London Borough Extension Report 2026: All 33 Ranked
We cross-referenced 299,000 Land Registry transactions, EPC data for every London borough, 17 planning constraint datasets, and an 11-region cost model to rank all 33 boroughs for home extensions. Nobody has done this before.
TL;DR
Inner London boroughs cost 2-3% more to build in than outer London, but property values are 2-5x higher - meaning extensions in inner boroughs deliver better ROI as a percentage of property value. The most constrained boroughs (Westminster, Kensington & Chelsea, Southwark) have the most conservation areas, listed buildings, and Article 4 directions - meaning most extensions require full planning permission. Outer boroughs like Havering, Bexley, and Sutton have fewer constraints and lower build costs, but also lower property values and therefore less value uplift from extending.
£3,000
Inner London cost/m²
Regional cost model
£2,928
Outer London cost/m²
Regional cost model
65
Conservation areas (Lambeth)
planning.data.gov.uk
3,455
Listed buildings (Westminster)
planning.data.gov.uk
Sources: HM Land Registry (2023-2026), DLUHC EPC data, planning.data.gov.uk, Mayfair Studio cost model
The Full Borough Comparison
Every London borough ranked by extension cost per square metre, with median property prices, constraint counts, and the percentage of properties that are freehold (which matters because leaseholders need freeholder consent to extend).
| Borough | Cost/m² | Median terraced | Conservation areas | Listed buildings | Freehold % |
|---|---|---|---|---|---|
| Kensington & Chelsea | £3,000 | £3,000,000 | 41 | 2,609 | 35% |
| Westminster | £3,000 | £2,350,000 | 56 | 3,455 | 18% |
| Camden | £3,000 | £1,650,000 | 40 | 1,967 | 28% |
| Islington | £3,000 | £1,375,000 | 41 | 1,049 | 32% |
| Hammersmith & Fulham | £3,000 | £1,400,000 | 47 | 264 | 37% |
| Hackney | £3,000 | £1,112,000 | 31 | 555 | 37% |
| Wandsworth | £3,000 | £1,040,000 | 46 | 306 | 50% |
| Lambeth | £3,000 | £845,470 | 65 | 949 | 34% |
| Southwark | £3,000 | £875,000 | 56 | 909 | 28% |
| Tower Hamlets | £3,000 | £820,000 | 61 | 904 | 22% |
| Lewisham | £3,000 | £605,000 | 30 | 364 | 47% |
| Newham | £3,000 | £452,000 | 7 | 124 | 39% |
| Haringey | £3,000 | £715,000 | 30 | 224 | 41% |
| Richmond | £2,928 | £868,000 | 87 | 812 | 60% |
| Barnet | £2,928 | £630,000 | 16 | 651 | 51% |
| Ealing | £2,928 | £625,000 | 32 | 307 | 49% |
| Brent | £2,928 | £650,000 | 23 | 582 | 39% |
| Merton | £2,928 | £618,000 | 28 | 243 | 52% |
| Kingston | £2,928 | £560,000 | 28 | 162 | 56% |
| Bromley | £2,928 | £500,000 | 69 | 412 | 62% |
| Hounslow | £2,928 | £518,000 | 27 | 511 | 47% |
| Harrow | £2,928 | £535,000 | 27 | 295 | 54% |
| Hillingdon | £2,928 | £480,000 | 34 | 435 | 53% |
| Enfield | £2,928 | £465,000 | 24 | 300 | 50% |
| Redbridge | £2,928 | £525,000 | 19 | 134 | 51% |
| Waltham Forest | £2,928 | £625,000 | 15 | 118 | 47% |
| Greenwich | £2,928 | £473,515 | 14 | 539 | 44% |
| Croydon | £2,928 | £425,000 | 20 | 181 | 52% |
| Sutton | £2,928 | £475,000 | 15 | 209 | 58% |
| Havering | £2,928 | £425,000 | 11 | 146 | 62% |
| Bexley | £2,928 | £405,000 | 31 | 114 | 63% |
| Barking & Dagenham | £2,928 | £385,000 | 4 | 48 | 49% |
| City of London | £3,000 | £2,043,000 | 28 | 634 | 5% |
Source: HM Land Registry, planning.data.gov.uk, Mayfair Studio cost model • Data as of 2023-2026
Extension Costs: Inner vs Outer London
Build costs in London are driven by two factors: labour rates (40-50% of total cost) and material transport/access. Our composite regional model, built from ConcreteMath trade rates and Costmodelling construction indices, gives inner London a 1.25x multiplier and outer London 1.22x - both against a Midlands baseline.
