Rear extension in London
A rear extension pushes the ground floor (occasionally two storeys) into the rear garden, almost always to create a larger kitchen-dining space opening onto the garden. On London's terraced and semi-detached stock it is the single most common domestic project — and the one with the deepest planning precedent record on nearly every street.
Value comes from added floor area and from reconfiguring the dark middle of a Victorian plan into one usable space. Typical London schemes add 12–20m² with full-width glazing; the build interacts with drainage, party walls on both sides, and — in conservation areas — close scrutiny of brick, bond and roofline.
The planning route — PD or permission?
Permitted development (GPDO Class A) covers single-storey rear extensions up to 3m beyond the original rear wall on attached houses and 4m on detached, with a maximum height of 4m. The 'larger home extension' route extends this to 6m/8m through prior approval with neighbour consultation — but that larger route is not available in conservation areas.
In conservation areas, the basic 3m/4m single-storey PD allowance usually survives unless an Article 4 direction removes it — which is exactly what Article 4 directions in areas like Hampstead and Muswell Hill do. Flats have no PD rights at all. Two-storey rear extensions in conservation areas always need full permission.
Practical rule for the five boroughs Siteline covers: check the address first. If a conservation area or Article 4 direction applies, budget for a full householder application decided on design, neighbour daylight (the 45-degree test) and materials.
What it really costs
| Cost per m² (low — straightforward site) | £3,000 |
| Cost per m² (expected) | £3,800 |
| Cost per m² (high — conservation spec, hard access) | £4,600+ |
| Typical project (12–18m² single storey) | £42,000 – £128,000 |
| Professional fees, surveys, party wall (add) | 10–18% of build |
Indicative London ranges calibrated from Hampstead Renovations project data. Conservation-area specifications (matching stock brick, lime mortar, bespoke glazing) and restricted rear access are the two biggest cost drivers we see. VAT not included.
Realistic timeline
| Design and drawings | 4–8 weeks |
| Planning decision (full application) | 8–10 weeks |
| Prior approval route, where available | 42 days |
| Party wall agreements | 4–10 weeks (parallel) |
| Build | 3–5 months |
What catches people out
- Article 4 directions silently removing the PD route — always verified at address level, never assumed from the borough.
- The 45-degree daylight test trimming depth on tight terraced gardens.
- Party wall awards on both flanks adding cost and programme before a brick is laid.
- Drainage build-over agreements where the extension crosses a shared sewer.
- Conservation-area material conditions (brick match, roof finish) discovered after pricing rather than before.
Rear extensions, borough by borough
Precedent-driven: hold to your street's established projection depth and match the stock brick.
Camden planning guide →Mostly flats and listed stock — assume a full application and check for listed building consent.
Westminster planning guide →The 45-degree daylight test shapes rear extensions here more than any design policy.
Islington planning guide →London's most active rear-extension market; contemporary design with good brickwork is mainstream.
Hackney planning guide →PD carries many schemes in the east; Muswell Hill and Highgate need full applications.
Haringey planning guide →Rear extensions, asked straight
How much does a rear extension cost in London?
Do I need planning permission for a rear extension?
How long does the whole process take?
What adds the most value?
Do I need a party wall agreement?
Related reading
Almost every London extension, loft and basement engages this Act. Here's how it actually works.
Read the guide →The size limits, the exceptions, and the postcode-level traps.
Read the guide →Real ranges from real projects — not brochure numbers.
Read the guide →Could you build one at your address?
The rules above bend at address level — conservation areas and Article 4 directions change the route entirely. Run the live constraint check before you spend on drawings.
Siteline provides planning and cost intelligence for early feasibility only. It is not legal, planning, valuation, architectural, structural, or surveying advice. All estimates are indicative and must be verified by qualified professionals before purchase, design, planning submission, or construction.
Cost estimates are indicative only — not a quotation. Final price depends on survey, specification, structure, access, party wall matters, VAT, professional fees, and contractor availability.
Planning outcomes are not guaranteed. Local planning authorities make final decisions.