Building cost in Westminster: 2026 rates.
Westminster building costs sit at the top end of the UK market. Rear extensions run £3,600–£5,000/m²; loft conversions £90k–£170k; full refurbs £3,000–£4,600/m². Mews, mansion-block and listed-terrace stock dominates, and Westminster's strict basement and heritage policies add 8–15% versus outer-London comparables. Costed acts as your client-side cost expert through quote, build and closing out the build.
| Project type | Typical 2026 cost |
|---|---|
| Rear extension | £3,600–£5,000/m² |
| Loft conversion (total) | £90,000–£170,000 |
| Full refurbishment | £3,000–£4,600/m² |
| Basement (per m²) | £5,800–£9,000/m² |
All figures exclude VAT (20% on most refurb/extension work), professional fees (8–14% typical), party wall awards and statutory costs.
- Westminster's UDP Policy ENV13 limits basements under gardens and listed terraces
- Mansion-block leasehold consent under Section 19 LTA 1985 adds 6–14 weeks before works start
- Conservation areas cover ~75% of Westminster, most external changes need full planning
- Restricted hours under the City Inspectors' Code of Practice for Deconstruction & Construction lengthen programmes
- Listed building consent is its own application, refusal rates above the London average
Leasehold consent, listed-building consent and basement policy ENV13 are the three biggest cost drivers. Get a cost expert briefed before quote, not after.
Frequently asked, Westminster
How much does a Westminster mansion-block refurbishment cost?
A full strip-and-reinstate refurb of a 150m² Westminster mansion-block flat costs £450,000–£700,000 on 2026 rates, plus VAT, fees and leaseholder licence costs. Lift access, weekday delivery windows and freeholder-approved contractors typically add 12–20% versus a freehold equivalent.
What permissions do I need for a Westminster terrace extension?
A rear extension on a Westminster terrace usually needs full planning permission, party wall awards under the 1996 Act, and, if listed, listed building consent. Permitted development rights are removed by Article 4 across most of the borough's conservation areas.
How much does a Party Wall surveyor cost in Westminster?
Party Wall Act 1996 surveyor fees in Westminster typically run £1,400–£4,000 per adjoining owner, depending on whether each side appoints separately. The building owner pays both sides under the Act for basement and structural works.
Know what your project should cost. Before you build it.
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