House extension architect near me — London plans

Which extension type gives you the space you need?

Extensions are usually described by where they go rather than by what they cost, but in London the two correlate closely because the planning route differs by type.

• Single-storey rear — the default. Permitted development allows 3 metres beyond the original rear wall on a terraced or semi-detached house and 4 metres on a detached house, with a maximum height of 4 metres and eaves no higher than 3 metres within 2 metres of a boundary.
• Larger single-storey rear under prior approval — doubles those depths to 6 and 8 metres, decided in 6 weeks at £249 after a 21-day neighbour consultation. Not available on flats, and withdrawn wherever an Article 4 direction applies.
• Side return — the classic Victorian terrace move, infilling the alley beside the original closet wing. Typically adds 8 to 14 square metres and transforms a narrow galley kitchen into a full-width room. Often combined with the rear extension in one wrap-around.
• Two-storey rear — permitted development caps this at 3 metres deep and requires 7 metres between the new rear wall and the rear boundary, with the roof pitch matching the existing. Most London two-storey extensions exceed one of those and need a full application.
• Side extension — limited under permitted development to single storey, half the width of the original house, and a maximum 4 metres high. Forward of the principal elevation it is never permitted development.
• Wrap-around — side return plus rear in one, the highest-value option on a London terrace and almost always a full householder application because the combined footprint breaches the permitted development geometry.

How far can you extend without planning permission?

Permitted development for extensions is governed by Class A of the General Permitted Development Order, and the limits are cumulative against the house as originally built — or as it stood on 1 July 1948 for older properties. A previous owner’s extension eats your allowance, which is why the planning history of the address matters before anything is drawn.

Beyond the dimensions, permitted development is lost entirely in several situations London owners meet constantly: the property is a flat or maisonette rather than a house; it is listed; it sits in a conservation area where side extensions and cladding are restricted; or the borough has made an Article 4 direction removing the right along your street. Camden, Islington, Haringey, Hackney and Waltham Forest all operate directions that catch residential alterations.

Where the work is permitted development, we still recommend applying for a Lawful Development Certificate at £274. It converts an assumption into a council-issued document, and it is the paperwork a buyer’s solicitor will demand when you sell.

What will your extension do to the neighbours?

Residential amenity is the reason most London extension refusals are written, and it comes down to daylight, outlook and overlooking. Boroughs test daylight with the 45-degree rule: a line drawn at 45 degrees in both plan and section from the midpoint of the nearest habitable-room window on the neighbouring property. If your extension crosses that line, expect an objection to carry weight.

The 25-degree test is the stricter version some boroughs apply in section, taken from the centre of the neighbour’s ground-floor window. Where a scheme fails, the usual fixes are pulling the depth back by half a metre, dropping the eaves height, or switching from a flat-roofed box to a mono-pitch falling away from the boundary — all decisions that cost nothing on the drawing and a great deal after a refusal.

Party wall matters run in parallel and are entirely separate from planning. Under the Party Wall etc. Act 1996 you must serve notice on the adjoining owner for work to a shared wall, for excavation within 3 metres of a neighbouring structure to a lower depth than its foundations, and within 6 metres where the excavation falls below a 45-degree line from their foundations. Notice periods are one month for party structure work and two months for excavation, so this is a programme item, not an afterthought.

Which building regulations apply to a house extension?

Every extension is notifiable, whether or not it needed planning permission. Planning and building control are separate consents with separate submissions, and an extension built under permitted development still requires full building control approval.

• Part A, structure — foundation depth against the soil type and nearby trees, lintels and steel beams over new openings, and calculations for anything spanning. London clay with mature trees close by commonly drives foundations to 2.5 metres or deeper, or to a piled solution.
• Part L, energy — U-values for the new walls, roof and floor, and a limit on the total area of glazing relative to the new floor area. Large sliding doors are achievable but have to be compensated elsewhere in the fabric.
• Part H, drainage — where you build over or within 3 metres of a public sewer, a build-over agreement with Thames Water is required, and that lead time regularly exceeds the planning decision.
• Part B, fire — an extension that encloses an existing escape route, or a two-storey extension creating an inner room, changes the escape strategy for the whole house.
• Part M and Part K — level thresholds at the new external door, and glazing, guarding and stair geometry where levels change.

Buying them separately from different practices means paying twice for the same existing-condition information and reconciling two drawing registers when the builder finds a discrepancy.

What do extension drawings cost, and what does the build cost?

Alongside our fee, budget the statutory application fee of £548 for a householder application or £274 for a certificate, building control charges of roughly £700 to £1,200 for an extension, a structural engineer for the beams and foundations, and a party wall surveyor where notice is served and the neighbour dissents.

Build costs at 2026 London rates run approximately £2,200 to £3,000 per square metre for a good-quality single-storey rear extension, rising with structural glazing, a basement element or a difficult site access. A 20 square metre rear extension is therefore commonly a £45,000 to £60,000 project before kitchen fit-out.

A registered architect in London working to a percentage fee would typically charge £3,000 to £6,000 for the planning-stage package on the same project. We are roughly 30% below typical London architect rates because the fee is fixed against a defined scope rather than scaling with a build cost that has not been tendered.

House extension questions London homeowners ask