UK extension costs · 2026
Single storey extension cost in the UK (2026 prices)
A single storey extension is the most common way to add space to a UK home. Costs are usually quoted per square metre of new floor area, then adjusted for access, ground conditions, glazing and the fit-out you choose. The ranges below are for a standard builder's finish on a semi-detached house.
Typical single storey extension cost by size
| Size | Indicative cost |
|---|---|
| Small (about 12 m²) | £18,500 – £23,500 |
| Medium (about 20 m²) | £31,000 – £39,500 |
| Large (about 30 m²) | £46,500 – £59,500 |
| Very large (about 45 m²) | £70,000 – £89,000 |
Indicative only, based on typical UK build rates for a standard finish. Your own quote depends on your property, access and specification.
What is included in a single storey extension price
A typical builder's quote covers site set-up, foundations, drainage alterations, brick and block walls, a flat or pitched roof, standard windows and doors, plastering, first and second fix electrics and plumbing, and basic decoration.
It usually excludes kitchens, bathrooms, upgraded flooring, landscaping and professional fees. Ask every builder to itemise these so you are comparing like with like.
Do you need planning permission?
Many single storey rear extensions fall under permitted development, subject to depth, height and boundary limits, and to your property not being a flat, a listed building or in a conservation area. Larger rear extensions may still be possible under the prior approval (neighbour consultation) route.
Building regulations approval is always required, even when planning permission is not.
What affects the price most
- Floor area in square metres — the single biggest driver of the total price.
- Access to the rear or side of the property for machinery, deliveries and waste.
- Ground conditions, drainage runs, manholes and any nearby trees affecting foundation depth.
- Structural openings: a wide steel beam into the existing house costs more than a standard doorway.
- Glazing choice — bifold or sliding doors, roof lanterns and large windows add several thousand pounds.
- Internal fit-out: kitchens, bathrooms, underfloor heating, flooring and decoration.
- Professional fees: architect or designer, structural engineer, building control and any planning application.
- Where you are in the UK — London and the South East typically run 10–20% above national averages.
Frequently asked questions
- How much does a single storey extension cost per m²?
- Most UK single storey extensions work out between roughly £1,600 and £2,300 per m² for a standard finish, before kitchens, bathrooms and professional fees.
- How long does a single storey extension take to build?
- Allow around 10–16 weeks on site for a typical single storey extension, plus design, approvals and lead-in time beforehand.
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