The real cost difference between boroughs comes not from build costs but from specification. Prime boroughs (Kensington, Westminster, Camden) have higher-spec expectations - better kitchens, more complex glazing, higher-end finishes. The GLA viability study classifies boroughs into value bands A-E, and Band A areas typically see 8-10% higher build costs from specification uplift alone.
| Borough | Region | Build cost (low) | Build cost (high) | All-in inc. fees |
|---|---|---|---|---|
| Kensington & Chelsea | Inner | £40,500 | £58,500 | £75,000-£105,000 |
| Camden | Inner | £40,500 | £58,500 | £72,000-£100,000 |
| Islington | Inner | £40,500 | £58,500 | £70,000-£95,000 |
| Wandsworth | Inner | £40,500 | £58,500 | £68,000-£92,000 |
| Richmond | Outer | £39,528 | £57,096 | £65,000-£90,000 |
| Bromley | Outer | £39,528 | £57,096 | £60,000-£82,000 |
| Croydon | Outer | £39,528 | £57,096 | £58,000-£78,000 |
| Havering | Outer | £39,528 | £57,096 | £55,000-£75,000 |
| Barking & Dagenham | Outer | £39,528 | £57,096 | £53,000-£72,000 |
Source: Mayfair Studio cost model (ConcreteMath + Costmodelling indices) • Data as of 2024/25 rates
Property Prices: Where Extensions Make Most Sense
An extension typically adds 10-20% to property value. That means a £70k rear extension in Kensington (median terraced: £3M) could add £300-600k in value - an 4-8x return. The same extension in Barking (median terraced: £385k) might add £38-77k - barely breaking even after fees.
This explains why inner London homeowners extend despite higher costs and stricter planning. The ROI calculation overwhelmingly favours high-value areas.
| Borough | Detached | Semi-detached | Terraced | Flat |
|---|---|---|---|---|
| Kensington & Chelsea | £7,925,000 | £7,000,000 | £3,000,000 | £950,000 |
| Westminster | £6,500,000 | £5,350,000 | £2,350,000 | £858,900 |
| Camden | £3,775,000 | £2,525,000 | £1,650,000 | £677,000 |
| Hammersmith & Fulham | £1,744,000 | £1,650,000 | £1,400,000 | £635,099 |
| Islington | £2,225,000 | £1,920,000 | £1,375,000 | £590,000 |
| Hackney | £1,640,000 | £1,400,000 | £1,112,000 | £525,000 |
| Wandsworth | £2,300,000 | £1,500,000 | £1,040,000 | £545,000 |
| Haringey | £1,935,000 | £1,218,500 | £715,000 | £449,999 |
| Merton | £2,100,000 | £780,000 | £618,000 | £385,000 |
| Richmond | £1,600,000 | £1,053,000 | £868,000 | £465,000 |
| Barnet | £1,295,000 | £790,000 | £630,000 | £396,660 |
| Lambeth | £1,320,000 | £1,000,000 | £845,470 | £465,000 |
| Southwark | £1,422,500 | £1,245,000 | £875,000 | £470,000 |
| Lewisham | £861,750 | £720,000 | £605,000 | £367,000 |
| Ealing | £1,300,000 | £690,000 | £625,000 | £407,340 |
| Brent | £915,000 | £650,000 | £650,000 | £400,000 |
| Waltham Forest | £800,000 | £650,000 | £625,000 | £388,000 |
| Kingston | £1,100,000 | £725,000 | £560,000 | £385,000 |
| Enfield | £1,000,000 | £637,500 | £465,000 | £305,000 |
| Bromley | £860,000 | £605,000 | £500,000 | £325,000 |
| Harrow | £982,750 | £625,000 | £535,000 | £331,875 |
| Redbridge | £870,000 | £625,000 | £525,000 | £300,000 |
| Hounslow | £725,000 | £555,000 | £518,000 | £350,000 |
| Hillingdon | £790,000 | £545,000 | £480,000 | £303,000 |
| Greenwich | £741,000 | £560,000 | £473,515 | £395,000 |
| Sutton | £860,000 | £610,000 | £475,000 | £285,000 |
| Tower Hamlets | £1,150,000 | £832,000 | £820,000 | £465,000 |
| Croydon | £740,000 | £531,500 | £425,000 | £277,000 |
| Havering | £645,000 | £487,250 | £425,000 | £265,000 |
| Newham | £552,500 | £485,000 | £452,000 | £397,425 |
| Bexley | £635,000 | £482,000 | £405,000 | £260,250 |
| Barking & Dagenham | £495,000 | £422,250 | £385,000 | £249,250 |
Source: HM Land Registry Price Paid Data • Data as of January 2023 - March 2026
Planning Constraints: Which Boroughs Are Hardest?
