House Extensions in South London: Costs, the Planning Rules Borough by Borough, and What Each Type Involves
Updated: Oct 28
By Anderson & the LCCL Construction Team | Finchley N3
South London is terrace country. From Clapham and Balham across to Dulwich and Peckham, and south through Tooting, Streatham and Wimbledon, the housing stock is Victorian and Edwardian terraces with a narrow side return, a back addition and a long garden — the three things that make a ground-floor extension worth doing. The question is never whether the house can take one; it is which type, what it costs, and whether it needs a planning application, and the answer to the last of those changes from one borough to the next and sometimes from one side of a street to the other. This guide covers the four extension types that suit South London houses with their from-prices, the permitted development position, what Wandsworth, Lambeth, Merton and Southwark each ask for when a scheme does go to planning, and what our own South London projects show about the programme.
As LABC Building Excellence Award Winners 2024 for Best Residential Extension in London — and London Regional Winners again in 2026 for Best Conversion or Alteration to an Existing Home — we design and build house extensions across South London from our base in Finchley — one contract covering the drawings, the Lawful Development Certificate or planning application, the structural engineering, the party wall notices and the build. Not sure whether your house needs planning permission?
The four extension types that suit South London houses
Rear extension. The most common: the original back wall comes out and a single-storey room runs 3 metres into the garden (6 metres by prior approval), giving an open-plan kitchen-diner across the width of the house. From £42,000 inc. VAT (£35,000 + VAT) at the economy shell-and-plaster specification; our rear extension cost guide covers the range by size. Side-return infill. The narrow alley beside a Victorian back addition is roofed over and brought inside, usually with a glazed roof so the middle of the house does not go dark; from £42,000 inc. VAT. Wraparound. Side return and rear in one L-shape — the biggest ground-floor gain a terrace can make; from £72,000 inc. VAT (£60,000 + VAT). Double-storey rear. Rooms on both floors, usually a bedroom and en-suite over the kitchen; it suits the larger semis of Wimbledon, Streatham and Balham more than a narrow terrace, from £90,000 inc. VAT (£75,000 + VAT). Every figure is postcode dependent and fixed in writing after the survey; the cost examples guide shows where the money goes on four real projects.
What a house extension costs in South London
Rear extension (single storey): £42,000 – £90,000 inc. VAT (£35,000 – £75,000 + VAT).
Side-return infill: £42,000 – £93,600 inc. VAT (£35,000 – £78,000 + VAT).
Wraparound: £72,000 – £153,600 inc. VAT (£60,000 – £128,000 + VAT).
Double storey: £90,000 – £198,000 inc. VAT (£75,000 – £165,000 + VAT).
All four include the design, the certificate or application, the structural calculations, building control and the build. Inner South London — Wandsworth, Lambeth, Southwark — sits a little above the outer boroughs because of access, parking and skip licences on terraced streets, which is why the instant estimate applies your postcode. The one item people most often leave out of their own budget is the steel that opens the original back wall into the new room — on a Victorian terrace typically £4,800 – £14,400 inc. VAT — and it is in every LCCL price from the first figure.
Do you need planning permission in South London?
Usually not for the first two types. On a terrace outside a conservation area, a 3 metre rear extension and a side-return infill are permitted development under Class A of the General Permitted Development Order — no planning application, just a Lawful Development Certificate, which we apply for on every one. A 6 metre rear extension goes through the 42-day prior approval route rather than a full application. Two things take a South London extension into a householder planning application: conservation areas, of which South London has a great many — Clapham, Abbeville Road, Brixton, Dulwich Village, Camberwell Grove, Peckham Rye among them — where side extensions and anything two-storey lose their permitted development rights; and wraparounds, which have to meet the rear and side limits at once and in practice almost always go to planning. Our rear extension permitted development guide sets out the national rules; what follows is what each borough adds.
The rules borough by borough
Wandsworth (Clapham Junction, Balham, Tooting, Putney, Battersea) keeps its extension guidance qualitative — subordinate to the house, sited away from the neighbour's boundary — judges daylight under the BRE guide rather than a fixed 45-degree rule, has only street-specific Article 4 directions (Commondale, Westover Road, Wandsworth Common West Side, the Heaver Estate), and offers the cheapest pre-application advice of any borough we work in: £146 for a written response, with a free duty planner by email. Lambeth (Clapham, Brixton, Streatham, Stockwell) applies its August 2023 Design Guide: single-storey rear extensions subordinate to the house with no metre limit, infills single storey, side extensions kept a metre off the boundary above ground, and — usefully — no Article 4 directions on the Clapham, Abbeville Road or Brixton conservation areas, so permitted development still applies there for a single-storey rear. Merton (Wimbledon, Raynes Park, Mitcham) decides under policy D12.4 of its November 2024 Local Plan, qualitative on form, scale and bulk, with householder Article 4 directions only on John Innes Merton Park and Wilton Crescent. Southwark (Dulwich, Peckham, Camberwell) uses its 2025 Householder SPD — extensions subservient to the host, assessed case by case, traditional brick and slate in its 48 conservation areas — and runs an Article 4 direction over the Sunray Estate. All of them decide a householder application in eight weeks from validation, at a fee of £548 from April 2026. Our planning rules by council page carries the detail for all 27 councils we work in and is reviewed each spring.
