House Extension Cost in Morden: 2026 Price Guide

A side-return infill or a full double-storey extension is the next step up for a lot of Morden 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 Morden, within the London Borough of Merton, sits inside the Upper Morden conservation area, 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 Morden Hall Park, a National Trust property 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 Merton. 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.