TL;DR — Key Facts
- Build cost per sqm: £3,000 – £4,500 (inner London premium borough)
- Council planning fee: £548 (householder application, 2026)
- Typical projects: rear extension, side-return, two-storey, wraparound
- Conservation areas: 56 designated areas in Westminster
- Permitted Development: Restricted by Article 4 in some areas
Extension costs in Westminster
Once the budget works, the next step is the drawing set: house extensions in Westminster.
House extension build costs in Westminster typically range from £3,000 to £4,500 per square metre, reflecting Westminster's position as an inner London premium borough. These figures cover the construction cost only and do not include professional fees, planning fees, or VAT.
Westminster costs are dominated by consent rather than construction. Listed building consent applies to much of the residential stock and reaches internal work; specialist trades and matched historic materials follow from that. Where the project goes downwards instead, the underpinning, plus an award agreed with each flanking owner, load the job with engineering and professional fees far exceeding the construction.
Typical extension costs in Westminster (build only)
On top of the build cost, you will need architectural drawings. These fees are 30% below typical London architect rates.
What affects extension costs in Westminster
Westminster is one of the two slowest authorities in London for householder work - 74.5% decided inside eight weeks and 16.2% beyond thirteen (MHCLG planning statistics, retrieved 1 September 2026) - and it issued 419 enforcement notices, more than any other borough. Not a single deep rear extension went through the neighbour-consultation route across that window, which tells you the ordinary rear extension is not really what happens here. Basement and internal work is.
Conservation areas
Westminster has 56 designated conservation areas, including Mayfair, Belgravia, Marylebone, Pimlico, Soho, Covent Garden, St John's Wood. If your property is in a conservation area, Permitted Development rights are restricted, meaning you may need a full planning application (£548 council fee). Conservation area applications also tend to require higher-quality materials and more sympathetic design, which can increase build costs by 10-20%.
Article 4 Directions
Westminster has Article 4 Directions in place. Multiple Article 4 Directions — office-to-residential restrictions, HMO conversions, retail to residential in shopping zones. Where Article 4 applies, works that would normally be Permitted Development require a full planning application. This adds £548 to the council fee and typically 2-3 months to the project timeline, which can affect overall costs if you are renting alternative accommodation during the build.
Property type
The housing stock in Westminster is predominantly grand georgian and regency terraces in belgravia and pimlico; victorian and edwardian mansion blocks; heritage-rich townhouses throughout; very high listed building density. Victorian and Edwardian terraces typically suit side-return and rear extensions, while 1930s semi-detached properties are well suited to two-storey rear extensions. Detached houses offer the most flexibility but often have larger footprints, meaning higher absolute costs even if the per-sqm rate is similar.
Access and logistics
Belgravia, Pimlico and Marylebone are Georgian, Regency and mansion-block stock on streets where a skip, a suspension and a delivery slot are each arranged rather than assumed. There is no side return on this housing and frequently no rear vehicular access. Where the work goes downwards instead, the excavation volume and the removal route become the largest single element of the groundworks cost.
Our drawing fees for Westminster extensions
At Architectural Drawings London. No hourly billing, no surprises. Our fees cover everything needed for a valid planning submission to Westminster City Council and for building regulations approval.
Drawing fees for extensions in Westminster
Every package includes a measured survey of your existing property, existing floor plans and elevations, proposed floor plans and elevations, a site plan, and a design and access statement where required. Our MCIAT chartered architectural technologists handle the full process from survey to submission, and we liaise with Westminster City Council on your behalf.
We cover all Westminster postcodes: SW1, W1, W2, W9, NW1, NW8, WC2.
Planning vs Permitted Development in Westminster
Many house extensions in Westminster can be built under Permitted Development (PD) rights without needing a planning application. Under PD, single-storey rear extensions can extend up to 6 metres from the original rear wall for terraced and semi-detached houses, or 8 metres for detached houses (under the Prior Approval process). However, Westminster has Article 4 Directions that may remove some PD rights. Multiple Article 4 Directions — office-to-residential restrictions, HMO conversions, retail to residential in shopping zones.
Across Westminster's 56 conservation areas, an Article 4 Direction and widespread listed status leave little permitted development to restrict. The terraces have no side to extend into, so the operative controls are materials and the rear elevation. Where a building is listed, consent is needed for internal work too, and 419 enforcement notices in three years indicate this is checked.
Two-storey extensions need full permission, and on most Westminster addresses listed building consent as well. The Belgravia and Pimlico terraces are heritage assets in their own right, rear elevations are frequently as controlled as the fronts, and the council issued 419 enforcement notices over three years (MHCLG planning statistics, retrieved 1 September 2026), more than any other London borough. Proceeding on an assumption is not viable here.
Westminster granted 533 certificates of lawful development in three years (MHCLG planning statistics, retrieved 1 September 2026). Set that against 85 certificates for proposed works to listed buildings - more than ten times any other borough - and the shape of the borough is clear: the live question here is far more often listed building consent than permitted development, and a lawful development certificate speaks only to the second.
Frequently asked questions
How much does a house extension cost in Westminster?
Build costs for a house extension in Westminster typically range from £3,000 to £4,500 per square metre, as Westminster is an inner London premium borough. A typical single-storey rear extension of 15-20 sqm would cost between £45,000 and £90,000 for the build alone. On top of build costs.
Can I extend under Permitted Development in Westminster?
Across Westminster's 56 conservation areas, combined with an Article 4 Direction and listed status on a large share of the stock, the working assumption should be that no permitted development right survives. Across three years the deep-extension consultation route produced no determinations here whatsoever, while 419 enforcement notices were issued - a higher count than any other London borough reached.
How long does a house extension take in Westminster?
A typical house extension project in Westminster takes 5-8 months from initial instruction to completion. This breaks down as: 2-3 weeks for measured survey and drawing preparation, 8 weeks for planning determination by Westminster City Council (if required), 2-4 weeks for building regulations approval, and 8-16 weeks for construction depending on the size and complexity of the extension.
Do I need a Party Wall Agreement for an extension in Westminster?
Terraced throughout, so two adjoining owners, and any excavation below a neighbour's foundation level within three metres is notifiable - which covers essentially all basement work. With 419 enforcement notices and 433 contravention notices on the record (MHCLG planning statistics, retrieved 1 September 2026), starting before the award is agreed is a poor bet in this borough specifically.
What experience do you have with extensions in Westminster?
We prepare extension drawings for properties across all Westminster postcodes (SW1, W1, W2, W9, NW1, NW8, WC2). Our MCIAT chartered architectural technologists understand Westminster City Council's policies, design guides, and local precedents. We have a council-ready drawing review across all 33 London boroughs.
