Single storey extension cost by size and finish
Build costs below cover the structure, insulation, plastering, and basic glazed door. Internal fit-out, kitchens, bathrooms, and professional fees are additional. London/South East add 20-30%.
| Size | Basic finish (£1,500/m2) | Standard finish (£2,000/m2) | Premium finish (£2,500-£3,000/m2) |
|---|---|---|---|
| 15 m2 | £22,500 | £30,000 | £37,500 to £45,000 |
| 20 m2 | £30,000 | £40,000 | £50,000 to £60,000 |
| 30 m2 | £45,000 | £60,000 | £75,000 to £90,000 |
| 40 m2 | £60,000 | £80,000 | £100,000 to £120,000 |
Add architect fees £2,000-£5,000, structural engineer £500-£1,500, building regs £800-£1,500, party wall surveyor £700-£2,000 if needed, and VAT at 20%.
What affects the cost of a single storey extension?
The same size extension can cost dramatically different amounts depending on these factors:
- Specification and finish quality: a basic extension uses standard brickwork, a simple flat or lean-to roof, and off-the-shelf doors and windows. A premium extension includes bifold or sliding glazed doors, a glass roof lantern, underfloor heating, and high-specification internal finishes.
- Roof design: a flat roof with a rooflight is the most affordable option. A pitched roof matching the main house, a vaulted roof, or a large glass lantern all increase cost significantly.
- Glazing: bifold doors typically cost £3,000 to £10,000 depending on width, frame material, and specification. Crittal-style steel-framed windows and doors are fashionable and expensive. Rooflights add £500 to £3,000 each.
- Structural complexity: any extension that requires removing a load-bearing wall to connect to the main house needs a structural steel beam (RSJ), typically £1,500 to £3,500 supply and install, plus structural engineer design fees.
- Foundations: most extensions use standard strip or raft foundations. Difficult ground (made-up ground, clay subject to shrinkage, proximity to trees or drains) can significantly increase foundation costs.
- Internal fit-out: an extension for a new kitchen, utility room, or bathroom requires additional plumbing, electrical, and fitting costs on top of the build price.
- Party wall: extensions that involve work on or within 3 m of a shared boundary or party wall require a Party Wall Notice and potentially a Party Wall Award. Budget £700 to £2,000 for a party wall surveyor if your neighbour appoints one.
- Location: London and the South East command a 20-30% labour premium. Remote or difficult-access sites may add delivery and logistics costs.
Planning permission and permitted development
Many single storey rear extensions can be built under Permitted Development (PD) without a planning application, but the rules are specific. A single storey rear extension can extend up to 4 m from the original rear wall of a detached house, or 3 m for a semi-detached or terraced property, and must not exceed 4 m in height. Extensions beyond these limits (up to 8 m detached, 6 m semi/terraced) can be built under the Larger Home Extension Scheme (prior approval), which requires a neighbour consultation.
PD rights do not apply to listed buildings, flats, or properties in conservation areas where additional restrictions apply. Side extensions and those that extend beyond the principal elevation fronting a highway almost always require full planning permission.
Building regulations approval is always required for a habitable extension, regardless of whether planning permission is needed. This covers structural integrity, insulation to Part L standards, fire safety, ventilation, and drainage. Submit a Full Plans application to your local authority building control before work starts and obtain a Completion Certificate on completion - essential for resale and re-mortgaging.
Professional fees and additional costs
The build cost per square metre is only part of the budget. A realistic all-in cost must include professional fees and ancillary costs that are easy to overlook at the planning stage.
Architect or architectural designer: most single storey extensions benefit from professional design input, particularly where planning permission is needed or internal layout is complex. Expect to pay £2,000 to £5,000 for a typical residential extension project, covering initial design, planning drawings, and building regulations drawings.
Structural engineer: required for foundation specification and beam design. Fees are typically £500 to £1,500. Some builders will use a standard engineer, but for unusual ground conditions or complex structures, appoint your own.
Building regulations: local authority building control fees for a single storey extension are typically £800 to £1,500 depending on the value of the work.
VAT: most building work on residential extensions is subject to VAT at 20%. Some conversions and alterations may qualify for a reduced rate - check with your accountant.
Contingency: always budget 15-20% of the build cost as a contingency for unforeseen groundworks, structural discoveries, or specification changes during the build.
Pitfall: quotes that exclude VAT, fees, and fit-out
A common shock for first-time extenders is receiving a builder's quote for, say, £35,000 and later discovering that VAT (£7,000), architect fees (£3,500), structural engineer (£800), building regs (£1,000), and kitchen fit-out (£15,000) push the real total to over £62,000. Always ask builders to confirm whether their quote includes or excludes VAT, and produce a full project cost schedule before committing. Our planner helps you map all these items from the start.
Set your budget before you buy
Our planner helps you set a realistic refurbishment reserve before you buy. If you are purchasing a home with the intention of extending, model the full project cost - structure, fit-out, fees, VAT, and contingency - so you know your true all-in spend and can negotiate accordingly.