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

A side-return infill or a full double-storey extension is the next step up for a lot of Harrow on the Hill homeowners who want real extra square footage without moving - especially on terraced streets, where the side return is often dead space anyway.

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

Because so much of Harrow on the Hill, within the London Borough of Harrow, sits inside 8 conservation areas covering the Hill - over a third of all Harrow's conservation areas, among the earliest designated in England, extensions here face real constraints on both design and materials - expect matching brick, slate or stone detailing to cost more than a modern finish would, and the planning timeline to run longer as a result.

Tighter terraced streets bring the same access problem here as with any extension: scaffolding, skips and deliveries near Harrow School and St Mary's Church often need a council permit, and a builder who already works locally will usually have been through that process 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.