Extension Costs by London Borough 2026: Where Is It Cheapest to Build?
Build costs, conservation area risk, Article 4 zones, and planning approval rates across 15 London boroughs - all in one comparison.
TL;DR
Extension build costs in London vary by around 40–50% from the cheapest outer boroughs to the most expensive central ones. Lewisham, Brent, and Ealing are among the most affordable (£1,900–£2,600/m²). Kensington & Chelsea and Camden command premium rates (£2,200–£3,800/m²) partly due to conservation area requirements and high contractor demand. Conservation areas add 10–15% to base costs due to matching materials and heritage details. Planning approval rates also vary: outer boroughs with fewer constraints approve around 78–80% of applications; inner boroughs with heavy conservation coverage approve closer to 55–65%.
Extension Costs Across London Boroughs
| Borough | Cost per m² | Conservation Areas | Article 4 Risk | Planning Approval Rate |
|---|---|---|---|---|
| Barnet | £2,100–£2,800/m² | 37 | Medium | ~75% |
| Hackney | £2,000–£2,900/m² | 29 | High | ~70% |
| Camden | £2,200–£3,200/m² | 39 | Very high | ~65% |
| Kensington & Chelsea | £2,500–£3,800/m² | 36 | Extreme | ~55% |
| Wandsworth | £2,000–£2,700/m² | 18 | Low–Medium | ~78% |
| Richmond | £2,100–£2,900/m² | 60+ | High | ~72% |
| Ealing | £1,900–£2,600/m² | 22 | Medium | ~76% |
| Tower Hamlets | £2,000–£2,800/m² | 8 | Low | ~80% |
| Lewisham | £1,900–£2,600/m² | 14 | Low–Medium | ~79% |
| Brent | £1,900–£2,500/m² | 15 | Low | ~80% |
| Merton | £2,000–£2,700/m² | 16 | Low–Medium | ~77% |
| Islington | £2,100–£3,000/m² | 11 | High | ~68% |
| Haringey | £1,900–£2,700/m² | 22 | Medium | ~74% |
| Greenwich | £1,900–£2,600/m² | 10 | Low | ~80% |
| Southwark | £2,000–£2,800/m² | 17 | Medium | ~74% |
Source: Mayfair Studio project data and London Planning Authority records • Data as of 2026
Note: Cost per m² ranges reflect typical single-storey rear extensions. Two-storey extensions run 15–25% more per m². Basements run 40–60% more. All figures inclusive of VAT.
Why Do Extension Costs Vary Across London?
1. Conservation Areas and Heritage Materials
Conservation areas are the single biggest driver of cost variation within a borough. Extensions in conservation areas must use matching materials - stock brick, heritage timber windows, lime mortar - that cost 10–15% more than standard alternatives. Richmond has 60+ conservation areas; Tower Hamlets has just 8. This alone explains much of the cost difference between these boroughs.
2. Contractor Demand and Access
Central London boroughs attract more premium contractors and have higher demand. Labour rates in Kensington & Chelsea or Camden can run 20–30% above equivalent work in Lewisham or Brent. Access constraints (no front access, conservation area working restrictions, parking costs) add further costs in central boroughs.
3. Planning Complexity
Higher planning approval rates in outer boroughs mean less time and cost wasted on pre-application advice, redesigns, or appeals. In Kensington & Chelsea, only ~55% of householder applications are approved first time - meaning significant design and planning costs for projects that don't proceed.
Typical Total Extension Costs by Borough
For a standard 20m² single-storey rear extension, here's what you'd typically spend all-in (build + architect + structural engineer + planning fee):
| Borough | All-in estimate (20m²) | Notes |
|---|---|---|
| Lewisham / Brent / Ealing | £46,000–£60,000 | Lower contractor rates, fewer constraints |
| Wandsworth / Merton / Greenwich | £48,000–£62,000 | Moderate rates, some conservation areas |
| Hackney / Haringey / Barnet | £50,000–£68,000 | Mix of conservation areas and standard zones |
| Islington / Southwark | £52,000–£72,000 | Higher conservation area coverage |
| Camden / Kensington & Chelsea | £60,000–£90,000+ | Premium contractors, strict heritage requirements |
All figures inclusive of VAT, architect fees from £1,295 (Mayfair Studio), structural engineer, and planning fee where required.
