Single-storey rear extension: costs, stages and hiring a builder

A practical guide to commissioning a single-storey rear extension: the right order of professionals, realistic costs, and how to avoid being ripped off.

MW-01What the job actually involvesA single-storey rear extension adds space at the back of the house, usually for a bigger kitchen or diner. It means new foundations, walls, a roof, and often removing or altering the existing rear wall to open the space up. It's a proper building project with groundworks, structural steels, and new services, not a weekend job. Expect disruption for several months, not days.

MW-02Check whether you need planning permissionMany single-storey rear extensions fall under 'permitted development' and don't need full planning permission, but this depends on size, height, how close you are to the boundary, and whether you've extended before. Conservation areas, listed buildings, and flats usually need permission regardless. Even if you qualify for permitted development, it's worth getting a Lawful Development Certificate from the council as proof for when you sell.

RULES

KNOW THE RULES

Planning permission rules in England are set out on GOV.UK. HomeGraft has a 'Know the rules' guide on planning permission — check it before committing to a design.

MW-03Get a designer and structural engineer in firstBefore you speak to a builder, you need a design. An architect or architectural designer draws up the plans, and a structural engineer works out the steel beams and foundations needed. Builders quote against a finished design, not vague ideas — skipping this step is how costs spiral later. Typical cost: £1,500–£4,000 for design, plus £500–£1,500 for structural calculations.

MW-04Building Regulations approvalAlmost every extension needs Building Regulations approval, separate from planning permission. You can apply for full plans approval before work starts, or use a building notice once you've chosen a builder. A building control inspector (from the council or a registered building control approver) checks the work at key stages and issues a completion certificate at the end — keep this safe.

RULES

KNOW THE RULES

Building Regulations approval is a legal requirement for all extensions in England. HomeGraft has a 'Know the rules' guide explaining the process.

MW-05Check if you need a Party Wall noticeIf your extension's foundations are near or on a shared boundary with next door, the Party Wall Act may require you to formally notify your neighbour before work starts. This isn't optional and ignoring it can lead to work being halted or legal disputes. For foundations dug near next door's building, notice must be given at least one month before work starts; for work on a party wall or a new wall on the boundary line, it's two months. Agreement, or a surveyor's award, is needed before that work starts.

RULES

KNOW THE RULES

The Party Wall Act 1996 applies where work is near a shared boundary. HomeGraft has a 'Know the rules' guide on Party Wall notices.

MW-06Realistic costs and timescalesFor an average three-bedroom house outside London, a single-storey rear extension (around 15–25m²) typically costs £30,000–£70,000 in 2026, including building work, basic kitchen reinstatement, and professional fees. London and the South East usually run higher — often 20–40% more for the same job. What pushes a job to the top of the range: bifold or sliding doors, a roof lantern, underfloor heating, difficult ground conditions, or a high-spec kitchen fit-out. Design and planning typically take 2–4 months; the build itself usually takes 8–14 weeks once it starts.

MW-07What a good quote should containA proper quote itemises labour, materials, and fees separately, not one lump sum. It should state exact specifications (which bricks, which doors, which insulation), a start and estimated finish date, and who is responsible for submitting Building Control applications. It should also show whether VAT is included and confirm the builder's public liability insurance. Get at least three quotes and compare like-for-like, not just the bottom line.

MW-08Questions to ask a builder, and red flagsAsk to see recent similar jobs, speak to past customers, and confirm they're used to working with Building Control inspectors. Ask how they handle unexpected costs and delays, and get this in writing. Red flags: pressure to pay a large deposit in cash, no fixed business address, no insurance documents, or reluctance to give a written contract. A builder who won't put anything in writing is a warning sign, however good they sound.

MW-09Contracts and payment stagesNever pay the full amount upfront. Agree staged payments tied to completed milestones — for example, after foundations, after the roof is on, after plastering. For larger jobs, a written contract (such as a JCT Homeowner Contract) protects both sides and sets out what happens if things go wrong or run late. Keep a small retention (often 2.5–5%) until snagging is finished.

MW-10What commonly goes wrongHidden problems like old drains, poor ground conditions, or unexpected damp are common and can add cost once work starts. Disputes often arise from verbal 'extras' that were never priced or agreed in writing. Builders occasionally overrun badly, or in rare cases go out of business mid-project — staged payments protect you here. Neighbour disputes over noise, access, or boundary works are also common if the Party Wall process was skipped.

MW-11Final sign-off and keeping recordsAt the end of the job, you should receive a Building Regulations completion certificate — this is essential proof for future buyers and mortgage lenders. Keep all plans, structural calculations, certificates, and warranties (including any structural guarantee insurance) in one folder. Don't release the final payment until snagging issues are fixed and all paperwork is in hand.

RULES

KNOW THE RULES

A Building Regulations completion certificate is a legal document confirming the work meets the regulations — solicitors will ask for it when you sell the house.

MW-12If you live in Wales, Scotland or Northern IrelandIn Wales, planning rules are broadly similar to England but permitted development limits differ — check with the Welsh Government or your local council. In Scotland, there's no 'Building Regulations' system as such; instead you need a building warrant from your local authority before work starts, and a completion certificate afterwards. In Northern Ireland, planning permission and Building Regulations are handled separately through your local council, with their own forms and timescales. Always check the current rules for your nation before commissioning work.

RULES

KNOW THE RULES

Scotland requires a building warrant before work begins — this is a legal requirement, not optional. Rules and bodies differ by nation; check GOV.WALES, mygov.scot, or nidirect for current details.

KNOW THE RULES

Before you start: Do I need planning permission for a single-storey rear extension? · Do I need a party wall agreement?

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