Loft conversion cost, worked out properly.
Rooflight, dormer or mansard — an all-in London budget with engineer, building control, party wall and contingency, so the quote you get has nowhere to hide.
- ✓All-in budget incl. fees and contingency
- ✓Programme length on site
- ✓Planning route for each loft type
A 3 m × 6 m extension is 18 m². A typical London loft adds 25–35 m².
Total budget · inc. VAT
£85,300 – £129,000
£2,843–£4,300 per m² all-in · 8–12 weeks on site
- Build cost (shell, structure, finishes, M&E)£72,000–£100,800
- Design, drawings and applicationsArchitect or architectural technologist£3,600–£8,100
- Structural engineer£1,100–£2,000
- 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£6,300–£13,800
What moves the number
- →Moving the water tank, boiler or soil stack adds cost; head height on the new stair is often the deciding factor.
- →A detailed, itemised quote is the only real number — the range above is for budgeting.
Questions
How much is a dormer loft conversion in London?+
A rear dormer adding around 30 m² typically costs £70,000–£110,000 all-in at standard spec in inner London, including VAT, fees and contingency. Rooflight conversions are cheaper; mansards cost more and need planning.
Do I need a party wall agreement for a loft conversion?+
Almost always on a terrace or semi: inserting steels into the party wall is notifiable under Section 3 and needs two months’ notice. Use our free party wall notice generator.
Does a loft conversion need planning permission?+
Rear dormers within 40 m³ (terraces) or 50 m³ (semis and detached) are usually permitted development, unless you are in a conservation area or under an Article 4 direction. Plan Check confirms it for your address.
How long does a loft conversion take?+
Six to nine weeks for rooflights, eight to twelve for a dormer and ten to fourteen for a mansard, once on site.
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