Planning constraints directly affect whether you can extend under permitted development or need full planning permission. Conservation areas remove most PD rights. Article 4 directions can remove specific PD rights even outside conservation areas. Listed buildings require separate listed building consent for any alteration.
We counted every constraint feature across 17 datasets from planning.data.gov.uk, supplemented by direct council website data for boroughs not yet publishing to the national platform. The results show massive variation: Waltham Forest has 807 Article 4 restriction areas submitted to the national platform, while Richmond - the most conservation-area-heavy borough - has 119 individual Article 4 directions protecting properties within its conservation areas. Richmond also leads on conservation areas at 87, more than any other London borough.
| Borough | Conservation areas | Listed buildings | Article 4 directions | TPO zones |
|---|---|---|---|---|
| Richmond | 87 | 812 | 119* | 1,025** |
| Bromley | 69 | 412 | 48* | 2,000+** |
| Lambeth | 65 | 949 | 5* | 226 |
| Tower Hamlets | 61 | 904 | 3* | 1 |
| Westminster | 56 | 3,455 | 10* | 263 |
| Southwark | 56 | 909 | 11* | 724 |
| Hammersmith & Fulham | 47 | 264 | 48* | 450** |
| Wandsworth | 46 | 306 | 11* | 250** |
| Kensington & Chelsea | 41 | 2,609 | 102* | 41 |
| Islington | 41 | 1,049 | 14* | - |
| Camden | 40 | 1,967 | 19* | - |
| Hillingdon | 34 | 435 | 4* | - |
| Ealing | 32 | 307 | 12* | 600** |
| Bexley | 31 | 114 | 7* | - |
| Hackney | 31 | 555 | 13* | 585** |
| Haringey | 30 | 224 | 7* | - |
| Lewisham | 30 | 364 | 4* | 345** |
| City of London | 28 | 634 | 1+* | - |
| Kingston | 28 | 162 | 3* | - |
| Merton | 28 | 243 | 5* | 600+** |
| Harrow | 27 | 295 | 4* | - |
| Hounslow | 27 | 511 | 3* | 604** |
| Enfield | 24 | 300 | 15* | 365** |
| Brent | 23 | 582 | 15* | 301 |
| Croydon | 20 | 181 | 8* | - |
| Redbridge | 19 | 134 | 11* | - |
| Barnet | 16 | 651 | 12* | 3,213 |
| Sutton | 15 | 209 | 4* | 375** |
| Waltham Forest | 15 | 118 | 6* | 159 |
| Greenwich | 14 | 539 | 2* | 500** |
| Havering | 11 | 146 | 3* | - |
| Newham | 7 | 124 | 4* | - |
| Barking & Dagenham | 4 | 46 | 2* | - |
Source: planning.data.gov.uk; council websites (March 2026) • Data as of March 2026
Listed buildings: Historic England NHLE via planning.data.gov.uk. Article 4 directions: * values are distinct direction instrument counts verified directly from council websites across all 33 London boroughs in March 2026. National platform (planning.data.gov.uk) counts are not shown as they reflect geographic polygons rather than instruments - one direction can generate hundreds of polygons (e.g. Waltham Forest: 807 polygons for 6 instruments; Southwark: 480 polygons for 11 instruments). TPO zones: unmarked values are polygon zone counts from planning.data.gov.uk submissions; ** values are TPO order counts from council websites or FOI responses (March 2026) - not directly comparable as council counts reflect distinct orders, while national platform counts individual polygons. Most boroughs hold TPO data in interactive map viewers only and do not publish total counts; dashes indicate no count publicly available.
Freehold vs Leasehold: Why It Matters for Extensions
If you're leasehold, you need your freeholder's consent to extend - and they can charge for it. Central London boroughs have much higher leasehold rates: Westminster is 82% leasehold, City of London 95%, Kensington 65%. Outer boroughs are overwhelmingly freehold: Bexley 63%, Havering 62%, Bromley 62%.
This is a hidden cost that many homeowners overlook. Freeholder consent for an extension can cost £5,000-£20,000+ in legal and surveyor fees, and some freeholders impose conditions on the design or take a share of the value uplift.
