Home Extension Guide UK — Costs & Planning 2026
Extending your home is usually cheaper and less disruptive than moving. A well-planned extension can add 10–20% to your property value while giving you the space you need. Here is the practical guide to making it happen.
Types of Extension and Costs
Single-Storey Rear Extension
The most popular type. Extends the kitchen, living room, or creates an open-plan kitchen-diner. Size typically 3m – 8m deep.
Double-Storey Rear Extension
Same footprint as single-storey but with an additional room above (usually a bedroom or bathroom). Better value per square metre than single-storey because the foundations and roof are shared.
Side Return Extension
Uses the narrow alley down the side of a terraced or semi-detached house. Usually extends the kitchen by 1–2 metres in width. Very popular in Victorian terraces.
Wrap-Around Extension
Combines a rear extension with a side return for an L-shaped footprint. Creates the most space and works brilliantly for open-plan living.
Conservatory / Orangery
A lighter, faster option. Modern orangeries with proper insulation and solid roofs are a step up from the old uPVC conservatories. Can be used year-round with proper heating.
Planning Permission
Many extensions fall under permitted development (no planning permission needed), but there are strict limits:
| Rule | Detached | Semi / Terraced |
|---|---|---|
| Max rear depth (single) | 8m * | 6m * |
| Max rear depth (double) | 3m | 3m |
| Max height (single) | 4m | 4m |
| Max eaves height (double) | 3m | 3m |
| Side extension width | Half the original house | Half the original house |
* Under the larger home extension scheme (prior notification required). Standard permitted development limits are 4m (detached) and 3m (semi/terraced).
You need planning permission if: You exceed these limits, live in a conservation area, listed building, or AONB, or your permitted development rights have been removed. See our planning permission guide.
Building Regulations
All extensions need building regulations approval. This covers structural safety, fire protection, insulation, ventilation, drainage, and electrics. Your builder or architect will handle the application, and building control will inspect at key stages. Budget £500 – £1,000 for building control fees.
Hidden Costs to Budget For
| Cost | Typical amount |
|---|---|
| Architect / designer | £2,000 – £5,000 (or 7–15% of build cost) |
| Structural engineer | £500 – £1,500 |
| Planning application | £206 (England, 2026) |
| Building control | £500 – £1,000 |
| Party wall surveyor | £700 – £1,000 per neighbour |
| Kitchen fitting | £5,000 – £20,000 |
| Landscaping | £2,000 – £10,000 |
Add 10–15% contingency to your total budget for unexpected issues (drainage, foundations, ground conditions).
How to Maximise Value
Open-plan kitchen-dining-living is what buyers want. If extending for resale value, this is the layout to aim for.
Add a downstairs WC if you do not have one. High demand, relatively low cost (£3,000 – £5,000).
Bifold or sliding doors to the garden create the indoor-outdoor feel that adds perceived value beyond the cost.
Do not over-extend for the area. If your neighbours’ houses are worth £350,000, spending £100,000 on an extension will not make yours worth £450,000. There is a ceiling price for every street.
Get a completion certificate. Without it, you will have problems when you come to sell. The buyer’s solicitor will flag it.
Planning to finance your extension with a remortgage? Use our Mortgage Calculator on CalcPad to see what the additional borrowing would cost per month.
Last updated May 2026. Costs are estimates and vary significantly by location, specification, and ground conditions. Always get at least three quotes.