"How much will it cost?" is always the first question, and the honest answer is: it depends — but not so much that you can't plan around it. Build cost does most of the work, a handful of fees and approvals add to it, and a contingency protects you from the surprises that every building project has.
This guide gives a realistic 2026 budget for a single-storey rear extension in the East Midlands, and shows where the money goes. The figures are planning guides, not quotes — the only real price is the one a builder gives you against a finished set of drawings.
The build cost
For a typical single-storey rear extension in the East Midlands in 2026, a sensible mid-range planning figure is around £2,000 per square metre of floor area. That's for a decent, standard specification: a solid structure, a flat or pitched roof, good insulation, standard windows and doors, and a basic finish.
| Size | Floor area | Approximate build cost |
|---|---|---|
| Modest (3m × 4m) | 12m² | £24,000 |
| Typical (4m × 5m) | 20m² | £40,000 |
| Generous (5m × 6m) | 30m² | £60,000 |
Treat the figure as a starting point. Costs vary with where you are in the country (London and the South East are well above the Midlands), with how busy builders are locally, and above all with specification. The same footprint can cost half as much again if you choose large sliding or bi-fold doors, a glazed roof, underfloor heating, or a high-end kitchen.
If you're considering building up as well as out, see Single-Storey vs Double-Storey Extensions.
What's usually not in the build cost
Check carefully what a figure includes. Many headline prices don't cover:
- The kitchen or bathroom fit-out — units, worktops, appliances and sanitaryware are often priced separately
- Floor finishes, decoration and landscaping
- VAT — building work on extensions is generally charged at 20%, so make sure every quote clearly states whether VAT is included, and compare like with like
- Professional fees and approvals (covered below)
- Unforeseen ground or drainage work
Professional fees and approvals
These are smaller than the build, but they're real, and they need budgeting from the start.
Design and drawings. Architectural design, planning drawings (where needed), construction drawings and structural calculations. This is typically a few percent of the build cost, with minimum fees on smaller projects. It's the part of the budget most worth getting right — good drawings are what let builders price accurately and avoid expensive variations later.
Planning fees (if you need them). If your extension needs a householder planning application, the council fee in England is £548 in 2026. The Planning Portal also charges a separate service charge (around £90) if you apply through it. If your extension is within permitted development, you may need no application at all — though a Lawful Development Certificate (half the planning fee) is worth considering for the paperwork when you sell. See Do You Need Planning Permission for a Rear Extension?
Building control. Every extension needs building regulations approval, whether or not it needs planning permission. A typical fee for a domestic extension is around £700–£900, depending on the provider. See Planning Permission vs Building Regulations.
Water authority approval. If your extension is built near or over a public sewer, you'll need a build-over agreement with the water company. Application fees are typically in the region of £150–£200, though a CCTV drainage survey may also be needed.
Party wall costs. If you're building on or close to a shared boundary, the Party Wall etc. Act may apply, and a surveyor may be involved. Costs vary widely, so allow for it if you're building on the boundary.
Community Infrastructure Levy. Rarely an issue for a standard extension, but extensions adding over 100m² of floor area may be liable.
A worked example
For a 4m × 5m (20m²) single-storey rear extension in the East Midlands:
| Item | Approximate cost |
|---|---|
| Build cost (20m² at around £2,000/m²) | £40,000 |
| Design, drawings and structural calculations | £1,000–£2,000 |
| Householder planning application, if needed | around £640 |
| Building control | £700–£900 |
| Water authority approval, if needed | £0–£200 |
| Contingency (10% of build cost) | £4,000 |
| Total | roughly £46,000–£48,000 |
That's before the kitchen fit-out, finishes and VAT treatment, which can add significantly depending on your choices.
Why you need a contingency
Every extension has unknowns until the ground is opened up: unexpected drainage, poor soil needing deeper foundations, a wall that turns out to be built differently from how it appears, or a steel beam bigger than expected once the structural engineer has done the sums. A contingency of around 10% of the build cost is standard practice, and more for older houses. If you don't spend it, great — but you'll rarely regret having it.
What makes the price go up
- Opening up the back of the house — removing a load-bearing wall needs a steel beam and structural design
- Complex roofs and lots of glazing
- Difficult access — limited site access makes deliveries and labour slower and costlier
- Poor ground conditions — requiring deeper or specialist foundations
- Moving drainage or services — manholes and pipes in the way of the new extension
- Changes during the build — the most expensive way to design is on site
How to keep the budget under control
- Get the design settled before you build. Changes during construction cost far more than changes on paper.
- Get at least three quotes against the same drawings and specification, so you can compare like with like.
- Be clear about your priorities. Spend on structure, insulation and the elements that are hard to change later; save on fittings you can upgrade at any time.
- Understand what's in and out of every quote, particularly VAT, provisional sums and fit-out.
Get a figure for your house
Our online estimator builds a cost breakdown for your project from your postcode and the size of extension you have in mind, including professional fees and the main approvals, so you can see the whole picture rather than just the build cost. It's a starting point, not a fixed quote — if you'd like to go through it properly, get in touch.