If you’re planning a house extension in South London, the first question is almost always the same: what’s it going to cost? It’s a reasonable thing to want to know before you commit to anything — and it’s a question that deserves a straight answer. This guide gives you realistic 2026 figures for the most common extension types across South London, explains what drives costs up or down, and covers the additional expenses that often catch homeowners off guard.
These aren’t national averages padded with caveats. They’re figures based on what we actually build across South East and South West London — in Bromley, Greenwich, Lewisham, Eltham, Beckenham, and the surrounding boroughs.
What Does a House Extension Cost in South London in 2026?
The honest answer is that extension costs vary — but not as wildly as some builders would have you believe. The main variables are size, specification, and complexity. Here’s a starting point:
| Extension Type | Typical Size | South London Cost Range |
|---|---|---|
| Single storey rear | 15–30m² | £25,000–£60,000 |
| Double storey rear | 15–25m² per floor | £45,000–£90,000 |
| Side return | 8–15m² | £18,000–£35,000 |
| Wraparound (rear + side) | 25–45m² | £55,000–£100,000+ |
| Garden room extension | 15–25m² | £25,000–£50,000 |
These are fully finished costs — structure, insulation, electrics, plumbing, plastering, and decoration. Not shell builds. Not “supply only.” What you move into at the end.
Single Storey Rear Extension Costs in South London
A single storey rear extension is the most commonly built extension across South London — and for good reason. It’s the most straightforward way to add a large, open-plan kitchen and living space to a Victorian terrace or 1930s semi, and the permitted development rules mean many homeowners don’t need planning permission at all.
In South London, expect to pay between £1,500 and £2,200 per square metre for a well-built single storey rear extension, fully finished. That works out as:
- 15m² extension: £22,500–£33,000
- 20m² extension: £30,000–£44,000
- 30m² extension: £45,000–£66,000
What Pushes the Cost Up?
The lower end of that range applies to a straightforward build on a flat site with simple foundations, a standard lean-to roof, and a mid-range finish. Costs rise when:
– You’re in a conservation area and need to use specific materials (handmade brick, natural slate, timber windows rather than uPVC)
– The ground conditions are difficult — clay soil in much of South London can require deeper or more complex foundations
– You want a flat roof with high-quality glazing or a lantern roof — these significantly increase both the structure cost and the glazing bill
– The specification is high — bi-fold doors, underfloor heating, and premium finishes add up quickly
Double Storey Extension Costs in South London
A double storey extension adds space on both floors — typically a larger kitchen or living area below and an extra bedroom or bathroom above. It costs more in total than a single storey extension but less per square metre, because the foundations, roof, and some structural elements serve both floors.
Expect to pay between £1,800 and £2,500 per square metre for a double storey extension in South London. That works out as:
- 15m² per floor (30m² total): £54,000–£75,000
- 20m² per floor (40m² total): £72,000–£100,000
Double storey extensions always require full planning permission — permitted development doesn’t cover them. Bromley, Greenwich, and Lewisham councils each have their own character policies, so design quality matters and a good architect who knows the local planning department is worth the investment.
Side Return Extension Costs in South London
If you have a Victorian or Edwardian terrace, you almost certainly have a narrow alleyway running down the side of the property — the side return. A side return extension fills this in, dramatically widening the rear of the ground floor.
The transformation is hard to overstate. A typical Victorian terrace kitchen is dark, disconnected from the garden, and too narrow to be anything other than functional. A side return extension turns it into a wide, light-filled space that changes how the whole ground floor works.
Side return extensions typically cost between £18,000 and £35,000 in South London, depending on size and specification. Many fall under permitted development outside conservation areas.
Combining a side return with a rear extension — a wraparound — is where the real transformation happens.
Wraparound Extension Costs in South London
A wraparound extension combines a rear extension with a side return into a single L-shaped build. It’s the most expansive single-storey option for terraced and semi-detached properties, and the result — when done properly — can add 30–50% to a property’s usable ground floor area.
Wraparounds in South London typically cost between £55,000 and £100,000+, depending on the size of both the rear and side elements, the specification, and the planning context.
This is a project that always requires careful structural design. The corner junction between the rear and side elements, the roof drainage, and the structural opening into the existing house all require engineering input. Cut corners here and you create problems that are expensive to fix.
What Affects the Cost of an Extension in South London?
Beyond size and extension type, several factors specifically affect costs across South London.
Conservation Areas
South London has a high concentration of conservation areas — Bromley alone has around 40. Properties within conservation areas often need to use specific materials that match the existing building and local character. Handmade brick, natural slate, and timber-framed glazing cost more than standard alternatives. Some conservation areas also have Article 4 Directions that remove permitted development rights, adding planning fees and potentially an architect’s time to projects that would otherwise be straightforward.
Ground Conditions
Much of South London sits on London clay — a shrinkable clay that expands when wet and contracts when dry. Extensions on clay often require deeper strip foundations or engineered solutions that add cost. It’s not unusual for ground conditions to add £3,000–£8,000 to a project in affected areas. We check this before quoting.
The Property Type
A 1930s semi in Hither Green has different structural characteristics to an Edwardian terrace in Brockley or a Victorian villa in Beckenham. Suspended timber ground floors, lime mortar brickwork, and original structural configurations all affect how the extension connects to the existing house. Experience with the specific property types in each area matters — and shortcuts taken at this stage create expensive problems later.
Planning and Professional Fees
These are the costs that often catch homeowners by surprise. Budget for:
- Architect fees: £3,000–£10,000+ depending on scope
- Planning application fee: £258 for a householder application (England)
- Building control: £500–£1,500
- Party wall surveyor: £700–£2,000 (if required)
- Structural engineer: £800–£2,500
Not every project needs all of these — a permitted development single-storey rear extension on a standard property might only need building control and a structural engineer. But any project requiring planning, or involving a party wall, will need the full set.
Will a South London Extension Add Value?
In most cases, significantly. South London property values — particularly across Bromley, Lewisham, Greenwich, and the inner South East — are high enough that a well-built extension typically adds more in value than it costs to build.
A well-executed rear extension that creates an open-plan kitchen-living space commonly adds 10–20% to a property’s market value. On a £600,000 South London terrace, that’s £60,000–£120,000 of additional value — from a project that might cost £40,000–£60,000 to build.
The caveats are important though. Build quality matters enormously. A poorly executed extension — or one that looks out of place on a conservation area property — can cause problems at sale and ultimately cost more than it added. Buyers in South London’s market are discerning, and a bad extension is visible.
Getting an Accurate Quote for Your South London Extension
The figures in this guide give you a realistic starting point — but your actual costs depend on your specific property, the size and specification of what you want to build, and what the planning context allows.
The only way to get an accurate number is a proper site visit and a detailed quote. We visit properties across South London — Bromley, Greenwich, Lewisham, Eltham, Beckenham, and the surrounding areas — for free, with no obligation.
We’ll tell you what’s feasible, what planning route applies to your address, and what the project will realistically cost — before you commit to anything.