Home Extension Design in Merton: Planning, Costs and What's Realistic in 2026

A home extension in Merton typically costs £3,200 to £4,500 per m² to build in 2026, at mid-range specification, before VAT and professional fees. Planning is street-dependent: the borough has around 29 conservation areas, from Wimbledon Village to Mitcham Cricket Green, and the John Innes areas of Merton Park carry Article 4 Directions dating back to 1986 that remove permitted development rights for many alterations. This page covers what the council allows, what it costs, and how to check your own street before design work starts.

Studio MASS is a residential design practice led by an architectural technologist. We design and deliver extensions, loft conversions and refurbishments across London, from first feasibility check to completed build.

Do you need planning permission for an extension in Merton?

Many Merton extensions do not need planning permission. Outside conservation areas, a single-storey rear extension up to 3 metres beyond the original rear wall (4 metres on a detached house) usually falls under permitted development, as do most loft conversions within volume limits.

Merton's conservation areas change that picture locally. The council's list runs to around 29 areas, including Wimbledon Village, John Innes (Merton Park), John Innes (Wilton Crescent), Merton Hall Road, South Park Gardens and Cricket Green in Mitcham. Inside them, side extensions, rear extensions beyond 3 metres and any multi-storey rear extension need planning permission.

The John Innes areas carry the borough's strictest controls. Four Article 4 Directions apply across the John Innes Merton Park and Wilton Crescent conservation areas: the first made in 1986, a second endorsed by the Secretary of State in February 1990, and two further Article 4(2) directions made in July 2007. Together they require planning permission for changes that would elsewhere be permitted development. Merton also publishes a design guide or character assessment for most of its conservation areas, which shapes what gets approved. Checking your address against the council's Article 4 map and the relevant guide is part of our Stage Zero review, before any design work starts.

What types of homes do we extend in Merton?

Merton splits into distinct residential characters. Wimbledon Village keeps its historic core of period houses and cottages, with conservation area rules to match. Merton Park is the borough's showpiece: a planned Victorian and Edwardian garden suburb laid out by John Innes, and the most tightly controlled streets in the borough. Raynes Park offers Edwardian and interwar terraces and semis that suit rear extensions and loft conversions, and Mitcham provides more varied and affordable stock around the historic Cricket Green.

Anyone planning a basement should know Merton has published its own basement and subterranean planning guidance, dating from 2017, which sets expectations for scale, drainage and neighbour impact. For most projects, though, the practical question is simpler: which side of a conservation area boundary the house sits on, and whether a John Innes direction applies.

How much does a home extension cost in Merton in 2026?

Merton sits in London's mid-price construction zone. From our 2026 Extension Cost Guide, built from BCIS indices, FMB build cost data and our own completed projects:

A single-storey extension in Merton costs £3,200 to £4,500 per m² at mid specification in 2026. A loft conversion runs £3,200 to £3,800 per m². Conservation area projects carry roughly 15% on top for material restrictions and period-appropriate detailing. These figures are construction only; VAT at 20% and professional fees are additional.

Before a brick is laid, expect £8,000 to £16,000 in professional and statutory fees on a typical project, including the £548 planning application fee that applies from April 2026. The full breakdown by extension type and specification level is in our free Extension Cost Guide 2026, no email required.

To see how we design and deliver work like this, have a look at the Rathcoole Gardens project page and the Drakefield House project page, both completed London projects with the process shown from brief to build.

Who designs your extension if not an architect?

An architectural technologist designs buildings with a focus on how they are built: construction detail, building regulations, and buildability, alongside layout and light. In the UK "architect" is a legally protected title, so we don't use it. What matters for a Merton extension is the same either way: design within the local planning context, technical drawings a builder can price accurately, and someone accountable from first idea to finished build. That is what we do, through one team and five stages, starting with a Stage Zero review that tells you what is realistic for your budget and what planning will allow before design begins.

Start with a conversation

The best starting point is our short questionnaire. It takes a few minutes and tells us about your home, your brief and your budget. We review it and arrange a free thirty-minute call, with an honest view on feasibility either way.

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