House
extensions
in Westminster, SW1
Architects, Engineers, Surveyors & Builders Under One Roof.
One Studio. One Contract. One Team.
RICS surveying support for party walls, licence to alter, conservation-area works & neighbour mattersWe design and build Westminster house extensions across Smith Square, Great Peter Street, Lord North Street - side-return, rear, wraparound and double-storey additions for Georgian townhouses, Listed Westminster terraces and larger family homes where planning strategy, conservation detailing, Party Wall sequencing and structural engineering need handling properly.
House Extensions in Westminster: What You Need to Know
Every house extension project in Westminster is designed to respect the Smith Square / Westminster Conservation Area and the character of Georgian townhouses, listed Westminster terraces, Crown Estate mansion flats. Our in-house RIBA Chartered Architects handle Westminster planning applications, our chartered structural engineers specify any load-bearing work, and our directly-employed trades deliver the build under one fixed-price contract. We coordinate Building Control, Party Wall awards where needed, and all certifications through to handover.
In Westminster's conservation area, extensions must be designed with exceptional sensitivity. Westminster officers assess impact on the host building, neighbouring properties, and the wider streetscape. We secure planning approval through design excellence and thorough pre-application engagement.
Political heart of London with residential pockets around Smith Square, Lord North Street, and Vincent Square. Westminster conservation area with significant listed buildings. Our extensions in Westminster use materials and detailing that complement the existing architecture — whether matching traditional brickwork and stonework or introducing high-quality contemporary glazing and zinc cladding.
The residential heart of Westminster is its Georgian enclave, and it is some of the most tightly protected ground in the country. The streets around Smith Square and the Division Bell district fall within the Westminster Abbey and Parliament conservation areas; external change is exceptionally tightly controlled, listed-building consent applies widely, and security and access near Parliament add their own constraints. An addition here begins as a heritage and consent exercise rather than a building one.
The stock is exceptional and heavily listed. Smith Square and Lord North Street rank among the most intact Georgian streets in London, restored with authentic brick and joinery; Barton Street and Cowley Street carry fine small townhouses where listed-building consent governs interiors as well as facades; and Great Peter Street and Vincent Square add larger houses and mansion flats. So even internal reconfiguration is designed for consent at each step.
Cost is led by that control and by the logistics near Parliament. A Westminster rear extension hovers around £71,250 to £112,500 before VAT, a side return near £90,000 to £120,000, with heritage consent driving the balance.
House Extensions in Westminster: Applied to the Local Stock
Westminster — the streets around Smith Square and the Division Bell district — sits in the City of Westminster, within the Westminster Abbey & Parliament conservation areas, some of the most tightly-protected ground in the country. London-wide indicative market band for wraparound extension: £100,000–£200,000 (source VAT treatment unstated, shown as an indicative market band, not a normalised ex-VAT figure) (Resi wraparound extension guide, 2025).
Sources: Westminster City Council — Westminster Abbey and Parliament Square Conservation Area mini guide (May 2004) (2026); Wraparound extension (Resi wraparound extension guide, 2025) (2026)
What a House Extension Involves
An extension reshapes how the whole ground floor works, not just how big it is — the new structure, the opening into the existing house, the kitchen and the garden connection all have to be designed as one move. We take each project from feasibility drawings through consents, structure and fit-out under a single fixed-price contract. Most extensions we build include the scope below.
- Rear extensions — single-storey additions that open the back of the house to the garden
- Side-return & wraparound — infilling the side alley of a terrace, alone or combined with a rear addition
- Double-storey — new bedrooms or bathrooms above the ground-floor addition, designed to sit comfortably with the original roofline
- Foundations & steel — groundworks, beams and the opening into the existing house, engineered by our own structural team
- Glazing — rooflights, glazed roofs and slim-framed sliding or bi-fold doors specified for light and thermal performance
- Kitchen integration — the kitchen layout planned with the architecture from the first drawing, not squeezed in afterwards
- Services & heating — electrics, plumbing, drainage and underfloor heating built into the new floor
- External works — patio, drainage connections and garden reinstatement completed within the contract
How We Run Your Project
Six stages, with the consents and engineering settled before the first spade goes in.
- Feasibility visit — we assess the site, the boundaries and your brief, and identify the right consent route for your property.
- Design & approvals — our architects draw the scheme while planning or permitted development paperwork, structural calculations and party wall notices progress together.
- Fixed-price contract — every element specified and priced before construction, from foundations to door hardware.
- Groundworks & shell — excavation, foundations, walls, steel and roof, taken to weathertight while you carry on living in the house.
- Fit-out — insulation, services, glazing, kitchen, flooring and decoration, with the opening into the existing house formed once the shell is sealed.
