House Extension Cost in Highgate: 2026 Price Guide

A side-return infill or a full double-storey extension is the next step up for a lot of Highgate 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 Highgate, within the London Boroughs of Camden, Haringey and Islington, sits inside the Highgate conservation area, jointly overseen by all three boroughs it spans, 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.

Access is again a factor on the tighter terraced streets: scaffolding, skips and material deliveries on narrow roads near Waterlow Park and Highgate Cemetery 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 Boroughs of Camden, Haringey and Islington. 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.