For homeowners across south west London, the decision to extend is rarely a simple one. The potential is clear. More space, more light, a home that finally works the way your life does. But before architects are appointed and builders are called, one question sits at the centre of every conversation: how much is this house extension in south west London actually going to costs?
The honest answer is that extension costs south west London vary considerably depending on the type, size, specification and location of the project. What this guide sets out to do is give you a genuinely useful framework for understanding those costs, not vague ranges designed to avoid commitment, but the real figures that homeowners in Wandsworth, Battersea, Clapham, Fulham, Putney and across the wider area are working with in 2025.
At SM & Sons Construction we work with a carefully selected number of homeowners across south west London each year, delivering extensions that are built to the highest standard and priced with the transparency our clients deserve. Here is everything you need to know before your first conversation with a builder.
Why House Extension Costs in South West London Are Higher Than National Averages
Before getting into the numbers it is worth understanding why London costs sit where they do. South west London is not a cheap place to build. Labour costs are higher than anywhere else in the UK, materials need to be transported into a dense urban environment, access is often constrained, skip permits are required, parking for tradespeople is expensive and the quality of workmanship expected by homeowners in areas like Clapham, Wandsworth and Fulham is consistently high.
Add to this the fact that south west London’s housing stock is dominated by Victorian and Edwardian terraces that require careful, skilled work to extend without compromising their character, and you have a market where quality construction commands a genuine premium over the national average.
A figure that might build a decent extension in the Midlands will not build the same extension in SW4 or SW11. This is not a London tax, it is a reflection of real costs. Any builder quoting significantly below the figures in this guide should be asked very specific questions about why.
Types of House Extension and Their Costs in South West London
Single Storey Rear Extension
The most common extension type across south west London’s terraced and semi-detached properties. A single storey rear extension typically extends the kitchen or living space and opens the home up to the garden.
Cost range: £2,500 – £3,500 per m²
For a typical 4m x 5m rear extension of 20m², this translates to a project cost of approximately £50,000 to £70,000 excluding VAT. At the higher end of specification — premium glazing, underfloor heating, high quality finishes — costs can reach £80,000 or beyond for the same footprint.
What this typically includes: demolition of the existing rear wall, foundations and substructure, cavity wall construction, flat or pitched roof, new window and door openings, first and second fix mechanical and electrical works, plastering and basic finishes.
What it typically excludes: kitchen units, appliances, flooring, tiles and all client-supplied finishes.
Side Return Extension
Particular to Victorian and Edwardian terraces, a side return extension fills in the narrow alleyway that runs alongside the rear of the ground floor. The result is a significantly wider kitchen or ground floor space that transforms how the home functions.
Cost range: £65,000 – £100,000+
Side return extensions are slightly more complex than straightforward rear extensions due to the proximity to the neighbouring property and the party wall considerations this creates. The narrowness of the space also adds complexity to access and construction logistics.
Wraparound Extension
A combination of rear and side return extension creating an L-shaped addition that maximises ground floor space. Common in larger south west London properties where both rear and side return space exists.
Cost range: £90,000 – £160,000+
The additional complexity of the junction between the rear and side elements, combined with the greater overall area, pushes costs above a single rear extension. The result however is transformative, a wraparound extension typically creates an entirely new relationship between the ground floor living spaces and the garden.
Double Storey Extension
Where the property and planning position allow, a double storey extension adds additional bedroom and bathroom space above while expanding the ground floor below. The cost per square metre is lower than a single storey extension because the expensive foundations, roof and envelope costs are shared across two floors.
Cost range: £2,200 – £3,200 per m² of total floor area
For a 4m x 5m double storey extension creating 40m² of new floor area, a realistic budget sits between £72,000 and £112,000 excluding VAT.
Loft Conversion Combined with Extension
Many south west London homeowners choose to undertake a rear extension and loft conversion simultaneously to maximise disruption efficiency and achieve the greatest possible increase in living space in a single project.
Combined project costs vary significantly based on both elements but typically start at £120,000 and can reach £250,000 or more depending on the scale and specification. For detailed loft conversion costs see our separate guide to loft conversions in Clapham.
What Drives the Costs of a House Extension in South West London
Understanding what moves the needle on price helps you make better decisions about where to invest and where to save.
Size is the most obvious driver. More floor area means more of everything — more materials, more labour, more time. But size is not linear. There are fixed costs in every extension project that do not scale with area. Such as temporary works, building regulations, project management, site setup, meaning a larger extension often represents better value per square metre than a smaller one.
Specification has an enormous impact on final cost. The structural shell of an extension, foundations, walls, roof, represents perhaps 50 to 60 percent of the total project cost. The remaining 40 to 50 percent is determined by the quality of the windows and doors, the kitchen, the flooring, the tiles, the bathroom fittings and every other finish decision. A project with a standard kitchen and ceramic tiles costs substantially less than the same footprint with a bespoke kitchen and large format porcelain throughout.
Roof design significantly affects cost. A simple flat roof is the most economical option. A pitched tiled roof costs more. A glazed roof lantern or structural glazing system is a significant premium on top of that, typically adding £15,000 to £40,000 or more to the roof element alone depending on the size and specification. This is one of the most impactful single decisions in an extension specification.
