Harrow Extension Data 2026: Costs, Prices & Planning Constraints
We analysed 6,367 Land Registry transactions, EPC records, and planning constraint data for Harrow. With near-million-pound detached homes, 27 conservation areas, and outer London build costs, Harrow sits in an interesting middle ground for extension economics.
TL;DR
Harrow combines outer London build costs (£2,928/m²) with property values that approach inner London levels - median detached homes are nearly £983k. This creates a favourable ROI for extensions, particularly on the larger detached and semi-detached properties in areas like Pinner and Stanmore. The 27 conservation areas are concentrated around Harrow on the Hill and Pinner village, leaving the majority of suburban residential areas with full PD rights. At 54% freehold, more than half of all properties can extend without freeholder consent.
£535k
Median terraced price
HM Land Registry
54%
Freehold ownership
Land Registry tenure data
27
Conservation areas
planning.data.gov.uk
£2,928
Build cost per m²
Regional cost model
Sources: HM Land Registry (2023-2026), planning.data.gov.uk, Mayfair Studio cost model
Harrow in Numbers
From 6,367 residential transactions recorded between January 2023 and March 2026, here are the median prices by property type. Harrow's detached homes approach £1M - significantly higher than most outer London boroughs.
| Property type | Median price | Context |
|---|---|---|
| Detached | £982,750 | Premium areas: Pinner, Stanmore, Harrow Weald |
| Semi-detached | £625,000 | Dominant across Rayners Lane, North Harrow, Kenton |
| Terraced | £535,000 | Common in South Harrow, Wealdstone, Greenhill |
| Flat | £331,875 | Harrow town centre and station-adjacent conversions |
Source: HM Land Registry Price Paid Data • Data as of January 2023 - March 2026
Property Profile
Harrow's housing stock is dominated by interwar suburban development - the 1930s semi is ubiquitous across Rayners Lane, North Harrow, and Kenton. These were built as part of the Metroland expansion, following the Metropolitan Railway into what was then rural Middlesex.
EPC data confirms predominantly cavity-wall construction with generous plot sizes by London standards. Many semis and detached homes have 90-120m² of existing floor space and large rear gardens - ideal for both ground-floor rear extensions and loft conversions.
Freehold ownership at 54% reflects the mix of houses and flats. The suburban areas are overwhelmingly freehold, while flat-heavy zones around Harrow town centre and along the Metropolitan line stations are predominantly leasehold.
Planning Constraints
Harrow has 27 conservation areas - a moderate number that places it alongside Hounslow (27) and Kingston (28) in the middle tier of outer London constraint density. The key conservation areas are concentrated around Harrow on the Hill (including the famous Harrow School campus), Pinner village, and parts of Stanmore.
Harrow has 4 Article 4 directions (sourced from Harrow council website, March 2026). The national planning dataset also records 295 listed buildings for Harrow, with no TPO zones published (via spatial query on planning.data.gov.uk). The overall constraint picture is moderate - most of the suburban belt between Rayners Lane and Kenton has full permitted development rights.
The Harrow on the Hill conservation area is the most significant planning consideration. Properties on and around the Hill face stricter scrutiny, and the elevated position means roof extensions are more visible and more likely to be refused.
Extension Costs in Harrow
Harrow falls within the outer London cost region at £2,928/m² baseline. As a GLA value band D borough, specification expectations are moderate - above the budget end but not at the premium levels seen in bands A-C.
| Extension type | Size | Build cost (low) | Build cost (high) |
|---|---|---|---|
| Rear extension | 15m² | £39,528 | £57,096 |
| Rear extension | 25m² | £65,880 | £95,160 |
| Loft conversion | 40m² | £105,408 | £152,256 |
| Side return | 10m² | £26,352 | £38,064 |
Source: Mayfair Studio cost model (outer London, band D) • Data as of 2024/25 rates
Build costs only. All-in costs including kitchen, fees, VAT, and contingency are typically 1.8-2.2x the build cost.
Cross-Dataset Insights
Combining Land Registry, EPC, and planning data reveals patterns specific to Harrow:
- 1.Harrow's detached homes are priced like inner London but cost outer London rates to extend. At £982,750 median, detached homes in Pinner and Stanmore are valued above the London-wide average. But build costs remain at £2,928/m² - meaning extension ROI is better here than in inner London boroughs with comparable property values.
- 2.The semi-to-detached price gap is £357,750. At £625k vs £982,750, extending a semi in Harrow to create detached-equivalent living space could save significantly versus trading up. Stamp duty alone on a £983k purchase would cost £39,137 - nearly enough to fund a 15m² rear extension.
- 3.Metroland housing is ideal for extensions. The interwar semis and detached homes that dominate Harrow have generous plot ratios, cavity-wall construction, and standard roof pitches - all of which simplify extension design and construction. Loft conversions are particularly straightforward in these properties.
- 4.Pinner's conservation status creates a two-tier market. Properties within Pinner village conservation area face tighter planning controls, but those just outside benefit from the area's desirability without the constraints. Extension activity is highest in the streets immediately adjacent to conservation boundaries.
Frequently Asked Questions
How much does an extension cost in Harrow?
Build costs in Harrow are approximately £2,928/m² (outer London rate). A 15m² rear extension costs £39,500-£57,000 for the build, with all-in costs typically reaching £70,000-£125,000 depending on specification.
Do I need planning permission for an extension in Harrow?
Most extensions in Harrow qualify as permitted development outside the 27 conservation areas. Key conservation areas are around Harrow on the Hill and Pinner village. Our free AI chat checks your exact address against all constraint datasets instantly.
Which areas of Harrow have conservation areas?
Harrow's 27 conservation areas are concentrated around Harrow on the Hill (including the Harrow School campus), Pinner village, and parts of Stanmore. The suburban areas of Rayners Lane, North Harrow, Kenton, and South Harrow are largely unconstrained.
Is Harrow a good borough for extending?
Harrow is excellent for extensions. Outer London build costs (£2,928/m²) combined with near-inner-London property values (detached at £983k) create strong ROI. The Metroland housing stock is well-suited to extensions, and 54% freehold ownership means most house owners can proceed without freeholder consent.
What is the average house price in Harrow?
Based on 6,367 Land Registry transactions (2023-2026), median prices in Harrow are: detached £982,750, semi-detached £625,000, terraced £535,000, flat £331,875. Premium areas like Pinner and Stanmore trade above these medians.