If you’ve started researching a house extension in the UK, you’ve almost certainly encountered a bewildering range of figures. The truth is that house extension cost per m² in the UK varies considerably depending on your location, the type of extension, the specification of finishes, and the complexity of the build. This guide cuts through the noise and gives you a clear, honest breakdown of what UK homeowners are realistically paying in 2026 and, crucially, why the figures differ so much.
Whether you’re dreaming of a light-filled open-plan kitchen-diner, an extra bedroom above the garage, or a side return that finally makes your Victorian terrace feel spacious, understanding the cost per square metre is the foundation of every sensible budget.
What Is the Average House Extension Cost Per m² in the UK?
As of 2026, house extension costs in the UK typically range from £1,500 to £3,300 per m², depending on factors like location, finish quality, and project complexity. Higher-end finishes or projects in London and the South East often exceed £3,500, reaching £4,000–£5,000+ per m². A typical single-storey extension averages £1,800–£3,000 per m², while double-storey extensions can provide better value per m² (often £1,500–£2,800) due to shared foundations and economies of scale. These figures exclude VAT, professional fees, and site-specific costs—consult local builders for accurate quotes.
It is important to note that most builder quotes are for the shell build only. Always budget an additional 20–30% for professional fees (architects, structural engineers, building control), VAT at 20%, and a contingency of around 10–15% for unexpected ground conditions or making good to the existing structure.
Single vs Double-Storey Extension Cost: Which Offers Better Value?
One of the most common questions homeowners ask is whether a double-storey extension is worth the additional upfront cost. The answer, in most cases, is yes, at least in terms of cost per square metre.
Single-Storey Extensions
Single-storey rear extensions remain one of the most popular types in the UK as of 2026, frequently used to expand kitchen-diners or family rooms with direct garden access for enhanced open-plan living. Costs typically range from £1,800 to £3,300 per m² nationally (varying by finish quality and location), rising to £3,000–£4,500 per m² in London and the South East. A standard 20m² kitchen extension would therefore cost between £36,000 and £66,000 before VAT, professional fees, and site-specific extras; consult local quotes for precision.
The single-storey route is generally faster to build (around 12–16 weeks for the structure) and easier to gain planning consent for, as many single-storey rear extensions qualify under Permitted Development Rights without the need for a full planning application.
Double-Storey Extensions
As of 2026, double-storey house extensions in the UK typically cost between £1,800 and £3,000 per m², often 10–20% less per square metre than single-storey extensions (£2,000–£3,300 per m²). This cost efficiency stems from shared foundations and roofing across both levels, which spreads significant fixed expenses over a larger total floor area. That said, the total project cost is naturally higher for double-storey builds due to the expanded overall space.
For families seeking extra downstairs living space alongside upstairs bedrooms or offices, a double-storey extension delivers strong value per pound invested. A well-designed 30m² double-storey addition (e.g., 15m² per floor) might cost £54,000 to £90,000 before VAT, professional fees, and site-specific factors, compared to £60,000–£99,000 for the equivalent total floor area built at single-storey rates. Costs can rise 20–50%+ in London or the South East due to higher labor and material prices; always obtain localized quotes for accuracy.
The trade-off is that double-storey extensions almost always require full planning permission, take longer to build (typically 20–26 weeks), and involve more complex scaffolding and structural work.
Side Return Extensions
Side return extensions — popular in Victorian terraced and semi-detached homes tend to sit at the higher end of the cost range at £2,500–£3,500/m² nationally, due to the narrow site access, steel work typically required, and the glazed rooflight specifications that make them so desirable.
Extension Cost London vs the Rest of the UK
Location is the single biggest variable in any UK extension budget. London and the South East command a premium of 25–40% above the national average, driven by higher labour costs, more expensive materials, complex site logistics (often on narrow plots with restricted access), and greater planning scrutiny, particularly in conservation areas.
An architect-designed house extension in London typically costs between £3,000 and £5,000 per square metre, compared with £2,000–£3,500/m² elsewhere in England. In practical terms, a 20m² single-storey kitchen extension that might cost £50,000 in Birmingham could easily reach £70,000–£80,000 in South or West London for the same specification.
London vs UK Extension Costs Per m² (2026)
| Extension Type | UK Average/m² | London/m² | Premium Uplift % |
|---|---|---|---|
| Single-Storey | £1,800 – £3,300 | £2,500 – £4,500 | 25–50% |
| Double-Storey | £1,500 – £2,800 | £2,200 – £4,000 | 20–40% |
| Side Return | £2,000 – £4,000 | £2,800 – £5,000+ | 20–40% |
Sources: Architecture for London, HOA, EasyEstimate (2026). All costs exclude VAT.