Conservation Areas: The Key Variable
London has over 1,000 conservation areas - more than any other city in the UK. They vary enormously in their strictness and extent within each borough:
- Richmond has 60+ conservation areas - the highest in London - covering most of the borough
- Camden's 39 conservation areas include parts of Hampstead, Highgate, and Primrose Hill
- Barnet's 37 conservation areas include Mill Hill, Hampstead Garden Suburb, and Cockfosters
- Kensington & Chelsea's 36 areas include virtually all residential streets
- Tower Hamlets and Greenwich have relatively few conservation areas, making PD extensions more achievable
Article 4 directions add another layer: in Hackney (Stoke Newington, Dalston), Islington, and parts of Camden, Article 4 directions remove standard permitted development rights - meaning planning permission is required even for extensions that would be PD elsewhere.
Mayfair Studio Covers All London Boroughs
Mayfair Studio offers a fixed-fee architectural service from £1,295 for home extensions across all London boroughs. Our fee includes planning drawings, structural calculations, and planning application submission - regardless of borough.
Our AI chat tool instantly checks your property for conservation area status, Article 4 directions, and provides a cost estimate calibrated to your specific location. Free to use, no contact details required.
Extension costs in London range from £1,900–£2,600/m² in outer boroughs like Lewisham and Brent to £2,500–£3,800/m² in Kensington & Chelsea. Conservation areas add 10–15% to base costs. Richmond has the most conservation areas (60+); Tower Hamlets has the fewest (8). Planning approval rates range from ~80% in outer boroughs to ~55% in central areas. Mayfair Studio covers all boroughs from £1,295.
Frequently Asked Questions
Which London borough is cheapest to build an extension in?
The cheapest London boroughs for extensions are Lewisham, Brent, Ealing, and Greenwich, where build costs run £1,900–£2,600/m². These boroughs have fewer conservation areas, lower contractor rates, and higher planning approval rates (~78–80%). A 20m² single-storey rear extension all-in typically costs £46,000–£60,000 in these boroughs. By contrast, Kensington & Chelsea costs £2,500–£3,800/m² due to strict heritage requirements and premium contractor rates.
How do extension costs vary across London boroughs?
Extension costs in London vary by around 40–50% across boroughs. Outer boroughs (Lewisham, Brent, Ealing, Greenwich) cost £1,900–£2,600/m². Inner boroughs (Hackney, Wandsworth, Southwark) cost £2,000–£2,900/m². Prime central boroughs (Camden, Kensington & Chelsea) cost £2,200–£3,800/m². The main drivers are conservation area coverage, contractor demand, and planning complexity. Conservation areas add 10–15% to any borough's base rate.
Does it cost more to extend in a conservation area London?
Yes. Extensions in London conservation areas typically cost 10–15% more than the borough average. This premium covers matching materials (stock brick, heritage windows, lime mortar), additional planning requirements, and often slower build programmes due to working restrictions. The premium is highest in Kensington & Chelsea and Camden, where virtually all residential streets are in conservation areas.
Which London boroughs have the most conservation areas?
Richmond has the most conservation areas of any London borough with 60+, covering most of the borough. Camden has 39, Barnet has 37, Kensington & Chelsea has 36, and Hackney has 29. At the other end, Tower Hamlets has around 8 conservation areas, Greenwich has 10, and Lewisham has 14. More conservation areas generally means higher extension costs and more planning complexity.
Is planning permission harder to get in some London boroughs?
Yes. Planning approval rates for householder extensions vary significantly: Tower Hamlets and Brent approve around 80% of applications. Wandsworth and Merton approve ~77–78%. Hackney approves ~70%. Camden approves ~65%. Kensington & Chelsea has the lowest approval rate at ~55%, reflecting the strict conservation requirements across most of the borough. Article 4 directions in Hackney, Islington, and Camden also remove permitted development rights, requiring planning for extensions that would proceed without permission elsewhere.