Extension cost per m² in London
Extension cost by size in London
Each row multiplies the current £1,800–£3,000 per m² national benchmark by floor area and then applies the same London factor.
What can change a real London quote?
- Floor area and number of storeys
- London and regional labour rates
- Steelwork and structural changes
- Kitchen or bathroom installation
- Glazing and door specification
- Finish level, access and contingency
Local planning considerations
Dense access, parking, deliveries and contractor overheads can widen the spread between quotations, so use the figures as an early budget rather than a quote.
- Allow separately for parking, loading, permits or restricted delivery windows where a property has difficult street access.
- Flats and managed buildings can add landlord, freeholder or building-management requirements that are outside a basic works estimate.
- Party-wall, structural and access costs should be checked independently on extensions, lofts and major internal alterations.
How the regional model works
The 1.28× factor places this planning estimate 28% above the national planning baseline. We keep that modelling assumption separate from the dated national benchmark so users can see exactly which part comes from an external source and which part is UK Home Costs modelling.
Sources
House Extension: Cost Breakdown 2026
Published / checked: June 2026
Used for: Extension cost-per-m² benchmarks and common add-on costs
Construction output price indices
Published / checked: 13 August 2026
Used for: Construction price trend and update checks
Frequently asked questions
How much does a house extension cost in London in 2026?
Our modelled London planning rate is about £2,300–£3,850 / m², based on the current national £1,800–£3,000 per m² benchmark and the London regional factor. This is not an observed city-wide quote average.
How much is a 20m² extension in London?
Applying the same regional model to the current national benchmark gives about £46,100–£76,800 before VAT for a straight-line 20m² example. Structural work, glazing, kitchens, bathrooms and access can move a real quote outside that range.
Why is the London estimate different from the UK benchmark?
UK Home Costs applies a 1.28× London planning factor, which places the model 28% above the national planning baseline. The factor is our modelling assumption rather than an external city price index.
Does the extension figure include VAT and professional fees?
The national benchmark used here is stated before VAT. Do not assume architect, structural engineer, planning, building-control, party-wall or specialist survey fees are included unless a contractor quote says so explicitly.