- Completion — building control inspection, snagging, external works and handover with the full certification pack.
Read the full house extension guide for extension types, permitted development limits and the build programme stage by stage.
Many rear extensions can proceed under permitted development — broadly up to 3m deep on attached houses and 4m on detached, extendable to 6m and 8m through the prior approval scheme — but conservation areas, Article 4 directions and listed status change the picture, and every extension needs Building Regulations approval regardless of the planning route. Where foundations come within 3m of a neighbouring building, the Party Wall etc. Act 1996 obliges you to give adjoining owners a minimum of two months’ written notice before construction can begin. Our planning guide library sets out the rules for each borough.
Why Hampstead Renovations
We hold £10 million public liability insurance with Aviva; the certificate is available on request.
House Extensions in Westminster: Questions
Why is extending near Smith Square and Westminster among the hardest in London?
Because it is some of the most tightly protected ground in the country. The streets around Smith Square and the Division Bell district fall within the Westminster Abbey and Parliament conservation areas; external change is exceptionally tightly controlled, listed-building consent applies widely, and security and access near Parliament add their own constraints.
How much of this part of Westminster is listed?
A great deal. Smith Square and Lord North Street rank among the most intact Georgian streets in London, restored with authentic brick and joinery, and on streets such as Barton Street and Cowley Street listed-building consent governs interiors as well as facades. So an addition or internal alteration is designed for the heritage case throughout.
What does an extension or reconfiguration cost in Westminster?
London-wide guidance has a Westminster rear extension hovering around £71,250 to £112,500 before VAT. A side return is dearer, near £90,000 to £120,000, with a wraparound at an indicative £100,000 to £200,000. Given the near-total listed control and security-driven access near Parliament, the consent strategy usually leads a scheme's figure.
Does access near Parliament really affect building work?
Yes. Security and access near Parliament add real constraints, so deliveries, storage and site set-up in the Georgian enclave are planned carefully. Leading with the consent strategy keeps a Westminster scheme on track, and the logistics around the Division Bell district are built into the programme.
What is the property stock like in this part of Westminster?
The residential heart of Westminster is its Georgian enclave: Georgian townhouses, listed Westminster terraces and mansion flats. Smith Square and Lord North Street rank among London's most intact Georgian streets, Barton Street and Cowley Street carry fine small townhouses beloved of the political world, and Great Peter Street and Vincent Square add larger houses and mansion flats.
Local character & heritage: extending in Westminster
Westminster sits in the City of Westminster, and extending here means working with period terraces and mansion blocks in the historic civic heart beside Parliament. Where houses and gardens allow, side-return and rear extensions open up the living space; in the many flats, the equivalent is internal reconfiguration and, where possible, lower-ground additions.
Permitted-development rights are usually restricted in Westminster's conservation areas, so most extensions need a full householder application, and the area is among the most tightly protected in London, much of it listed. We design every extension to respect the period frontage and to win consent.
Westminster heritage & architecture: Roof developments & conservation · Conservation areas · Listed buildings · Renovating a period terrace · Period facades · Facade planning rules
House Extensions in Westminster: Planning & Local Context
Westminster City Council · SW1
Planning for house extensions in Westminster
Rear and side-return extensions are popular in Westminster, but permitted-development rights are frequently restricted within Westminster's conservation areas, so most extensions need a full planning application to Westminster City Council with design that respects the prevailing building line and materials.
Westminster City Council context
Westminster SW1 sits within Westminster's conservation areas, and Westminster applies a demanding conservation and listed-building regime, and many mansion blocks and townhouses here are listed or sit within tightly controlled estates such as the Grosvenor, Howard de Walden and Crown estates. Our in-house RIBA Chartered Architects prepare and submit every application to Westminster City Council and handle pre-application discussions where early planning advice is useful.
Building Regulations
House Extensions in Westminster must satisfy Building Regulations — covering structural design to Part A, thermal performance to Part L, and drainage and foundations agreed with Building Control. Our in-house engineers manage Building Control from initial notice to completion certificate.
Recent work & local links
See recent Westminster work: Ebury Road, Westminster. Explore our Kitchen Renovation Loft Conversions in Westminster, or the Westminster hub · Refurbishment Company in Westminster.
Our in-house planning team prepares and submits every Westminster City Council application and manages building control end to end. View our planning track record →
House Extensions in Nearby Areas
More Services in Westminster
Book a Free Consultation
Visit our studio or invite us to survey your home. We’ll assess scope, discuss design possibilities and provide an honest budget framework — completely free.
Free Consultation
We survey your property, discuss your vision and provide a clear budget framework.