What will your extension really cost?
Every line a London extension carries — build, design, engineer, building control, party wall and contingency — at 2026 prices. Adjust the size and spec; the total updates live.
- ✓Every line: build, design, engineer, party wall
- ✓Standard, mid or high-end spec
- ✓Inner, outer or central London pricing
A 3 m × 6 m extension is 18 m². A typical London loft adds 25–35 m².
Total budget · inc. VAT
£83,200 – £121,600
£4,160–£6,080 per m² all-in · 12–16 weeks on site
- Build cost (shell, structure, finishes, M&E)£62,400–£81,600
- Bifold / sliding / steel-look doors£8,000–£14,000
- Design, drawings and applicationsArchitect or architectural technologist£3,100–£6,500
- Structural engineer£1,200–£2,200
- Building control£1,000–£1,800
- Lawful development certificate (recommended)Proves permitted development when you sell£270
- Party wall surveyors (1 neighbour)Only if a neighbour dissents£1,000–£2,200
- Contingency (8–12%)Ground conditions, drainage, hidden defects£6,200–£13,000
What moves the number
- →Foundations are the biggest unknown: clay, trees and drains can add £3k–£15k.
- →A detailed, itemised quote is the only real number — the range above is for budgeting.
Questions
How much does a single-storey rear extension cost in London in 2026?+
At standard spec most land between £3,200 and £4,500 per m² all-in, including VAT, design, engineer, building control, party wall and contingency. A typical 20 m² extension is roughly £65,000–£90,000 before a new kitchen or large glazing.
Is VAT included?+
Yes. Every figure includes VAT at 20%. Builders under the VAT threshold may quote less, but check they are not leaving items out instead.
Why is my builder’s quote so different?+
Usually because items are missing (steels, party wall, drainage, decoration) or priced as provisional sums. Quote Check compares every line against the standard scope and London benchmarks.
How accurate is this?+
It is a budgeting range based on 2026 London project data, not a quote. Ground conditions, access and specification choices move the final number — a detailed, itemised tender is the only firm price.
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