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

A home extension in Wandsworth typically costs £3,500 to £4,500 per m² to build in 2026, at mid-range specification, before VAT and professional fees. Basement space costs more: £8,000 to £12,000 or more per m² under the house. Whether you need planning permission depends on the street. Wandsworth has 46 conservation areas, and eight of them carry Article 4 Directions that remove permitted development rights for external alterations. This page covers what the council allows, what it costs, and the Tooting basement project we took over mid-build.

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

Do you need planning permission for an extension in Wandsworth?

Many Wandsworth 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.

Inside one of the borough's 46 conservation areas the position changes: side extensions, rear extensions beyond 3 metres and any rear extension of more than one storey need planning permission. Eight conservation areas also carry Article 4 Directions restricting external works that would otherwise be permitted development: Dover House Estate, Heaver Estate, Latchmere Estate, Putney Lower Common, Roehampton Village, Shaftesbury Park Estate, Totterdown Fields Estate and Wandsworth Common. Several of these, such as the Heaver Estate by Tooting Bec Common and Totterdown Fields in Tooting, cover exactly the red-brick Victorian and Edwardian terraces people most want to extend.

The practical rule: never assume permitted development applies in Wandsworth until someone has checked your street against the council's conservation area map and your property's planning history. That check is part of our Stage Zero review, before any design work starts.

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

A single-storey extension in Wandsworth costs £3,500 to £4,500 per m² at mid specification in 2026, with basic specification starting nearer £3,000 per m². A loft conversion runs £3,200 to £3,800 per m². Conservation area projects, common across Putney, Battersea and the streets between Wandsworth and Clapham Commons, carry roughly 15% on top for material restrictions and period detailing. These figures are construction only; VAT at 20% and professional fees are additional.

Wandsworth spans travel zones 2 and 3, from the Victorian terraces of Tooting and Balham to the larger houses of Putney and the riverside streets of Battersea, so build prices sit at the top of London's mid band and can approach inner-London rates of £4,000 to £5,500 per m² on the most constrained sites.

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.

Can you build a basement in Wandsworth?

Yes, and basement extensions have become common in Wandsworth, but the council publishes clear limits. Planning permission is usually required, and always for flats, listed buildings and lightwells. Council guidance says to retain at least half the rear garden undug, keep a minimum 2 metre depth of front garden (normally a requirement in conservation areas), and it states plainly that multi-level basements are not appropriate in Wandsworth. Many properties also sit in flood risk areas, so groundwater reports and an independently verified construction method statement are expected.

On cost, basement space runs £6,000 to £8,000 or more per m² under the garden and £8,000 to £12,000 or more per m² under the house. A basement is rarely the cheap way to add space. It is the way to add space when the loft is converted and the garden matters too much to lose.

What does a real Wandsworth basement look like?

At Drakefield House in Tooting, we designed and delivered a full-footprint basement of roughly 100 m² beneath a Victorian terrace, as part of a whole-house transformation built for around £750,000 in construction cost, excluding the client-supplied kitchen, fixtures and fittings. Studio MASS was brought in mid-build to take control of a struggling project, and the design includes a fire-engineered strategy that allowed the open-plan stair to stay. The full story, with real costs, is on the Drakefield House project page.

That project is also a useful lesson in accountability. Basements punish gaps between design, engineering and site management, and a rescue mid-build is far harder than getting the team right at the start.

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 Wandsworth 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.

Start Here →