UK extension costs · 2026
Rear extension cost in the UK (2026 prices)
A rear extension extends the back of the house into the garden and is the most popular extension type in the UK. Prices below are for a single storey rear extension on a semi-detached house with a standard builder's finish.
Typical rear extension cost by size
| Size | Indicative cost |
|---|---|
| 3 m deep across part of the rear (about 15 m²) | £23,500 – £29,500 |
| 3 m deep full width (about 22 m²) | £34,000 – £43,500 |
| 4 m deep full width (about 30 m²) | £46,500 – £59,500 |
| 6 m deep full width (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.
How deep can a rear extension be?
Under permitted development, single storey rear extensions are generally limited to 3 m for an attached house and 4 m for a detached house, with larger depths possible via the prior approval route where your council allows it.
Depth affects more than planning: the deeper the extension, the more you need to think about natural light, roof structure and how the new space connects to the original house.
Drainage and boundaries
Rear extensions frequently build over or near existing drains, which may require a build-over agreement with your water company and a deeper foundation design.
Building right up to a boundary usually means a party wall notice and careful detailing so scaffolding and maintenance are still possible.
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 3m rear extension cost?
- A 3 m deep full-width rear extension of around 22 m² typically costs in the region of £35,000 to £50,000 for a standard finish, before kitchen or bathroom fit-out.
- Does a rear extension add value?
- A well-designed rear extension that keeps a usable garden generally adds value, particularly where it creates an open-plan kitchen-dining space.
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