House extension architects in London: what changes borough by borough

A house extension in London is any addition to a home, such as a rear room, a side-return infill or a wraparound of both. The design questions are the same as anywhere. The approval questions are not, because boroughs layer their own rules over the national ones. Start with the council that covers your postcode, which GOV.UK's council finder will give you, then work out which of the three routes below applies. Our London hub guide covers the wider borough picture, and the national extensions page explains the routes themselves.

Does a London conservation area change the answer?

It usually does. The Planning Portal's extensions page lists conservation areas as article 2(3) land, and on that land every side extension needs householder permission, as does any rear extension of more than one storey (Planning Portal). The 6 metre prior approval route for a larger single-storey rear extension is also unavailable there (GPDO Schedule 2 Part 1). So a deep ground-floor extension that a house in a non-designated street might build with a neighbour consultation will need a full householder application inside a conservation area.

Boroughs can go further. Islington says it has removed permitted development rights in 40 of its 42 conservation areas (Islington), and Lambeth publishes a dataset of its Article 4 directions that you can download (Lambeth). These directions often target front-facing changes, but you should read the direction itself, because the list of restricted works differs between areas.

What does a typical London terrace mean for the design?

About 28.9% of London's dwellings were built before 1919 (VOA), and many of the houses among them are narrow terraces with a back addition and a side passage. That plan shapes the usual London brief. A side-return infill fills the gap beside the back addition. A wraparound combines that with a rear extension. Both change daylight to the neighbour's kitchen window, and councils tend to test the effect on neighbours' outlook closely, so a drawing that shows the 45-degree line to the nearest window earns its keep.

Where the borough publishes householder guidance, it is the document officers will hold your design against. Camden, for example, publishes planning guidance on home improvements, dated January 2021, and Kensington and Chelsea has a Basements SPD adopted on 14 April 2016 for the subterranean variant (Camden planning guidance; RBKC; Camden basement guidance). A garden-level lower-ground extension can count as basement development under some boroughs' policies, which brings engineering reports with it. Ask your architect which document governs your own street.

How does the Party Wall Act affect an extension?

Most London extensions touch a shared wall, build up to a boundary or dig foundations near a neighbour. Each of those can bring the Act into play. The Government's booklet says notice for work to a party structure is due at least two months before the planned start, and one month for certain excavations (GOV.UK). If a neighbour dissents, surveyors prepare an award, which adds cost and time. Plan the notice at the same time as the planning application, not after the approval letter arrives. Our party wall page explains the process.

What does the council cost and how long does it take?

A householder application for a single dwelling costs £548 from 1 April 2026 (Planning Portal fee guide). London's authorities decided 23,967 householder applications in the year to June 2026 and granted 85.8% (MHCLG). Those are totals for 33 authorities, so look up your own borough's decision history in its online planning register to see how similar schemes on your street fared. Pre-application advice is a separate, optional charge in many boroughs; Richmond upon Thames, for instance, runs a written service for non-listed homes without an officer meeting (Richmond).

Could the extension go down instead of out?

Some London owners with small gardens look at a lower-ground floor or basement instead of a rear addition. Boroughs treat this as a distinct category. Camden asks for technical assessment of drainage, groundwater and structural stability with basement applications, and Kensington and Chelsea has its own supplementary document, so the engineering input starts before the planning submission rather than after it. Expect longer lead times and a larger consultant team than for a ground-floor extension, and ask your borough for its basement checklist before you decide the idea is affordable.

How should you sequence the approvals?

A sensible London order is to settle the designations first, agree a brief and budget, test the idea with the planning team if the borough sells pre-application advice, and only then commission the full drawings. Building regulations drawings and party wall notices follow the planning decision, although the notice can be served earlier if the design is stable. Skipping the early check is the most common way a London owner pays for detailed drawings of a scheme that the borough's guidance would never have supported.

What should you ask an architect before hiring?

• Which borough guidance applies to my street, and which approval route do you expect me to need?
• How will you handle neighbours' daylight and outlook, and when will you show me that analysis?
• Do you include party wall advice, or do I need to appoint a surveyor separately?
• What is in your fee for each stage, and what is excluded, such as structural engineering or the council's fee?
• Can you show me comparable decisions from the same borough on its planning register?

Check any architect you shortlist on the ARB's register of architects before you sign terms. Our guide to costs explains how fees are built up, and the blog post on planning permission for extensions covers the national rules in more depth.