Move
Total: £117,660
- SDLT (HMRC)
- £78,750
- Estate-agent fee
- £12,660
- Survey
- £1,500
- Legal (sale + purchase)
- £2,500
- Removals
- £2,000
- Chain contingency (1.5%)
- £20,250
Stamp duty vs extension costs in Westminster — 2026 guide
For homeowners in Westminster (Westminster City Council). Updated 2026-06-11.
Inner central London. Georgian, Regency and Victorian stucco-fronted terrace and mansion-block stock — the most architecturally restricted residential area in the UK alongside RBKC. Extensive listed buildings. Side returns and lofts almost always require full planning and heritage-led design.
The borough covers the NW1, NW8, SW1A, SW1E, SW1H, SW1P, SW1V, SW1W, SW1X, SW1Y, W1A, W1B, W1C, W1D, W1F, W1G, W1H, W1J, W1K, W1S, W1T, W1U, W1W, W2, W9, W10, WC1H, WC2A, WC2B, WC2E, WC2H, WC2N, WC2R postcode districts. Terraced homes here trade around £2,950,000 against the borough-wide £844,000 average.
In short
In Westminster, the average home is valued at £844,000. Moving up by 60% to a £1,350,000 property triggers approximately £78,750 in Stamp Duty Land Tax (SDLT). By contrast, a 22 m² high-spec rear extension (single storey) here costs around £144,716 all-in. Westminster has 56 conservation areas — among the most of any London borough.
Your numbers
Adjust the inputs to see your scenario. The page updates instantly as you adjust the inputs.
Your result
Moving is £27,056 cheaper than extending
Extend
Rear extension (single storey) — high spec
Total: £144,716
- Build cost (22 m² × £4,400/m²)
- £96,800
- Professional fees (15%)
- £14,520
- Subtotal
- £111,320
- VAT (20%)
- £22,264
- Contingency (10%)
- £11,132
An extension of this type and size could add roughly £109,720 (≈13%) to your home's value — Nationwide Building Society, October 2025.
Your tipping line
Where moving starts costing more than the extension you selected. The chart follows your inputs above.
Up to about £1,574,000 of target price, moving is the cheaper route in Westminster; beyond that line, extending wins.
Extending in Westminster: what's different here
Local planning picture
- Conservation areas
- 56
- Householder decisions within 8 weeks
- 50%
- Build-cost tier
- prime central London
- Typical housing stock
- Victorian and Edwardian terraces
Decision speed: MHCLG householder statistics, year ending September 2025, excluding agreed extensions of time.
Worth knowing in Westminster
- Westminster has 56 conservation areas — among the most of any London borough.
- Westminster has a borough-wide Article 4 direction removing permitted development rights for basements.
- The average home here is around £844,000 (ONS, 2026).
- Extension types suited to Victorian and Edwardian terraces: Rear extension (single storey), Kitchen extension, Loft conversion (mansard), Side return extension, Basement / lower-ground conversion.
How the £78,750 stamp duty stacks up
HMRC bands applied to a £1,350,000 purchase — the default Westminster trade-up.
| Price band | Rate | SDLT due |
|---|---|---|
| £0 – £125,000 | 0% | £0 |
| £125,000 – £250,000 | 2% | £2,500 |
| £250,000 – £925,000 | 5% | £33,750 |
| £925,000 – £1,500,000 | 10% | £42,500 |
What each suitable type costs vs the £117,660 move
High specification at typical size, prime central London rates, all-in.
| Extension type | All-in cost | vs moving |
|---|---|---|
| Rear extension (single storey) (22 m²) | £144,716 | £27,056 more than moving |
| Kitchen extension (22 m²) | £144,716 | £27,056 more than moving |
| Loft conversion (mansard) (30 m²) | £116,610 | saves £1,050 vs moving |
| Side return extension (12 m²) | £93,288 | saves £24,372 vs moving |
| Basement / lower-ground conversion (40 m²) | £418,600 | £300,940 more than moving |
Why this comparison matters
"Homeowners that add a loft conversion or extension, incorporating a large double bedroom and bathroom, can add as much as 24% to the value of a three-bedroom, one-bathroom house."
Frequently asked questions
- Is it cheaper to extend or move in Westminster?
- At Westminster's average £844,000 home moving up to £1,350,000, the move costs about £117,660 — including £78,750 of HMRC Stamp Duty. A 22 m² high-spec rear extension (single storey) runs roughly £144,716 at prime central London rates. Moving is cheaper here by about £27,056. Your real figure depends on extension size, specification and planning constraints.
- How long does planning permission take in Westminster?
