What we build in Cowley
Most Cowley projects we scope fall into three buckets: a side-return or rear infill that fixes a dark galley kitchen, a full-width rear extension that opens the ground floor to the garden, and a rear-plus-loft package that adds a bedroom while the scaffold is already up. 1930s semis and detached, larger Arts & Crafts in Ruislip and Northwood.
Extension costs in Cowley
A mid-spec 25 m² extension in Cowley typically lands between £89,000 and £144,000 all-in — that figure includes professional fees, planning and Building Control, party wall, a 12% contingency and VAT at 20%. Ex-VAT build rates run £2625–£4000 per m² depending on glazing, structural spans and kitchen spend.
| Project | Size | Cowley all-in |
|---|---|---|
| Rooflight conversion | 15–20 m² | £49k – £79k |
| Rear dormer kit | 20–28 m² | £89k – £144k |
| L-shaped dormer kit | 28–36 m² | £120k – £194k |
| Hip-to-gable + dormer | 30–40 m² | £134k – £216k |
Planning permission in Cowley
One of the most PD-friendly boroughs in London. Northwood and Ruislip Manor conservation areas are the exceptions.
Before you assume permitted development applies to your street, check whether your street falls inside a conservation area or an Article 4 direction — both are common in Hillingdon and both remove PD rights for rear and side extensions. Our London planning guide walks through the tests in order.
Party wall and neighbours
Semi-detached — one party wall typical, often none for detached homes.
Serve notice at least two months before you break ground. Budget £1,200–£2,400 per adjoining owner if they dissent and appoint their own surveyor — see the Party Wall Act guide.
Access, logistics and site set-up
Access is typically through the house, so expect dust protection, a boarded route and a skip permit from the council.
Every quote we issue for Cowley prices skip permits, parking suspensions, protection and welfare explicitly in the preliminaries — so the number you sign is the number you pay.
Value added in Cowley
Well-executed extensions here tend to return a healthy share of their cost at resale, particularly where they add a genuine third or fourth bedroom.
Typical programme
- Weeks 0–4Measured survey, design and structural calcs
- Weeks 4–12Planning or lawful development certificate, party wall notices
- Weeks 12–16Tender, builder selection and contract signing
- Weeks 16–34On site: groundworks, shell, watertight, first and second fix
- Week 35Building Control sign-off, snagging and handover
Full detail in the London extension timeline guide.
Why homeowners in Cowley use us
- KYB-verified builders only
- Live Hillingdon project experience
- Itemised fixed-price specifications
- Insurance, warranty and accreditation checks