Average house extension costs per m² in the UK
In the UK, house extension costs typically range from £1,600 to £2,600 per square metre (m²) for single-storey builds and £1,400 to £2,200 per m² per floor for two-storey extensions. Two-storey extensions offer superior economies of scale because the expensive ground excavation, foundations, and roof structure are shared across two floors.
Costs vary significantly depending on interior fit-out. Adding an open-plan kitchen-diner with structural steel beam knock-throughs and bifold doors substantially increases budget requirements compared to simple dining room or bedroom extensions.
Planning permission vs Permitted Development
Many single-storey rear extensions fall under UK Permitted Development rights, allowing you to extend up to 4 metres for a detached house or 3 metres for an attached house (or up to 8m/6m under the Prior Approval Neighbour Consultation Scheme) without full planning permission.
However, all extensions must comply with UK Building Regulations covering structural stability, thermal insulation (Part L), ventilation (Part F), drainage (Part H), and fire safety (Part B). Building control fees, architectural drawings, and structural engineer calculations typically account for 10% to 15% of the total spend.
Worked example: 30m² single-storey kitchen extension in Greater London
Consider a 30m² single-storey rear extension in Greater London with good quality finishes, including a new mid-range kitchen (£12,000) and ground-floor WC (£4,500).
The core shell construction at London rates (£2,200/m²) totals £66,000. Combined room fit-outs add £16,500, bringing construction to £82,500. Professional fees (architect and structural engineer at 12%) add £9,900. Including statutory 20% VAT (£18,480) brings the total project investment to approximately £110,880.
Inputs for the extension cost calculator
Extension style
Single storey rear, two storey, wrap-around, or side return.
Floor area (m²)
Total gross internal footprint of the new extension in square metres.
Specification level
Finish standard: budget/standard builder finish, good contemporary quality, or premium luxury.
New kitchen fit-out
Include supply and trade installation for a new kitchen suite in the extended space.
New bathroom fit-out
Include plumbing, sanitaryware, and tiling for a new shower room, bathroom, or downstairs cloakroom.
Region of the UK
Select your location to apply realistic regional labor multipliers.
CALCULATOR QUESTIONS
UK house extension cost calculator questions
Do I need planning permission for a house extension in the UK?
Not always. Many single-storey extensions fall within Permitted Development rights if they meet specific size and boundary criteria. Two-storey extensions, wrap-arounds, or properties in conservation areas usually require full planning permission.
Is a two-storey extension twice as expensive as a single-storey?
No. A two-storey extension typically costs only 50% to 60% more than a single-storey extension of the same footprint, because the foundation and roof costs are spread across twice the floor area.
How much should I set aside for a contingency fund?
You should always budget a 10% to 15% contingency fund above your builder quotes to cover unforeseen ground condition issues, drain diversions, or hidden structural defects.
Do extension builders charge VAT?
Yes. Residential extension work by VAT-registered contractors is subject to standard 20% VAT on both labor and materials.
Do I need a Party Wall Agreement?
If your extension excavations are within 3 metres of a neighbour structure and deeper than their foundations, you must serve a formal Party Wall notice under the Party Wall etc. Act 1996.