Westminster · W1 · SW1 · NW1
House Extensions in Westminster
Central borough with listed and conservation stock — interior and basement work dominates. Single and double-storey extensions delivered as a fixed-scope design and build package.
Postcodes covered
Quick answer
Builderr delivers rear and side extensions across Westminster — W1, W2, W9, NW1, NW8, SW1 and WC2. Typical Westminster extension cost £95,000–£260,000 over 16–26 weeks. The borough's Georgian and early Victorian stucco terraces in Mayfair, Marylebone, Pimlico and Belgravia require heritage-grade design with matching stucco, sashes, slate and ironwork. Basement extensions are restricted under the Westminster Basements SPD.
Westminster's 56 conservation areas cover nearly all residential stock. Article 4 directions remove most PD rights. Pre-app engagement is mandatory and typically takes 6–8 weeks.
Westminster
House Extensions specialists across Westminster.
We deliver house extensions projects across the full Westminster postcode set — W1, SW1, NW1, NW8. Central borough with listed and conservation stock — interior and basement work dominates.
Every Westminster project includes a free desk-based planning assessment before we quote, so you know upfront whether permitted development, prior approval, or full planning applies to your specific address. Our local project manager for Westminster has detailed knowledge of the planning department's preferences, the conservation area boundaries, and the common construction details for the prevailing housing stock here.
Quotes for Westminster house extensions are fixed-scope after a site survey, valid for 60 days, and never carry provisional sums or open-ended day rates. Every project includes our 10-year structural warranty and 12-month defects period.
Planning in Westminster
Local planning rules for house extensions in Westminster.
Westminster contains 56 conservation areas covering virtually all residential stock — Mayfair, Belgravia, Bayswater, Marylebone, Pimlico, Soho, St John's Wood, Maida Vale and Fitzrovia among them. Thousands of properties are statutorily listed (Grade I, II* or II). PD rights are removed under Article 4 across most of the borough. The City Plan and individual Conservation Area Audits govern alterations. Basement extensions are heavily regulated under the Basement SPG (single-storey only on most plots, no extensions under gardens beyond 50% footprint). Mansard roof extensions are typically the preferred upward route over rear dormers. Pre-application consultation is essential — Westminster's planning officers operate a structured pre-app service and outright refusals without engagement are common. Listed Building Consent applications run in parallel with planning and require specialist conservation input.
Our planning team handles the full submission for Westminster addresses including drawings, Design and Access Statements where required, and pre-application consultation with the council. We tell you whether your house extension is permitted development, prior approval, or full planning before you sign anything.
Builderr vs other London builders.
The construction industry has a wide distribution of operators. Here's what changes between a directly-employed, fixed-scope outfit and the alternatives.
| Criterion | Builderr | Typical London builder | Cowboy outfit |
|---|---|---|---|
| Labour model | Directly employed team (PAYE) | Mixed subcontract gangs | Day-rate cash labour |
| Pricing | Fixed-scope itemised quote | Estimate + provisional sums | Verbal price + variations |
| Design & engineering | In-house architect + SE | Outsourced, separate billing | Builder draws on the back of an envelope |
| Planning + LDC handled | Yes — included in price | Often charged extra | Builder asks you to apply |
| Party wall surveyors | Instructed by us | Your responsibility | Skipped (illegal) |
| Building control | Plans + site inspections booked by us | Building Notice route | Not registered |
| Project management | Dedicated PM, weekly photo updates | Foreman doubles up | Owner-manager juggles 5 jobs |
| Payment schedule | Stage payments against signed-off milestones | Weekly invoices | Cash up front |
| Insurance | £10M PL + 10yr structural warranty | £2–5M PL only | No documented cover |
| Snags at handover | <3 typical | 20–30 typical | Walk-off mid-job common |
| Variation creep | 0% — fixed scope | +15–25% over original quote | +40%+ regularly |
Save £27,000–£60,750 on a house extension in Westminster.
Industry data (FMB, RICS, Which? Trusted Trader 2024) shows the average London construction project overruns by 18–22% on cost and 25–35% on time. Fixed-scope contracts with a single accountable team eliminate that variance. The savings above assume a typical project at £135,000.
FAQ
House Extensions in Westminster.
How much does a house extension cost in Westminster?
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Most house extensions projects in Westminster cost £50,000–£220,000 depending on size, spec and structural complexity. Inner London postcodes typically sit toward the upper end; outer borough projects trend mid-range. All Builderr Westminster quotes are fixed-price with no provisional sums.
Do I need planning permission for a house extension in Westminster?
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Planning rules vary across Westminster. We check permitted development eligibility, conservation area status and any Article 4 directions affecting your specific postcode before quoting. Many house extensions fall under permitted development, but conservation areas and listed buildings need full planning.
How long does a house extension take in Westminster?
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Typical timeline is 10–20 weeks on site, plus 4–8 weeks for design, structural engineering and approvals. If full planning is required (conservation areas, mansards, larger extensions), add 8–12 weeks for the planning decision before site start.
Do you have house extension references in Westminster?
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Yes — we maintain a live reference list for Westminster and can connect you with two or three completed clients within the borough. We've delivered projects across nearly every Westminster postcode since 2022, so most prospective clients can visit a completed project nearby.
What's included in a Builderr house extension quote in Westminster?
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Every Westminster quote covers structural design, party wall notices and surveyor coordination, planning or prior approval submission where required, building control, full build, snagging and a 10-year structural warranty. Kitchen units, sanitaryware and bespoke finishes are itemised separately so you can flex spec without losing fixed-price certainty on the build.
Other services in Westminster
Get a Westminster house extension quote.
Senior consultant call within one business hour. Free desk-based planning assessment for W1, SW1, NW1 and surrounding postcodes. Fixed-scope price — no provisional sums.
