low-carbon hot water
We'll size the heat source and cylinder to your home, sort the grants, and certify the install.
The Cost of Getting It Wrong
Don't Cut Corners
Breaching these Acts isn't just a paperwork issue — it brings criminal prosecution, unlimited fines, imprisonment for directors, and stop-work orders. Fines are set under the Sentencing Council's Health and Safety Offences guidelines, scaled to a company's turnover.
| Act / Regulation | Typical Breach | Summary (Magistrates) | Indictment (Crown Court) |
|---|---|---|---|
| Health & Safety at Work etc Act 1974 (s.33) | General H&S offence — the umbrella for CDM, Work at Height, COSHH, PUWER breaches | Fine up to £20,000 and/or 6 months' imprisonment | Unlimited fine and/or up to 2 years' imprisonment |
| Health & Safety (Offences) Act 2008 | Makes many offences triable either way and raised penalties for individuals | Up to £20,000 fine and/or imprisonment | Unlimited fine and/or imprisonment |
| Construction (Design & Management) Regs 2015 | No construction phase plan, failure to appoint duty-holders, poor site management | Prosecuted under HSWA — fines by sentencing guideline (org turnover) | Large firms: fines up to £10m+; directors may be disqualified |
| Work at Height Regulations 2005 | Unsafe roof work, scaffolding or lack of edge protection | HSWA penalties — falls are the leading cause of death | Unlimited fine and/or imprisonment |
| Gas Safety (Installation & Use) Regs 1998 | Gas work by an unregistered person, dangerous installation | Fine up to £10,000 | Unlimited fine and/or up to 2 years' imprisonment |
| Building Regulations 2010 / Building Act 1984 | Notifiable work without approval, non-compliant construction | Fine up to £5,000 (level 5) + daily continuing fine | Local Authority enforcement notice; rectify or demolish |
| Control of Asbestos Regulations 2012 | Disturbing asbestos without survey/assessment (pre-2000 homes) | Either-way offence — HSWA penalties | Unlimited fine and/or imprisonment |
| F-Gas Regulations 2015 (GB) | Uncertified refrigerant work, no leak checks, no records | Fine up to £5,000 (level 5) | Unlimited fine and/or up to 2 years' imprisonment |
| Party Wall etc Act 1996 | Works to a shared wall without serving notice (civil, not criminal) | Civil — injunction and damages | No criminal penalty; court-ordered rectification |
Current HSE Enforcement Examples
Father & son sentenced for illegal gas work
Unqualified teenager carried out a boiler replacement, causing a gas leak that put tenants — including a baby — at risk.
HSE reportPrincipal contractor fined after fall from height
Worker left 'physically ruined' after falling while dismantling pipework; no arrangements to manage contractors safely on site.
HSE reportGroundworks company fined — dumper overturn
Worker seriously injured when a dumper truck overturned on a housing site near Wigan; driver told not to wear a seatbelt and excavations were unprotected.
HSE reportScaffold tower collapses onto high street
Tower assembled incorrectly with sheeting that acted as a sail; the company had a previous prohibition notice for unsafe scaffolding.
HSE reportApprentice fatally crushed at renovation site
Teenage joiner crushed by unsecured board materials; working alone with no lone-working policy. Managing director handed a suspended prison sentence.
HSE reportContractor — suspended prison sentence
A contractor was given a suspended prison sentence after risking workers' lives and ignoring HSE action on unsafe work at height (roof work).
HSE reportCheck the Registers
Why we take compliance seriouslyEvery case above is a real prosecution from HSE's public records — homes, extensions and renovations included. Cutting corners on safety, gas or asbestos puts lives and liberty at risk. We build to the regulations so you're never one of these stories.
Construction Glossary
Common terms you'll see on drawings, quotes and Building Regulations — defined clearly so you can follow your project with confidence.
Damp-Proof Course (DPC)
abbr. DPCA horizontal barrier built into walls near ground level to prevent moisture rising from the ground into the structure. Typically a layer of waterproof membrane (bituminous, polythene or slate) placed at least 150mm above external ground level.
Damp-Proof Membrane (DPM)
abbr. DPMA continuous waterproof sheet laid under solid ground floors to stop ground moisture rising through the concrete. Distinct from a DPC, which is used in walls.
U-Value
abbr. UA measure of how well a building element conducts heat — the rate of heat transfer in W/m²K. The lower the U-value, the better the insulation. Approved Document L sets maximum U-values for walls, roofs, floors and windows.
Thermal Bridge
A point in the building fabric where insulation is interrupted or bypassed, creating a path for heat to escape (e.g. at junctions, lintels or steel beams). Can cause cold spots, condensation and mould. Designed out with continuity of insulation and thermal breaks.
Airtightness
How well a building resists uncontrolled air leakage through its envelope. Measured as air permeability (m³/h·m² @ 50Pa) under a blower-door test. Lower values mean fewer draughts, less heat loss and better performance — a target ≤5 is typical for low-carbon builds.
Cavity Wall
A wall built of two skins (outer leaf + inner leaf) with a gap between, usually filled with insulation. The cavity prevents moisture crossing from outside to inside while improving thermal performance.
