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

A home extension in Lewisham typically costs £3,200 to £4,500 per m² to build in 2026, at mid-range specification, before VAT and professional fees. Planning depends on the street more than the scheme: the borough has around 29 conservation areas, and many of them, including Blackheath, Brockley and Telegraph Hill, carry Article 4 Directions that remove permitted development rights for external changes visible from the street. This page covers what the council allows, what it costs, and how to check your own address 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 Lewisham?

Many Lewisham 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 a conservation area the rules tighten. Lewisham lists around 29 conservation areas, and side extensions, rear extensions beyond 3 metres and any multi-storey rear extension within them need planning permission. The borough also applies Article 4 Directions to many of its conservation areas, including Blackheath, Brockley, Telegraph Hill, Lee Manor, Hatcham, Culverley Green and Forest Hill. These directions apply to single dwellinghouses and require permission for external changes visible from public viewpoints: replacement windows and doors, front porches, paving over front gardens, roof material changes, chimneys, boundary walls and gates.

Where an application is needed, the council assesses householder schemes against its Alterations and Extensions supplementary planning document, adopted in April 2019. Checking your street against the conservation area list, the relevant Article 4 Direction and that guidance is part of our Stage Zero review, before any design work starts.

What types of homes do we extend in Lewisham?

Lewisham is one of London's best boroughs for period housing at sane prices. Blackheath holds Georgian and early-Victorian villas and terraces around the heath, with some of the borough's tightest Article 4 controls. Brockley is among London's largest Victorian conservation areas, street after street of generous mid-Victorian terraces and semis. Telegraph Hill offers remarkably consistent late-Victorian terraces built for the Haberdashers' Company estate, and Hither Green and Lee provide Victorian and Edwardian terraced streets where more of the stock sits outside conservation areas.

For most of these houses the brief is familiar: a side-return or wraparound kitchen extension, a loft conversion, or both in one coordinated project. What changes street by street is the consent route, and in Brockley or Telegraph Hill even a window replacement can need permission. The design answer starts with the constraint check, not the sketch.

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

Lewisham 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 Lewisham 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 Lewisham 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 →