What Does an Extension Cost in 2026?
“How much does an extension cost?” is the first question almost every homeowner asks — and the honest answer is that it depends on the type, the size, the specification and where you live. As a broad guide, most domestic extensions in the UK in 2026 fall between £25,000 and £100,000, with build costs working out at roughly £2,000 to £3,000 per square metre, rising higher in London and for high-spec finishes.
Below we break the cost down by extension type, then explain exactly where the money goes, what drives the price up or down, and what is usually excluded from the headline figure so you can budget with confidence.
Extension Costs by Type
These ranges reflect typical 2026 UK build costs for a completed extension to a reasonable standard. They exclude the kitchen fit-out, loose furnishings and (in many cases) VAT — see the breakdown sections below.
Rear single-storey
£30,000 – £50,000The most common extension type — a single-storey addition to the back of the property, typically opening up a kitchen, dining or living space onto the garden. Costs depend heavily on size, glazing and how much of the existing rear wall is removed.
Suits homeowners wanting a brighter, larger kitchen-diner or open-plan living space without going up a floor.
Side return
£25,000 – £40,000Fills in the narrow alley (the "side return") alongside a Victorian or Edwardian terrace, widening a galley kitchen into a usable room. Smaller in footprint but often involves complex structural and party wall work.
Suits terraced and semi-detached owners in London and the South East who want to square off an awkward kitchen.
Wrap-around
£50,000 – £80,000Combines a rear extension and a side return into a single L-shaped footprint, creating a large open-plan ground floor. More structural work and more glazing pushes the cost up, but the space gain is significant.
Suits owners who want a transformational ground-floor remodel rather than a single new room.
Double-storey
£60,000 – £100,000Adds space on two floors at once — typically an enlarged kitchen below and an extra bedroom or bathroom above. The cost per square metre is often lower than single-storey because you share the same foundations and roof across two floors.
Suits growing families needing both more living space and an additional bedroom or en-suite.
For the full picture on each option, see our house extensions service, along with side extensions, double-storey extensions and wrap-around extensions.
Where the Money Goes: A Full Cost Breakdown
An extension budget splits broadly into design and planning, structural work, and the build itself. Understanding each helps you see why two quotes for the same project can differ so much.
Design & planning costs
- Architect or architectural designer — roughly £2,000 to £5,000 for design and drawings, more for full architectural service
- Structural engineer — calculations for beams and foundations, typically £600 to £1,500
- Planning application fee — currently around £258 for a householder application in England (where permission is needed)
- Building Control — application and inspections, typically £500 to £1,000 via the council or an approved inspector
- Party wall surveyor — where required, often £700 to £2,000+ depending on the number of neighbours
Structural costs
- Steel beams (RSJs) — to support the structure where walls are removed, supplied and fitted with padstones
- Foundations — excavated and poured to the depth required by ground conditions and Building Control
- Underpinning — only where existing foundations must be strengthened (for example near trees or on poor ground); a significant added cost when needed
- Drainage diversions — where the extension sits over or near existing drains
Build costs
- Groundworks — excavation, foundations, oversite, damp-proof membrane and ground-floor slab
- Brickwork and blockwork — the external and internal walls, including cavity insulation
- Roof — flat or pitched, including structure, covering, insulation and rooflights
- Glazing — bi-fold or sliding doors, windows and rooflights (a major cost driver)
- Mechanical and electrical (M&E) — wiring, sockets, lighting, plumbing, heating and underfloor heating where specified
- Plastering, screeding and finishes — including internal joinery and decoration where included
Per-Square-Metre Guide
A useful way to sense-check any quote is the cost per square metre of new floor area. As a 2026 rule of thumb:
- Standard specification, outside London: roughly £1,800 – £2,400 / m²
- London and the South East: roughly £2,000 – £3,000 / m²
- High-spec finishes, structural glazing and complex sites: £3,000 / m² and above
For example, a 20m² rear extension in London at £2,500/m² works out at around £50,000 for the build — before the kitchen, finishes and VAT.
What Affects the Cost
Two extensions of the same size can differ in price by tens of thousands of pounds. These are the factors that move the number:
- Size — the larger the footprint, the higher the total, though the per-square-metre rate often falls slightly as size increases
- Ground conditions and foundations — clay soils, nearby trees, sloping sites or high water tables can deepen foundations or trigger underpinning
- Glazing — bi-fold and sliding doors, large rooflights and structural glazing add cost quickly compared with standard windows
- Specification level — the grade of materials, ironmongery, sanitaryware and finishes makes a substantial difference
- Kitchen fit-out — units, worktops and appliances are usually a separate budget and can range from a few thousand to £30,000+
- Site access — restricted access, no rear access, or the need to carry materials through the house increases labour time
- Structural complexity — removing a large amount of wall, spanning wide openings or building over drains adds steel and engineering cost
- London premium — labour and overheads in London and the South East typically run higher than the rest of the UK
What's Typically Included vs Excluded
The headline price for an extension rarely covers everything. These items are commonly excluded from a builder's quote and need to be budgeted separately:
Kitchen units and appliances
Almost always excluded from the build contract. The builder may include installation labour, but the units, worktops, appliances and tiling are a separate supply budgeted by the client.
Flooring and floor coverings
Screed is usually included, but the finished floor — tiles, engineered wood, resin or carpet — is frequently excluded and left to the client to specify and supply.
Decoration
Some quotes include painting and decorating; many price to plaster finish only and exclude final decoration. Always confirm which.
VAT
Most extensions to existing homes are standard-rated at 20%. Check whether the quoted figure is inclusive or "plus VAT" — it changes the total significantly.
Party wall surveyor and legal fees
Where a Party Wall Award is required, the surveyor's fees are usually a separate cost and are not part of the builder's quote.
RCB's Approach to Pricing
At RCB Design & Build we price extensions with transparent, itemised, fixed-scope quotes — not vague lump sums. Every quote sets out the scope line by line, states clearly what is and is not included, confirms the VAT position, and backs any provisional sums with a genuine site assessment rather than an optimistic guess.
That means you can see exactly where your money is going before work starts, compare our quote properly against any others, and avoid the mid-build surprises that come from low headline figures with hidden exclusions.
Why Homeowners Trust RCB
- Itemised, fixed-scope quotes — no single lump sums
- Clearly stated exclusions and assumptions
- VAT position confirmed in writing
- Realistic provisional sums backed by site assessment
- Stage-linked payment schedule tied to progress
- Fully insured and verified delivery team
Not Sure If Your Quote Is Fair?
If you have already received quotes for your extension and are unsure whether they represent fair value, RCB offers independent quote comparison reviews. We will go through the figures with you, identify gaps and inconsistencies, and give you an honest assessment.
Learn about our quote comparison service