House Extension Cost in Harrow on the Hill: 2026 Price Guide

Beyond the kitchen-focused rear extension, many homeowners in Harrow on the Hill look at side-return infills or full double-storey extensions to add real square footage without moving house - particularly on the terraced streets where a side return is otherwise wasted space.

Typical costs

• Side return infill: roughly £25,000-£40,000
• Single-storey rear extension: roughly £35,000-£70,000
• Double-storey extension: roughly £70,000-£130,000+

A double-storey extension isn't simply double the cost of a single-storey one - the roof, foundations and scaffolding are a large fixed cost, so the additional storey is often better value per square metre.

What affects the price locally

Harrow on the Hill sits within the London Borough of Harrow, and a meaningful share of its housing stock falls inside 8 conservation areas covering the Hill - over a third of all Harrow's conservation areas, among the earliest designated in England - which affects both what you're allowed to build and what materials you're expected to use. Matching original brick, slate or stone detailing on an extension in a conservation area typically costs more than a standard modern finish, and can extend the planning timeline too.

Access is again a factor on the tighter terraced streets: scaffolding, skips and material deliveries on narrow roads near Harrow School and St Mary's Church often need council permits, which a local builder will usually have already dealt with before.

Planning permission

Side return and single-storey rear extensions often qualify for permitted development if they stay within set size limits, but double-storey extensions almost always need full planning permission from the London Borough of Harrow. Get a pre-application check if your street falls within a conservation area - it's a small upfront cost that can save a redesign later.