- The statutory determination period for a householder application is 8 weeks. Westminster City Council decided 50% of householder applications within 8 weeks in the year to September 2025 (MHCLG figures, excluding agreed extensions of time) — among the slowest in London. Across London, authorities grant roughly low-to-mid 80s per cent of applications. A householder application costs £548 from April 2026, and a Larger Home Extension prior approval £240.
- Do I need planning permission for an extension in Westminster?
- Many single-storey rear extensions fall under Permitted Development — no planning permission needed. Westminster has the most restrictive planning regime in London alongside Kensington and Chelsea. Nearly all the borough is covered by conservation areas. Article 4 directions remove PD rights for almost every type of external alteration. Listed buildings are extensive. Assume full planning permission required for almost any external work. Westminster has 56 conservation areas, including Mayfair, Belgravia, Marylebone, where PD rights are curtailed. Side extensions, large rear extensions and most wraparounds need full planning regardless. Check the Westminster City Council planning portal first. Open the Westminster City Council planning portal
- How long does a rear extension (single storey) take in Westminster?
- Allow around 16 weeks on site for a typical rear extension (single storey) in Westminster, after design, party wall agreements with neighbours and the council's determination period. A kitchen extension runs nearer 14 weeks. End to end, plan 4–8 months from first call to completion — longer in conservation areas or where structural surveys are needed.
- Will an extension add value to my Westminster home?
- Nationwide Building Society (October 2025) found that adding a double bedroom can lift a home's value by about 13%, and a full loft conversion plus extension by up to 24%. On a Westminster average of £844,000, that's roughly £109,700 to £202,600 of capital uplift. Final return depends on the local market, the type of buyer in Westminster, and your extension's specification.
- Can I live in my home during the extension build?
- In Westminster's period stock, staying put depends on the project: a rear or kitchen extension is usually liveable with 2–3 weeks of real disruption, while mansard lofts and basements are far more invasive — basements in particular can mean months of structural works. Many Westminster owners decant for the heaviest phase and budget accordingly.
- Can I build a basement in Westminster?
- Yes, but not under permitted development: Westminster City Council has a borough-wide Article 4 direction removing PD rights for basement development, so every basement needs a full planning application. Local policy limits most schemes to a single storey and around 50% of the garden, with engineer-certified method statements required. Budget prime central London basement rates and 6–12 months of works.
- At what price does moving stop making sense in Westminster?
- Starting from Westminster's average £844,000 home with a 22 m² high-spec rear extension (single storey) (£144,716 all-in), the tipping line sits at roughly £1,574,000: buying above that price costs more in SDLT and moving fees than the extension itself. Below it, moving stays the cheaper route. The live chart on this page recalculates the line for your own inputs.
- How much stamp duty would I pay moving up in Westminster?
- On a typical Westminster trade-up from £844,000 to £1,350,000 (a 60% step up), Stamp Duty Land Tax (SDLT) is £78,750 — an effective rate of 5.83%. First-time buyers and additional-property buyers pay different rates. Use HMRC's official calculator at tax.service.gov.uk for your exact figure. HMRC Stamp Duty calculator
- What does a typical extension cost in Westminster in 2026?
- A 22 m² rear extension (single storey) in Westminster runs about £145,000 all-in at high specification, on prime central London build rates of around £4,400/m². A 22 m² kitchen extension comes to roughly £145,000. All-in figures include 15% professional fees, 20% VAT and a 10% contingency — final cost depends on access, specification and ground conditions.
Compare nearby London boroughs
Nearby by average property value — useful if you're weighing a move to an adjacent price tier. A comparable trade-up in City of London would carry about £79,750 of stamp duty — £1,000 more than in Westminster.
Methodology and sources
Stamp Duty Land Tax is calculated from the HMRC bands in force for the 2025–26 tax year. Verify any figure against HMRC's official calculator before committing. Extension costs use prime central London £/m² rates by extension type — industry-estimate midpoints, presented as approximations.
- Nationwide House Price Index (October 2025 release) — value-add commentary attributed to Andrew Harvey, Senior Economist.
- HM Land Registry / ONS UK House Price Index — borough average property values (March 2026 release).
- MHCLG planning statistics (PS2 dashboard) — householder decision speed, year ending September 2025.
- London 2026 £/m² grid by extension type and tier — Build Team benchmark compiled from BCIS/RICS, FMB and published architect guides (refreshed quarterly).
- Conservation-area counts and Article 4 status — individual London borough councils, verified 2026.
- HMRC SDLT calculator — tax.service.gov.uk/calculate-stamp-duty-land-tax
- GOV.UK Stamp Duty Land Tax guidance — gov.uk/stamp-duty-land-tax