Structural complexity varies considerably between properties. A straightforward rear wall opening with a standard steel lintel costs far less than a situation requiring multiple steels, significant temporary works or complex foundations due to ground conditions or existing drainage. Properties in some parts of south west London sit on clay soil that requires deeper foundations. Victorian properties often have shallow existing foundations that need consideration when extending.
Access matters more in south west London than almost anywhere else. Properties in dense terraced streets with no rear access, restricted parking and narrow passages for materials add cost in time and logistics that straightforward suburban properties do not carry.
Every project is different, to get a accurate price for your project you need to arrange a site visit with a construction company and discuss your plans in detail.
The Real Cost of Getting It Wrong
The cheapest quote is rarely the cheapest project. This is one of the most important things to understand before appointing a builder for an extension in south west London.
An extension is a significant structural intervention in your home. Foundations that are inadequately specified, steelwork that is under-engineered, roofing that is poorly weatherproofed or electrical and plumbing work that does not meet building regulations, all of these produce outcomes that are expensive, disruptive and sometimes dangerous to rectify after the fact.
The price differential between a careful, properly specified quote and a cheap one rarely reflects the same scope of work. It almost always reflects assumptions, omissions or quality reductions that only become visible once the work is underway or the project is complete. A detailed, itemised quote from a builder who has genuinely understood your project and priced every element properly is worth significantly more than a low headline number.
Getting the Right Quote for Your Extension
The quality of a quote tells you a great deal about the builder providing it. A thorough quote will itemise works by phase, describe what is included and excluded clearly, reference your specific drawings and structural calculations, include a programme of works and payment schedule and stand behind a fixed price for a defined scope.
A vague single-page quote with one total figure and no detail should be treated with caution regardless of the price.
When comparing quotes from different builders ensure you are comparing the same scope. The most common source of price discrepancies between quotes is not that one builder is genuinely cheaper than another, it is that they are quoting different scopes. One quote may include structural steelwork and another may not. One may include building regulations fees and another may exclude them. One may include kitchen fitting and another may price finishes seperatly.
The right question to ask any builder is not why are you more expensive than someone else but what exactly is included in your price and what is not.
VAT on Extension Works
Standard London residential house extension works are subject to VAT at 20 percent of the costs. This is a significant addition to the base build cost that is sometimes omitted from early budget conversations and can come as a surprise.
For a £80,000 extension the VAT liability is £16,000, bringing the total to £96,000. Always ensure you are clear on whether any figure you are working with is inclusive or exclusive of VAT.
There are some circumstances where reduced or zero VAT rates apply. Certain works to listed buildings, conversions from non-residential to residential use and some energy efficiency improvements, but standard residential extensions are subject to the full 20 percent rate.
How to Budget For The costs of your House Extension in South West London
A sensible budget framework for an extension project in south west London includes the following:
The construction cost itself as described above. A contingency of 10 to 15 percent on top of the construction cost to cover unforeseen works, variations and decisions made during the build. Architect and designer fees if applicable, typically 5 to 10 percent of construction cost. Structural engineer fees, typically £1,500 to £3,500 depending on complexity. Building regulations fees, typically £800 to £1,500 depending on the local authority and project size. Party wall surveyor fees if required, typically £800 to £1,500 per neighbour. Planning application fees if required, currently £258 for a householder application in England.
Including all of the above a realistic total project budget for a 20m² single storey rear extension in south west London sits between £70,000 and £95,000 all in at a standard specification. At a premium specification the same footprint can reach £110,000 to £130,000 including all associated costs.
Frequently Asked Questions
Does a house extension add value in south west London?
A well designed and properly built extension consistently adds value to properties across south west London. In areas like Wandsworth, Battersea, Clapham and Fulham where property values are high, the value added by a quality rear extension or wraparound frequently exceeds the cost of the works. The key phrase is well designed and properly built. A poor quality extension can actually detract from value.
How long does a rear extension take to build in south west London?
A standard single storey rear extension typically takes 10 to 16 weeks from commencement on site. Larger or more complex projects take proportionally longer. The pre-construction phase – design, planning and building regulations typically adds 3 to 6 months before site works begin.
Do I need planning permission for an extension in south west London?
Many extensions fall within permitted development rights and do not require formal planning permission. However south west London has a number of conservation areas where permitted development rights are restricted or removed, and properties within the Royal Borough of Kensington and Chelsea face specific additional policies. Always verify your planning position early. Building regulations approval is always required regardless of planning position.
Can I live in my house during an extension?
In most cases yes. A rear extension is generally manageable to live through, though there will be a period during the structural opening of the rear wall when additional disruption is unavoidable. Some homeowners choose to make alternative arrangements for two to four weeks during this phase.
Should I get multiple quotes?
Yes, ideally two to three quotes from builders who have properly reviewed your drawings and visited the site. Be cautious of quotes provided without a site visit, they cannot be accurate without one.
Planning Your Extension in South West London?
We work with homeowners across south west London including Wandsworth SW18, Battersea SW11, Clapham SW4, Fulham SW6, Putney SW15, Wimbledon SW19, Barnes SW13 and the surrounding areas.
We take on a select number of projects each year, chosen because we know we can do something exceptional with them. If you are planning an extension and want to work with a team that will bring the same care, precision and transparency to your project that you do, we would love to hear about it.