Hounslow Extension Data 2026: Costs, Prices & Planning Constraints
Hounslow spans from the affluent streets of Chiswick to the Heathrow corridor towns of Feltham and Hanworth. We analysed 7,252 property transactions and 27 conservation areas to build the complete extension data picture.
TL;DR
Hounslow is a tale of two halves. Eastern Hounslow (Chiswick, Brentford riverside) behaves like inner London - Victorian terraces commanding £700k-1M+, numerous conservation areas, and homeowners who extend to avoid moving. Western Hounslow (Feltham, Hanworth, Hounslow West) has lower prices, fewer constraints, and more straightforward extensions under permitted development. The borough-wide median of £518k for terraced houses masks this split. Build costs are £2,928/m² across the board, but specifications in Chiswick typically run 15-20% higher.
7,252
Transactions (2023-2026)
HM Land Registry
£518k
Median terraced price
Land Registry
27
Conservation areas
planning.data.gov.uk
47%
Freehold tenure
Land Registry
Sources: HM Land Registry (2023-2026), planning.data.gov.uk, Mayfair Studio cost model
Hounslow in Numbers
Median property prices by type from 7,252 Land Registry transactions across the borough, covering sales from January 2023 to March 2026.
| Property type | Median price | vs London average |
|---|---|---|
| Detached | £725,000 | Below average |
| Semi-detached | £555,000 | Below average |
| Terraced | £518,000 | Below average |
| Flat | £350,000 | Below average |
Source: HM Land Registry Price Paid Data • Data as of January 2023 - March 2026
Property Profile
Chiswick dominates the eastern end of the borough with Victorian and Edwardian terraces, many in conservation areas. Bedford Park - often called London's first garden suburb - is a prime example. These properties have narrow side returns and deep rear gardens, making side-return and rear extensions the most common project types.
Brentford and Isleworth have undergone substantial regeneration, with new riverside developments pushing up flat prices. The Great West Quarter and Brentford Lock West schemes have added thousands of new-build flats, which partly explains the lower-than-expected freehold rate of 47%.
Feltham, Hanworth, and Hounslow Central have more inter-war and post-war housing stock - 1930s semis and terraces with good plot sizes. These areas are generally outside conservation areas and offer the most straightforward extension opportunities in the borough.
Planning Constraints
Hounslow has 27 conservation areas, heavily concentrated in the eastern half of the borough. Chiswick alone accounts for several, including the Bedford Park Estate, Strand-on-the-Green, and Chiswick House grounds. Brentford has the Butts conservation area and several along the river.
Western Hounslow has far fewer designations. Feltham, Hanworth, and Cranford have minimal conservation area coverage, meaning most properties retain full permitted development rights. This east-west split in constraints mirrors the property price gradient.
| Constraint type | Count | Impact on extensions |
|---|---|---|
| Conservation areas | 27 | Removes most PD rights for extensions |
| Article 4 direction areas | 3 | Covers Bedford Park/Gunnersbury CAs, HMO conversions, and office-to-residential in town centres |
| Listed buildings | 511 | Restricts modifications; requires listed building consent |
| TPO zones | 604** | Sourced from FOI response to LB Hounslow (March 2026) |
Source: planning.data.gov.uk / LB Hounslow • Data as of March 2026
Conservation areas and listed building counts from planning.data.gov.uk via spatial query. Article 4 direction count sourced from LB Hounslow council website (March 2026) - 3 directions: A4D.1 (Bedford Park/Gunnersbury CAs), A4D.2 (HMO conversions), A4D.3 (office-to-residential). TPO count (604**) sourced from FOI response to LB Hounslow - double asterisk denotes council order count, not national platform polygon zones.
Extension Costs
Hounslow sits in the outer London cost region with a build cost of £2,928/m² and GLA value band D. In practice, Chiswick projects tend to cost 15-20% more than the baseline due to higher specification expectations, restricted site access on narrow Victorian streets, and conservation area requirements for matching materials.
| Extension type | Size | Build cost | All-in inc. fees |
|---|---|---|---|
| Small rear extension | 15m² | £39,528 - £57,096 | £55,000 - £75,000 |
| Large rear extension | 25m² | £65,880 - £95,160 | £85,000 - £120,000 |
| Loft conversion (dormer) | 20-25m² | £52,704 - £85,680 | £65,000 - £100,000 |
| Side return extension | 10-12m² | £26,352 - £42,768 | £40,000 - £60,000 |
Source: Mayfair Studio cost model • Data as of 2024/25 rates
All-in costs include professional fees, kitchen/bathroom fit-out where applicable, VAT, and 10% contingency. Chiswick projects may run 15-20% above these estimates due to specification uplift.
Cross-Dataset Insights
Combining Land Registry, planning constraint, and cost data reveals patterns specific to Hounslow:
- 1.Chiswick is inner London in all but name. Terraced house prices in Chiswick regularly exceed £1M, approaching Hammersmith & Fulham levels. Yet Hounslow is classified as outer London for build costs (£2,928/m² vs £3,000/m²). This makes Chiswick one of the best ROI locations for extensions - inner London property values at outer London build costs.
- 2.The freehold rate is suppressed by new development. At 47%, Hounslow's freehold rate is lower than similar outer boroughs (Hillingdon 53%, Ealing 49%). This is driven by large-scale flatted developments in Brentford and Hounslow town centre. For houses specifically, the freehold rate is much higher.
- 3.Conservation area concentration creates a planning postcode lottery. Two streets apart in Chiswick can have completely different planning regimes. One inside Bedford Park conservation area needs full planning permission; the next street over can extend under PD. This makes address-level checking essential.
Frequently Asked Questions
How much does a rear extension cost in Hounslow?
A 15m² rear extension in Hounslow costs approximately £39,500-£57,000 for the build alone, or £55,000-£75,000 all-in including fees, kitchen, VAT, and contingency. Chiswick properties typically cost 15-20% more due to higher specifications.
Do I need planning permission for an extension in Chiswick?
Many Chiswick properties sit within conservation areas (Bedford Park, Strand-on-the-Green, Turnham Green) where permitted development rights are restricted. You will likely need planning permission. Our free AI chat checks your exact address instantly.
Is Hounslow cheaper to extend than Ealing or Hammersmith?
Hounslow and Ealing share the same outer London build rate of £2,928/m². Hammersmith & Fulham is inner London at £3,000/m². The real cost difference comes from specification expectations - Chiswick projects tend to run higher spec than western Hounslow or most of Ealing.
What is the freehold percentage in Hounslow?
Hounslow is 47% freehold across all property types. This is lower than comparable outer London boroughs due to large new-build flat developments in Brentford and Hounslow town centre. For houses specifically, the freehold rate is significantly higher.
Does Heathrow affect extensions in Hounslow?
Properties near Heathrow (particularly in Cranford, Heston, and parts of Hounslow West) may fall within flight path noise contour zones. This can affect glazing specifications and potentially add £2,000-£5,000 for acoustic glazing requirements in extensions.
Related Articles
London Borough Extension Report 2026
All 33 London boroughs ranked for extensions.
Hillingdon Extension Data 2026
Extension data for neighbouring Hillingdon.
Extension Costs by London Borough 2026
Cost per m² compared across 15 boroughs.
Do I Need Planning Permission?
Decision framework for London homeowners.