Best Value Boroughs for Extensions
The best borough for an extension depends on your priorities. Here are three different rankings:
Best ROI (value added vs cost)
Kensington & Chelsea, Westminster, Camden. Despite the highest build costs, property values are so high that even a modest 10% uplift from a £70k extension delivers £200-300k+ in added value. The ROI is unbeatable - but planning is hardest here too.
Easiest planning
Barking & Dagenham, Newham, Havering. Fewest conservation areas and Article 4 directions. Most extensions qualify for permitted development. Straightforward process with fewer heritage constraints.
Lowest absolute cost
All outer London boroughs at £2,928/m² baseline. Barking & Dagenham, Havering, and Bexley also have GLA value band E classification, meaning spec expectations are lower and builders can be more competitive.
Cross-Dataset Insights
By combining these datasets, patterns emerge that individual sources can't show:
- 1.Constraint density correlates with property value. The 10 most constrained boroughs have a median terraced price of £915k. The 10 least constrained: £467k. Heritage protection and high values reinforce each other.
- 2.Waltham Forest is the Article 4 capital. With 807 Article 4 direction areas - more than any other London borough - many homeowners who think they have PD rights don't. This is a common surprise for buyers in E17 and E10.
- 3.Barnet has the most TPOs. 3,213 tree preservation order zones - meaning extensions often need arboricultural assessments. This adds £500-1,500 to project costs and can constrain foundation design.
- 4.Southwark is the hidden constraint giant. 480 Article 4 areas, 724 TPO zones, 56 conservation areas, and 909 listed buildings. Despite being perceived as less restrictive than Kensington or Camden, its constraint density is among the highest in London.
- 5.Richmond has the most conservation areas of any borough (87), yet property transactions remain strong at 8,180 sales in our dataset - suggesting heritage status doesn't deter buyers, it attracts them.
How We Built This Report
This report cross-references four major open datasets that nobody else has combined at this level:
- 1.HM Land Registry Price Paid Data - 299,471 London transactions from January 2023 to March 2026. Filtered by district (column 13), aggregated by property type (D/S/T/F). Median prices calculated per borough per type.
- 2.EPC Certificates - Domestic energy performance certificates for all 33 London boroughs. Floor areas, energy ratings, wall construction, and heating types extracted from DLUHC open data.
- 3.Planning Constraint GeoJSON - 17 datasets from planning.data.gov.uk covering conservation areas, listed buildings, Article 4 directions, TPOs, flood zones, green belt, and more. Features counted per borough via the organisation-entity field.
- 4.Regional Cost Model - Composite multiplier from ConcreteMath trade rates (2024/25) and Costmodelling construction indices, calibrated against DesignForMe's 2025 survey of 1,500+ completed projects.
Borough Deep Dives
We've published detailed data guides for every London borough. Each one includes property prices, EPC data, constraint maps, and borough-specific extension costs:
Frequently Asked Questions
Which London borough is cheapest for extensions?
Outer London boroughs all share the same baseline build cost of approximately £2,928/m². Within that group, Barking & Dagenham, Havering, and Bexley tend to have the lowest all-in costs because spec expectations are lower (GLA value band E) and there are fewer planning constraints that add delay and professional fees.
Which London borough has the most conservation areas?
Richmond upon Thames leads with 87 conservation areas according to planning.data.gov.uk data. Lambeth has 65, Tower Hamlets 61, Westminster and Southwark each have 56. Conservation areas remove most permitted development rights for extensions.
Do I need planning permission for an extension in London?
It depends on your specific property. Many extensions qualify as permitted development, but conservation areas, Article 4 directions, listed building status, and other constraints can remove these rights. Our free AI chat checks your exact address against all constraint datasets instantly.
How much does an extension cost per m² in London?
Build costs (structure only) are approximately £3,000/m² in inner London and £2,928/m² in outer London. All-in costs including kitchen, fees, VAT, and contingency are typically 2-2.5x the build cost, so £5,500-£7,500/m² total. This is based on our composite model using ConcreteMath trade rates and Costmodelling indices.
What data sources does this report use?
This report combines four datasets: HM Land Registry Price Paid Data (299k London transactions, 2023-2026), DLUHC Energy Performance Certificates for all 33 boroughs, 17 planning constraint GeoJSON datasets from planning.data.gov.uk, and our composite regional cost model built from ConcreteMath and Costmodelling data. All data is Open Government Licence.
Related Articles
Before you buy, bid or exchange, get the planning answer and the numbers on the specific site. Planning potential, risk, comparable approvals nearby and an indicative maximum bid - £395 fixed, delivered in 24 hours, every fact verified against primary public sources.
Get the planning due diligence report - £395, 24 hours