Local costs · London · September 2026
How much does an extension cost in Harrow?
A mid-range 25 m² single-storey extension in Harrow costs about £85,000 to £105,000 including VAT, or £3,400 to £4,200 per m² at London prices. Harrow charges £994 for building control on a 10 to 40 m² extension (full plans, including VAT): 18% above the typical council (£840). With design, a structural engineer, building control and a 15% contingency, budget £104,000 to £133,000.
Work out your project in Harrow
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Typical prices in Harrow
| Project | Building work | Budget to plan for |
|---|---|---|
| Single-storey extension, 25 m² | £85,000 to £105,000 | £104,000 to £133,000 |
| Double-storey extension, 40 m² | £123,000 to £156,000 | £149,000 to £193,000 |
| Dormer loft conversion, 30 m² | £76,000 to £94,000 | £93,000 to £118,000 |
| Kitchen, medium | £10,600 to £18,800 | £12,200 to £21,500 |
| Bathroom refit | £7,300 to £10,900 | £8,300 to £12,500 |
Building prices in London run about 35% above the UK average outside London. Kitchens and bathrooms vary less from place to place than extensions and lofts.
Building control fees in Harrow
| Work | Charge |
|---|---|
| Extension up to 10 m² (full plans) | £729 |
| Extension 10 to 40 m² (full plans) | £994 |
| Larger extension, 40-60 m² (full plans) | £1,259 |
| Extension 10 to 40 m² (building notice) | £994 |
| Loft conversion up to 40 m² | £927 |
| Garage conversion | £546 |
Harrow charges £994 for building control on a 10 to 40 m² extension (full plans, including VAT): 18% above the typical council (£840). Source: Harrow's building control charges, 6 April 2026. Inc VAT ("Rate of VAT 20% has been included").
Private approved inspectors set their own prices and are often competitive, so it is worth asking one or two for a quote as well. See how Harrow compares with every council we checked.
Notes on Harrow's schedule
Own schedule. For <=40 m2 the plan charge includes inspection.
Planning
If your extension needs planning permission, a householder application in England costs £548. Many single-storey rear extensions are permitted development and need no application, and a lawful development certificate (£274) confirms it.
Other councils in London
Building control for a 10 to 40 m² extension:
- Havering: £859
- Bromley: £1,008
- Richmond upon Thames: £1,069
- Wandsworth: £1,104
- Hillingdon: £1,122
- Redbridge: £1,320
- Southwark: £1,473
- Islington: £1,584
- Camden: £1,876
Questions people ask
How much does an extension cost in Harrow?
A mid-range 25 m² single-storey extension in Harrow costs about £85,000 to £105,000 including VAT for the building work, or £3,400 to £4,200 per m², based on London prices reviewed in September 2026. With fees and a 15% contingency, budget £104,000 to £133,000.
How much is building control in Harrow?
Harrow charges £994 (including VAT) for a 10 to 40 m² extension (full plans), according to its published schedule (6 April 2026).
How much does a loft conversion cost in Harrow?
A mid-range 30 m² dormer loft conversion in Harrow costs about £76,000 to £94,000 including VAT for the building work, and about £93,000 to £118,000 with fees and a 15% contingency.
Before you get quotes
- How to compare builders' quotes line by line
- Plan your whole budget, including the costs people forget
- Trades: price an extension element by element with editable unit rates