How the extension cost estimate works
Extensions are usually priced per m² of new floor area. The calculator uses a low and high build rate for your type of extension, adjusts it for region and finish, then adds the things build rates normally leave out:
Typical build rates (outside London, standard finish)
| Extension | £ per m² | 20m² | 30m² |
|---|---|---|---|
| Single-storey | £1,800–£2,800 | £36k–£56k | £54k–£84k |
| Two-storey | £1,600–£2,600 | £32k–£52k | £48k–£78k |
| Shell only | £1,200–£1,700 | £24k–£34k | £36k–£51k |
Rates are based on published 2026 UK cost guides and trade surveys. They differ between sources, especially for London (often 15–40% higher), so treat them as a starting point.
Typical extras
| Item | Typical cost |
|---|---|
| Kitchen (supply & fit) | £8,000–£25,000 |
| Bathroom / shower room | £5,000–£12,000 |
| Bifold or sliding doors | £4,000–£10,000 |
| Rooflights (each) | £1,000–£1,500 |
| Architect / drawings | £3,000–£6,000 |
| Structural engineer | £1,000–£2,000 |
| Building control | £1,000–£1,500 |
| Planning application (if needed) | £300–£600 |
| Party wall agreement | £1,000–£2,000 per neighbour |
This is a budgeting tool, not a quote. Ground conditions, drains, access, steel beams and how much of the existing house is altered can all change the price. Get at least three written quotes, and check exactly what each one includes.
Worked example: a 20m² kitchen extension
A 20m² single-storey rear extension with a standard finish, outside London and the South East:
- Build: 20m² × £1,800–£2,800 = £36,000–£56,000.
- Extras: a new kitchen, bifold doors and two rooflights add £14,000–£38,000. The kitchen is the biggest variable.
- Fees: drawings, structural engineer, building control and a planning or prior-approval application add £5,300–£10,100.
- Contingency: 10% of build and extras, £5,000–£9,400.
- VAT at 20% on top brings it to about £72,000–£136,000, a mid-point of around £104,000.
The range is wide because the finish and the kitchen make such a difference. Untick the extras to see the cost of the shell and structure alone, which is what most builders' headline "per m²" prices refer to.
The stages, in order
- Brief and budget: decide what the space is for and what you can spend, including the contingency.
- Design: measured survey and drawings. Check whether the work is permitted development or needs planning permission.
- Approvals: planning (or a lawful development certificate), Building Regulations approval, and party wall notices if they apply.
- Tender: send the same drawings and specification to at least three builders so the quotes are comparable.
- Build: groundworks and foundations, walls, roof, windows and doors, first fix, plastering, second fix, then decorating. A simple single-storey extension often takes a few months on site.
Ways to keep the cost down
- Keep the shape simple. A rectangle with a straightforward roof costs less than corners, angles and complex glazing.
- Avoid moving drains and services if you can. Building over or diverting a public sewer adds cost and needs water company approval.
- Do the finishing yourself. A shell-only contract leaves decorating, flooring and sometimes kitchen fitting to you.
- Fix the specification before you sign. Changes during the build are the most common cause of budget overruns.
Thinking of going up instead? Compare with the loft conversion cost calculator. For the groundwork quantities, see the foundation and skip size calculators, or browse all project cost calculators.