House Extension Cost London 2026

A single storey rear or side extension in London typically costs £2,500 to £4,000 per square metre in 2026, excluding VAT and professional fees. A typical 25 to 30 square metre kitchen extension lands around £55,000 to £80,000 at a mid-range specification, with a more basic finish achievable from around £40,000 for a smaller 20 square metre extension.

Design United Studio is a RIBA and ARB registered architecture practice based in Wimbledon Park, and we’ve taken house extensions through design, planning and construction across South West London. This guide breaks down what drives the cost, how single and double storey extensions actually compare, and where the money genuinely goes.

Key Takeaways

• A single storey rear or side extension typically costs £2,500 to £4,000 per m² in London in 2026, excluding VAT and fees.

• A typical mid-spec kitchen extension (25 to 30m²) costs roughly £55,000 to £80,000.

• Double storey extensions often cost less per square metre than single storey ones, since foundations and roof costs are shared across two floors.

• Central London runs 15 to 25% above outer London boroughs, and London as a whole runs 20 to 40% above the UK average.

• Professional fees, planning costs and VAT at 20% are all on top of the build cost itself.

Extension Cost by Type

These figures reflect London-wide averages. Central London boroughs typically run 15 to 25% higher than outer London, and London as a whole sits 20 to 40% above the UK average because of labour rates and the logistics of building on tighter, more built-up sites.

What Actually Drives the Cost

Size is the obvious factor, but it isn’t the only one. Structural complexity, steel beams to open up a kitchen-diner, underpinning near an existing foundation, a change in roof form, can add significantly to a project regardless of floor area. So can site access: a rear extension on a Victorian terrace with no side access for materials and machinery costs more to build than the same extension on a house with a driveway.

Specification matters just as much. Bi-fold or sliding doors, underfloor heating, bespoke joinery and a higher standard of kitchen fit-out all sit on top of the base build cost, and it’s easy for a project to drift from budget to mid-range to high-end without a single individual decision feeling like the one that tipped it over.

Why a Double Storey Extension Often Costs Less Per Square Metre

It seems counterintuitive, but a double storey extension frequently works out cheaper per square metre than a single storey one, because the foundations and roof, two of the most expensive elements of any extension, are shared across two floors of usable space rather than built for just one.

That doesn’t mean a double storey extension costs less overall. The total project cost is still higher because there’s more floor area and more structural work involved. It just means the cost-per-square-metre comparison between the two extension types isn’t as straightforward as it first looks, worth bearing in mind when comparing a single storey scheme against adding a second floor while you’re at it.

What’s Included Beyond the Build Cost

The build cost is the largest line item, but not the only one. Architect and structural engineer fees, planning application costs, a party wall surveyor where a shared wall is affected, and VAT at the standard 20% rate all sit on top. We’ve covered these in more detail, including specific figures, in our guide to loft conversion costs in London, most of which applies equally to an extension since the underlying cost categories are the same.

One extension-specific point worth flagging on VAT: almost all extension work to an existing home is standard-rated at 20%, including the professional fees, regardless of how the work is described on a quote. The reduced 5% rate only applies in specific circumstances, like converting a property into a different number of dwellings, which a standard home extension doesn’t qualify for.

Ways to Keep the Cost Under Control

A few decisions make a genuine difference to the final figure without compromising on a well-designed result. Getting the design and specification properly resolved before pricing avoids the most expensive kind of cost creep, changes made mid-build once walls are open and a contractor is already on site.

Choosing a single storey extension over a double storey one, where the brief allows it, keeps the total project cost down even though the per-square-metre rate might be marginally higher. Being realistic about finish level from the outset, deciding early whether bi-fold doors and bespoke joinery are genuinely part of the brief, rather than discovering the gap between budget and ambition partway through, also avoids a lot of late redesign.

Do You Need an Architect for an Extension

For a straightforward single storey extension within permitted development, some homeowners go directly to a designer or builder. For anything involving planning permission, a change to the roofline, or a wrap-around combining a rear and side extension, an architect’s input tends to pay for itself in a design that gets approved first time and drawings a contractor can price accurately. We’ve covered this question in more depth in our guide to whether you need an architect for an extension.

If you’re planning an extension, our house extension architects page covers how we approach the design and planning process, and our side return extension page goes into more detail on that specific extension type. We’ve worked on plenty of side return and wrap-around extensions on the narrower plots typical in areas like Fulham, and the same principles apply across most of London’s terraced housing.

Frequently Asked Questions

Does a house extension need planning permission in London?

Many single storey rear extensions fall under permitted development and don’t need a planning application, provided they stay within set size and height limits. Conservation areas, listed buildings and certain property types remove or restrict these rights, so it’s always worth checking before assuming a route applies.

How long does a house extension take to build?

A single storey extension typically takes 3 to 5 months on site once construction starts, with a double storey extension often taking longer given the additional structural work. Design and planning time, which can add several more months, comes before that.

Does a house extension add value to a London home?

It often does, particularly a well-designed kitchen extension that improves how a house actually functions day to day. How much value it adds depends on the local property market and the quality of the finished work, so it’s worth treating primarily as an improvement to the home rather than a guaranteed financial return.

What’s the cheapest type of house extension?

A single storey rear extension within permitted development, at a standard specification, is generally the most cost-effective option, since it avoids the added structural cost of a second storey and the fees associated with a full planning application.

Can I extend my house in phases to spread the cost?

Yes, in many cases. A loft conversion and a ground floor extension can often be designed together but built separately, so the cost is spread over time without either phase undoing decisions made in the other.

If you’re planning an extension and want a clearer sense of what’s achievable and what it’s likely to cost on your specific property, get in touch for an initial conversation. We’re based in Wimbledon Park and work across South West London and Surrey.

Sources

• Buildings and construction (VAT Notice 708) - GOV.UK — official HMRC guidance, used for VAT treatment of extension work.

• Permitted Development Rights - Planning Permission - Planning Portal — official planning guidance, used for permitted development context.