Is a double-storey extension cheaper per square metre than a single-storey?

Yes. A two-storey extension typically costs 20% to 30% less per square metre (roughly £1,500–£2,200/m²) because the expensive groundwork, foundations, and roof structure are shared across two levels of floor space.

How much does a 30 square metre extension cost all-in?

A standard 30 m² extension build costs £66,000–£84,000 before extras. Adding 20% VAT (£13,200–£16,800), architect and structural engineering fees (£5,000–£8,000), building control (£800–£1,200) and 10% contingency brings the realistic turn-key total to £90,000–£115,000.

How much does location affect extension cost per m²?

Greater London and the South East carry a 25% to 40% premium over UK average rates due to higher trades day rates (£350–£500/day vs £220–£300 in the North/Midlands), parking suspensions, skip permits, and access restrictions.

UK house extension building costs per square metre illustration

Construction8 min readPro Calculator UK Editorial

Extension Cost Per Square Metre: What UK Builders Actually Charge (2026)

Q: What is the average extension cost per square metre in the UK in 2026?

In 2026, UK builders charge £1,800–£2,300/m² for basic specification, £2,200–£2,800/m² for standard finish, and £2,800–£3,600+/m² for premium bespoke extensions. Figures exclude VAT and professional fees.

When planning a home extension in the UK, builders, quantity surveyors, and architects universally benchmark quotes in cost per square metre (£/m²). However, headline figures often cause confusion because some quotes include VAT and interior fit-outs, while others only cover the shell structure.

In 2026, building material inflation and rising skilled trade wages have set the baseline UK extension cost at £1,800 to £3,200 per m² for single-storey builds. This guide breaks down exact rates by specification, explores why double-storey extensions offer superior square-metre value, and details the hidden costs that take quotes over budget.

2026 Cost Per m² by Specification Level

Specification is the single greatest determinant of your rate per square metre. Below are realistic 2026 build costs (excluding VAT, architectural fees, and planning costs) based on standard UK ground conditions:

Specification Level	Cost per m² (excl. VAT)	Example 20 m² Build	Example 35 m² Build
Basic Flat mineral felt/EPDM roof, standard PVC French doors, radiator heating, basic electrics	£1,800 – £2,100	£36,000 – £42,000	£63,000 – £73,500
Standard Pitched tiled roof or roof lantern, aluminium bifolds, underfloor heating, recessed LED lighting	£2,200 – £2,700	£44,000 – £54,000	£77,000 – £94,500
Premium / Luxury Bespoke structural glazing, zinc or copper cladding, vaulted ceilings, high-end kitchen fit-out	£2,800 – £3,600+	£56,000 – £72,000+	£98,000 – £126,000+

Why Two-Storey Extensions Cost Less Per m²

Homeowners are frequently surprised to discover that a two-storey extension often costs only 40% to 50% more in total than a single-storey extension of identical footprint, which dramatically lowers the cost per square metre:

Shared Foundations: The expensive excavation, concrete footings, and drainage are identical whether supporting one storey or two.
Shared Roof: A single pitched or flat roof covers both floors, eliminating duplicate weatherproofing costs.
Average m² Rate: While a 25 m² single-storey build costs £2,200–£2,700/m² (£55k–£67k), a 50 m² double-storey build typically costs £1,600–£2,100/m² (£80k–£105k).

Regional Price Multipliers Across the UK

Day rates for bricklayers, carpenters, and electricians vary considerably across British regions. Use these regional multipliers against the national benchmark:

Inner London: +35% to +45% (congested access, parking suspensions, £400–£500 day rates)
Outer London & South East: +20% to +30%
East Anglia & South West: +5% to +15%
Midlands & Wales: UK National Average (0%)
North West, Yorkshire & North East: -10% to -15%
Scotland: -5% to +10% (depending on Edinburgh/Glasgow urban centers vs rural)

Hidden Costs Beyond the m² Build Rate

To avoid budgetary shock, ensure your total funds account for essential fees that builders do not include in their headline m² construction rate:

VAT (20%): Most home improvement and extension work is standard-rated at 20%. Always clarify whether quotes are inclusive or exclusive of VAT.
Architect & Structural Engineer (£2,500 – £7,000): Architectural drawings, structural calculations for RSJ steel beams, and building control submission packages.
Building Regulations Compliance (£700 – £1,500): Local council or approved private inspector stage inspections and final completion certificate.
Party Wall Agreements (£1,000 – £3,000): If building within 3 or 6 metres of a neighbouring structure, party wall surveyor fees may apply if neighbours dissent.
Groundworks Complications (£2,000 – £8,000): Discovery of public sewers requiring a Build-Over Agreement from Thames Water or United Utilities, clay soil requiring deeper trench footings, or tree roots.
Contingency Reserve (10%–15%): Essential buffer for unforeseen structural alterations when knocking through existing load-bearing walls.

Frequently Asked Questions

What is the average extension cost per square metre in the UK in 2026?

UK builders charge between £1,800 and £3,200 per square metre depending on finish specification. Standard specification averages £2,200–£2,700/m² before VAT.

Does an extension add more value than it costs?

In high-value areas (London, Surrey, Edinburgh, Bristol) where property values exceed £4,000–£8,000/m², an extension costing £2,500/m² yields an immediate net profit. In lower-value areas, always check your street's ceiling price.

Can I build an extension under Permitted Development?

Yes. Many single-storey rear extensions up to 4 metres (detached) or 3 metres (semi-detached/terraced) fall under Permitted Development, or up to 8m/6m under the Prior Approval scheme, subject to height limits.