SIP Panel
abbr. SIPStructural Insulated Panel — a factory-made sandwich of two oriented strand board (OSB) skins bonded to a rigid foam core. Forms walls and roofs in one highly insulated, airtight layer with low embodied carbon.
Metsec
A brand of lightweight galvanised cold-rolled steel framing. Used for stud walls, partitions and roof framing where speed, large spans and low weight matter.
Party Wall
A wall shared between two properties, or a wall standing on the boundary line. Work affecting a party wall is governed by the Party Wall etc. Act 1996 and requires formal notice to neighbours.
Foundations
The lower part of the structure that transfers the building's load to the ground. Types include strip, trench-fill, raft and piled foundations, chosen based on soil conditions and building loads.
Notifiable Work
Building work that must be reported to the local authority Building Control body before it starts, so it can be inspected and signed off. Contrast with non-notifiable work, which doesn't require an application but must still meet regulations.
Building Regulations
The legal standards buildings must meet for structure, fire safety, energy, drainage and accessibility, set out in Approved Documents A–S. Compliance is checked by Building Control and certified on completion.
Planning Permission
Local authority consent for the right to carry out development, judged on land use, appearance and impact — distinct from Building Regulations, which govern how it's built.
Permitted Development
abbr. PDA national grant allowing certain minor extensions and alterations to proceed without a full planning application, subject to size and location limits. Conservation areas and listed buildings are usually excluded.
Embodied Carbon
The greenhouse gas emissions associated with manufacturing, transporting and installing a material — before the building is ever used. Lower-carbon materials (timber, SIP, recycled aggregate) reduce a project's whole-life carbon.
Warm-Deck Roof
A flat or low-pitch roof where the main insulation sits above the structural deck, keeping the deck warm and eliminating condensation risk. Preferred over cold-deck construction for flat-roof extensions.
Project Preparation Checklist
Tick off the key steps as you prepare for your build — your progress is saved on this device so you can come back to it.
Pre-Construction
Builders Quotes & Contracts
Quality Standards & Finishes Agreement
Building Contracts, Stage Payments & Quality Control
For small domestic projects we keep paperwork proportionate but watertight — a clear contract, payments tied to inspected milestones, and a quality system that signs off each stage against an agreed finishing standard.
A Simple, Fair Contract
A short written agreement — typically a JCT Homeowner Contract or a bespoke simple contract — that sets out the scope, price, payment stages, programme and how variations are handled. No work starts without one signed by both parties.
Payments Tied to Inspections
You never pay for work that hasn't been inspected. Each payment stage is released only once that stage's inspection is signed off, so cash flow follows verified progress — not the calendar.
Inspected Against an Agreed Standard
Before work starts we agree a written finishing standard. At the close of each stage we jointly inspect against it, record the result, and only then release the stage payment.
Dated, Photographed Sign-Off
Every stage is recorded on a sign-off sheet with notes, photos and a date. You keep a copy, we keep a copy — a clear audit trail from foundations to final snag.
Indicative Stage Payment Schedule
Typical percentages for a domestic extension or renovation. Every stage is inspected and signed off before its payment is invoiced.
Mobilisation
Signed contract, deposit and site set-up confirmed before works begin.
Substructure
Foundations and drainage signed off by Building Control before backfill.
Superstructure
Walls, floors and roof weathertight — inspected and signed off.
First Fix
Structural, electrical and plumbing first fix complete and tested.
Second Fix & Finishes
Plastering, joinery and finishes inspected against the agreed standard.
Handover
Final snag, completion certificate and retention release.
How the Quality System Works
Four steps repeat at every stage of your project — so quality is verified, not assumed.
Agreed Finishing Standard
A written schedule defining materials, tolerances and workmanship expected at each stage — signed off before work starts.
Stage Inspection
At the close of each stage, we and the client jointly inspect against the agreed standard and record the outcome.
Sign-Off Sheet
Each stage is signed off (or snagged) on a formal sheet, with photos and a dated record retained by both parties.
Payment Release
The stage payment is only invoiced once that stage's inspection is signed off — so you pay for work that's been verified.
Key Documents for a Simple Contract
What you should hold before works begin
- Signed contract (e.g. JCT Homeowner or a bespoke simple contract)
- Scope of works and written specification
- Agreed finishing standard / schedule of finishes
- Itemised quotation with stage payment schedule
- Drawings and structural calculations (where applicable)
- Contractor's public liability and employer's liability insurance certificates
- Construction phase plan (CDM 2015) for notifiable works
- Warranties and guarantees (damp-proofing, roofing, windows, etc.)
- Building Control application reference and approval notice
- Party Wall agreement / award (if applicable)
Pre-Construction Contractor Checklist
Vet any proposed contractor against these
- Company registration and trading history (Companies House)
- Public liability insurance of at least £2m and employer's liability cover
- Relevant trade accreditations (e.g. Federation of Master Builders, TrustMark)
- References from at least three comparable recent projects — with site visits
- Evidence of competence for notifiable work (electrical Part P, gas Safe Register)
- Clear written quotation and itemised scope of works
- Proposed stage payment schedule tied to inspections
- Warranties offered and after-care commitment
- CDM duty-holder responsibilities understood and accepted
- No unresolved disputes or outstanding county court judgments
Want it in writing?
Our standard contract, stage payments and quality sign-off system come as standard on every project we quote.
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