A 3m single-storey rear extension is priced from four separate pots: the build itself, the design and compliance package, statutory fees, and the finishes you choose. Only the first is quoted per square metre, and it is the one that moves most between builders.
Last checked against current guidance on 2026-08-09. Rules differ by borough — the local pages below say how.
A rear extension budget splits into build cost, professional fees, statutory fees and finishes — and only build cost scales cleanly with floor area.
Most cost articles quote a single £/m² figure and stop. That figure is the builder's number, and it hides the three other pots that decide whether the project runs smoothly.
| Pot | What it covers | What moves it |
|---|---|---|
| Build | Groundworks, structure, envelope, first and second fix | Ground conditions, steel quantity, access, month of tender |
| Design & compliance | Measured survey, planning or lawful development drawings, building regulations package, structural calculations | Whether the scheme is permitted development or needs full planning; whether steels are involved |
| Statutory | Planning application or certificate fee, building control charge, party wall surveyor if the neighbour dissents | Set by regulation and by your local authority — not negotiable |
| Finishes | Glazing, floor, kitchen, heating, joinery | Entirely your specification; the widest range of the four |
The reason this matters more than a headline rate: two identical 3m extensions on the same street can differ by tens of thousands because one has a 5m structural opening and bifolds and the other has a masonry pier and a standard door set.
On a terraced or semi-detached house, a 3m single-storey rear extension usually falls within permitted development — provided the house has not already been extended, the height limits are met, and the property is not on designated land.
The relevant rules are in the General Permitted Development Order. For a single-storey rear extension the headline limits are 3m depth on a terrace or semi and 4m on a detached house, a maximum height of 4m, and a maximum eaves height of 3m where the extension is within 2m of a boundary.
Deeper schemes — 6m and 8m respectively — are possible through the prior approval route, where the council notifies your adjoining neighbours and determines the application only on the impact on their amenity.
Being within permitted development does not mean no paperwork. It means no planning application — and a strong reason to get a Lawful Development Certificate instead, which is what your buyer's solicitor will ask for.
Because they are pricing different information. A builder quoting from a sketch prices the risk; a builder quoting from a full drawing package prices the work.
Ask three builders to price a scheme described in words and you will get three different schemes back. Ask three builders to price the same set of drawings, with the structural openings, insulation build-ups and drainage runs already resolved, and the quotes converge — because they are pricing an identical scope.
That convergence is the most reliable saving available on a domestic project, and it happens before anyone digs.
Build cost moves with your builder, your specification and the month you tender. What you can pin down early is the design and compliance scope — how many drawing sets the council and building control will want, and whether structural input is in play. The estimator below works that out from the project type and floor area.
Move the controls for an indicative figure. It anchors your budget — it is not a quote. Every project is priced after a free survey.
Build costs are separate and quoted by your contractor. See our published fees.
Planning and building control fees are set by regulation and by your local authority; they are the same whoever prepares your drawings.
At the time of writing the householder planning application fee in England is £548, a lawful development certificate for a proposed use is half that, and prior approval for a larger home extension is £249. These are set nationally by regulation, reviewed periodically, and identical for every applicant.
Building control is charged separately by the local authority or a registered building control approver, and depends on the size and complexity of the work rather than on who drew it.
Neither fee is a design fee. If a quote bundles them without saying so, ask for the split.
Fix the things a builder cannot price around: the survey, the planning route, the structural openings, the drainage strategy and the insulation build-up. Leave genuinely optional items — glazing spec, floor finish, kitchen — as provisional sums you can move without redesigning.
That order also protects your programme. Councils determine most householder applications in eight weeks; building regulations approval can run alongside groundworks, but planning cannot.
No. It is within the depth limit for a terrace or semi, but the allowance also depends on height, boundary distance, previous extensions, whether the property is on designated land and whether an Article 4 Direction applies. Check all five before assuming.
It is not compulsory, but it is the only document that proves the work was lawful. Buyers' solicitors routinely ask for one, and obtaining it years later — once the evidence has gone — is far harder than applying before you build.
Local authorities aim to determine householder planning applications within eight weeks of validation, and prior approval for a larger home extension within 42 days. Validation itself can take a week or two if the submission is incomplete.
You can submit a building regulations application at any time, but building to a design that planning later changes wastes the work. On a scheme with any planning risk, resolve planning first.
If you are excavating within 3m of a neighbour's structure and deeper than their foundations, or building on the boundary line, then yes — the Party Wall etc. Act 1996 applies regardless of planning status.
