What can you build, and what does the council decide?
Permitted development gives you 3m on a terrace or semi and 4m detached, doubling under prior approval. Everything beyond that is an application, and London councils approved 83.0% of the 77,680 householder applications they decided in the three years to March 2026.
| Limit or figure | Value | Source |
|---|---|---|
| Rear depth, terrace or semi | 3m, or 6m under prior approval | GPDO 2015, Class A |
| Rear depth, detached | 4m, or 8m under prior approval | GPDO 2015, Class A |
| Side extension width | half the original house | GPDO 2015, Class A |
| Two-storey rear, distance to boundary | at least 7m | GPDO 2015, Class A |
| London householder approval rate | 83.0% | MHCLG PS1 and PS2 |
| Householder application fee | £548 | statutory, England 2026 |
| Prior approval fee | £249 | statutory |
Sources: the Town and Country Planning (General Permitted Development) (England) Order 2015, Schedule 2, Part 1, Class A; MHCLG District Planning Application Statistics (PS1 and PS2), twelve quarters to March 2026; fees from the Fees for Applications Regulations 2012 as amended, 2026 rates. Read at 1 September 2026. Build costs quoted on this page are our own 2026 London range and are an estimate.
Which type of extension suits your house?
London homes can be extended in several ways. The right choice depends on your property layout, budget, and planning constraints.
Rear extension
The most common type. A single-storey rear extension projects from the back of the house, typically creating a larger kitchen-diner. Under Permitted Development, you can extend up to 6 metres (semi/terrace) or 8 metres (detached).
Side return extension
Absorbs the narrow alleyway running along the side of a terraced house. Typically combined with a rear extension to create an L-shaped or full-width ground floor. See our Camden side return case study.
Wraparound extension
Combines side return and rear extension into a single structure. Creates the largest open-plan space. Usually requires full planning due to size. See our Wandsworth wraparound case study.
Double-storey extension
Extends at both ground and first floor. Costs 50–70% more than single-storey (not double — foundations and roof are shared). Always requires planning.
Flat roof extension
Most single-storey extensions in London use a flat roof with single-ply membrane. Rooflights bring light into deep plans.
Kitchen extension
The most popular reason for extending — a larger kitchen-diner.
What does a London extension cost in 2026?
- Single-storey rear: £1,800–2,800 per sqm (construction)
- Double-storey: £2,500–3,500 per sqm
- Side return only: £2,000–3,000 per sqm
- Wraparound: £1,900–2,600 per sqm
Do you need planning permission?
Many single-storey rear extensions fall under Permitted Development. Key limits: 6m (semi/terrace) or 8m (detached), 4m max height, matching materials. Conservation areas and Article 4 Directions typically require full planning.
We recommend a Lawful Development Certificate (£274) for any PD project. Read our planning permission guide.
Structural considerations
Most extensions require at least one RSJ beam. Our packages include structural engineer coordination. See our building regulations service.
Party Wall
If your extension involves work within 3 metres of a neighbour’s foundation, the Party Wall Act 1996 requires formal notices at least two months before work. Applies to most side return and rear extensions on terraced properties.
How much does the borough change the number?
Inner vs outer London (20sqm extension)
Browse extension services by borough:
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