How long a South London extension takes
From the first call to the first day on site is eight to twelve weeks on a permitted development extension — survey, certificate drawings, structural design and building regulations running in parallel, party wall notices served while the drawings are finished — and closer to sixteen where a householder application is needed. On site, a single-storey rear extension is 8 to 12 weeks; a wraparound or a full-width extension on a Victorian terrace with the ground floor opened up is 14 to 18. Our guide to how long a house extension takes breaks the programme down stage by stage.
Our South London projects
At Glendarvon Street in Putney SW15, a Victorian terrace, the back of the house became an open-plan kitchen-diner with rooflights and bifold doors onto a landscaped garden, with a new WC and pantry tucked into the old footprint — householder planning through Wandsworth, 14 weeks on site, 400 square feet added. At Westleigh Avenue in Putney SW15, a 1914 terrace, a rear dormer loft went through under permitted development in six weeks. At Chipstead Street in Fulham SW6, a Victorian terrace, the cellar was lowered into full living space, a loft converted and a rear extension added in one 65-week programme through Hammersmith & Fulham planning. And at Tetcott Road in Chelsea SW10, an Edwardian terrace, a wraparound extension and full renovation went through the Royal Borough in 14 weeks on site. They are all on our projects page; the South London kitchen extension guide covers the kitchen-led version of the same work.
Anderson's advice
“Wandsworth planning is much more helpful on pre-application than Islington in our experience.” On a South London terrace that matters more than anywhere, because the conservation-area line often runs down the middle of a street. If your house is on the planning side of it, or you want a wraparound, spend the £146 on Wandsworth's written pre-app before the drawings are detailed — the officer's view on depth and materials saves a redesign after the decision.
Why homeowners choose LCCL for a South London extension
One contract and one point of contact: our team confirms the planning route before the first call ends, designs the extension, obtains the Lawful Development Certificate or planning permission, produces the structural drawings, serves the party wall notices and builds it, with building control inspecting and a completion certificate at handover. We are LABC Building Excellence Award winners 2024 and London Regional Winners 2026, Best of Houzz Service winners and NFB members, and every project carries a 12-month workmanship guarantee and a 10-year LABC structural warranty. Already have drawings? We price the build as drawn within seven working days.
What our clients say
“LCCL Constructions was professional, trustworthy and worked to the highest standard. The process was smooth and I'm very happy with the result - it has transformed the quality of my home.”
— KashImaging, Trustpilot
Frequently asked questions
How much does a house extension cost in South London?
From £42,000 inc. VAT (£35,000 + VAT) for a 3 metre rear extension or a side-return infill at the economy specification, £72,000 for a wraparound and £90,000 for a double-storey rear extension, rising with size and finish to around £198,000 inc. VAT for the largest two-storey schemes. Every figure is postcode dependent and fixed in writing after the survey.
Do I need planning permission for an extension in South London?
Not for a 3 metre single-storey rear extension or a side-return infill on a terrace outside a conservation area — those are permitted development, confirmed with a Lawful Development Certificate. A 6 metre extension goes through prior approval. Conservation areas (Clapham, Dulwich Village, Camberwell Grove, the Heaver Estate and many more) remove permitted development for side and two-storey extensions, and a wraparound almost always needs a householder application.
Does LCCL Construction work in South London?
Yes. We are based in Finchley and build across London; our recent South London and south-west London projects include Glendarvon Street and Westleigh Avenue in Putney SW15, Chipstead Street in Fulham SW6 and Tetcott Road in Chelsea SW10. Consultations start with a phone call and the survey follows.
Get a figure for your own project
Not sure whether your project needs planning permission? Enter your postcode and tick what you want to build — we look up the conservation areas, Article 4 directions and listed buildings for you, and you see which parts are permitted development and the from-price for each. Lawful Development Certificate included on every permitted development job.
To book a free 30-minute consultation call about your South London house extension, call LCCL Construction on 020 8037 7332 or get an instant estimate online — six questions, about two minutes, and you see a real range for your postcode on screen. Finchley N3 based, covering all of London and Hertfordshire.
One team, one contract, one point of contact — from first sketch to handover.
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