For homeowners in Slough, Windsor, and the Thames Valley corridor, where Tacman Developers operates, costs typically sit at the mid-to-upper end of the national range, influenced by the proximity to London and the South East’s labour market. Budget between £2,500 and £3,500/m² for a well-specified extension in this area.
What’s Included in the Extension Cost Per m²?
When a builder quotes a price per square metre, it is essential to know exactly what that figure covers. A typical mid-specification build cost includes:
- Groundworks and foundations (strip or trench fill)
- External walls — brick and block construction
- Flat or pitched roof structure, felt, battens, and tiles or membrane
- Windows and external doors (uPVC or aluminium, double-glazed)
- First-fix electrics and plumbing
- Plastering and plasterboarding
- Basic decoration — walls primed, ceilings painted
- Structural steelwork where required
Items that are frequently excluded from shell-only quotes and need to be budgeted separately include:
- Architect and structural engineer fees: typically 7–15% of build cost
- Building Regulations approval and inspections: £800–£1,500
- Planning application fee: £258 for a householder application
- Party wall agreement: £700–£1,500 per neighbour if applicable
- Kitchen or bathroom fitting, appliances, and fixtures
- Underfloor heating, bi-fold doors, or premium glazing
- Landscaping and reinstatement of the garden
What Factors Affect the Extension Cost Per Square Metre?
Several variables can push your per-metre cost above or below the averages above:
Location: As discussed, London and the South East attract the highest premiums. Rural areas in the North, Wales, and Scotland tend to offer lower rates, though transport costs for materials and trades can offset some savings.
Specification: Bi-fold doors, underfloor heating, triple glazing, and high-end kitchen or bathroom finishes can add 30–50% to the cost per m² compared with a standard specification.
Ground conditions: Poor bearing capacity, clay soils prone to shrinkage, or proximity to trees can require piled or reinforced foundations, adding £5,000–£15,000 to groundwork costs.
Design complexity: Simple rectangular extensions cost significantly less per m² than designs with multiple angles, hipped roofs, or curved glazed elements. Every additional corner adds roughly 5–8% to foundation and roofing costs.
Size: Larger extensions often achieve a slightly lower cost per m² as fixed setup costs, such as scaffolding, plant hire, and skip hire, are spread across more floor space.
Does a House Extension Add Value in the UK?
A well-executed extension is one of the strongest value-adding investments a UK homeowner can make. According to Nationwide Building Society’s research, adding a large double bedroom and bathroom through an extension can add as much as 24% to the value of a three-bedroom home.
In general terms, each additional square metre of floor space adds approximately £2,000 to a property’s value across England and Wales. In London, where space is at an even greater premium, the figure can reach £4,000 per m². A well-planned 2026 extension typically returns between £1.20 and £1.50 for every £1 spent, making it a compelling alternative to moving house, particularly when stamp duty, legal fees, and removal costs are factored into the comparison.
FAQs
How much does a 20m² extension cost in the UK?
At the national average of £1800–£3,300/m², a 20m² single-storey extension costs approximately £44,000–£64,000 before VAT and professional fees. In London, the same extension is likely to cost £60,000–£90,000.
Is a double-storey extension cheaper per m² than single-storey?
Yes — typically 10–15% cheaper per m². Because foundations and roofing costs are shared across both floors, the cost per m² falls as you add more floor space. Total project costs are higher, but the value for money is generally better.
Do I need planning permission for a house extension?
Many single-storey rear extensions fall under Permitted Development Rights and do not require a full planning application. Detached houses can typically extend up to 4 metres, and terraced or semi-detached homes up to 3 metres, without planning permission. All extensions require Building Regulations approval, regardless of size.
Should I budget for VAT on my extension?
Yes. Most building contractors are VAT-registered and will charge 20% VAT on labour and materials. Always confirm whether quoted prices include or exclude VAT, and factor it into your overall budget from the outset.
Plan Your Extension with Tacman Developers
Understanding the cost per m² is just the starting point. At Tacman Developers, we provide fully transparent, itemised quotes, no hidden costs, no surprises, for homeowners across London, Slough, the Thames Valley, and beyond.
With over 55 years of combined experience in design and build, our in-house architects and construction teams handle everything from your initial design brief and planning application through to Building Regulations sign-off and final completion.