House extension types compared: cost, planning and build time in London

Rear, side return, wraparound, kitchen, ground floor, two-storey or a side extension on a semi — the right extension depends on the shape of your plot, what you want the ground floor to do and how much of the garden you are prepared to give up. This page puts the seven types side by side using our 2026 London design-and-build ranges, so you can compare them on one screen before opening the detailed page for the one that fits. Every figure is a range from our master price list, postcode dependent; your own home is surveyed and costed individually and the figure on your itemised estimate is the figure you pay — nothing is added afterwards.

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The seven types at a glance

Rear extension — £35,000–£75,000 · usually permitted development · 10–14 weeks on site.
Side return extension — £35,000–£78,000 · usually permitted development · 10–14 weeks on site.
Wraparound extension — £60,000–£128,000 · usually a planning application · 14–18 weeks on site.
Kitchen extension — £35,000–£128,000 for the build plus £15,000–£75,000 for the fit-out · depends on the shape · 12–16 weeks on site including the fit-out.
Ground floor extension — £35,000–£128,000 depending on the shape · rear and side return usually permitted development · 10–18 weeks on site.
Two storey extension — £75,000–£165,000 · usually a planning application · 16–24 weeks on site.
Side extension (semi-detached or detached) — from £35,000 single storey, £60,000–£128,000 with a rear extension, £75,000–£165,000 for two storeys · single storey usually permitted development, two storeys always a planning application · 10–24 weeks on site.

Allow 8–16 weeks of design and approvals before any of these start on site (12–16 weeks for two storeys). Kitchen and bathroom fit-outs are priced on top of the build ranges unless stated.

Rear extension

What it is: a single-storey extension across the back of the house, usually 3–6 m deep and full width. It adds depth: a kitchen extended into a kitchen-diner with garden doors across the back.
2026 range: £35,000–£75,000 design and build inclusive, kitchen on top. A 3 m full-width extension sits at £35,000–£50,000; 4–5 m at £50,000–£65,000; 6 m at £60,000–£75,000.
Planning: usually none. Permitted development covers 3 m on a terrace or semi (4 m detached), and 6 m / 8 m under the prior-approval route. A Lawful Development Certificate is included.
Build time: 10–14 weeks on site.
Best for: households that want the biggest kitchen-diner for the money and have garden depth to spare.
Rear extension service · Rear extension cost guide

Side return extension

What it is: filling in the narrow strip beside the outrigger on a Victorian or Edwardian terrace. It adds width rather than depth: a galley kitchen becomes full width under a glazed roof, and the garden keeps its length.
2026 range: £35,000–£78,000 design and build inclusive. Infill only is £35,000–£50,000; infill with a knock-through into the old kitchen £48,000–£65,000; infill plus a 1–2 m step-out beyond the outrigger £60,000–£78,000.
Planning: usually none. A single-storey side extension up to half the width of the original house is permitted development. Lawful Development Certificate included.
Build time: 10–14 weeks on site.
Best for: terraced houses with a dark, narrow kitchen and a short garden.
Side return extension service · Side return extension cost guide

Wraparound extension

What it is: a side return and a rear extension built together as one L-shaped room. It adds width and depth at once: a full-width kitchen, dining and living space with large glazing spans.
2026 range: £60,000–£128,000 design and build inclusive. Side return plus 3 m rear is £60,000–£85,000; plus 4–5 m rear £80,000–£110,000; plus 6 m rear £105,000–£128,000.
Planning: usually a planning application, because side and rear together normally exceed permitted development limits. Some wraparounds can be split into a permitted-development side return and a prior-approval rear.
Build time: 14–18 weeks on site.
Best for: families who want the whole ground floor rethought as one open-plan space and can wait for a planning decision.
Wraparound extension service · Wraparound extension cost guide

Kitchen extension

What it is: not a separate shape but a purpose: a rear, side return or wraparound extension designed around the kitchen, with the fit-out planned into the build from the first drawing rather than bolted on at the end.
2026 range: £35,000–£128,000 for the extension itself (rear £35,000–£75,000, side return £35,000–£78,000, wraparound £60,000–£128,000) plus £15,000–£75,000 for the kitchen fit-out.
Planning: follows the shape — rear and side return are usually permitted development; a wraparound usually needs a planning application.
Build time: 12–16 weeks on site including the fit-out.
Best for: anyone whose main reason for extending is the kitchen: it lets services, islands, extraction and glazing be designed together.
Kitchen extension service · Kitchen extension cost guide

Ground floor extension

What it is: the umbrella term for any single-storey extension: rear, side return or wraparound. Use this page when you know you want more ground-floor space but have not yet decided which shape suits the plot.
2026 range: £35,000–£128,000 depending on the shape: rear £35,000–£75,000, side return £35,000–£78,000, wraparound £60,000–£128,000.
Planning: rear and side return extensions are usually permitted development; a wraparound usually needs a planning application.
Build time: 10–18 weeks on site depending on the shape.
Best for: comparing shapes on your own plan with our team before committing to one.
Ground floor extension service · House extension cost guide

Two storey extension

What it is: an extension on two floors, most often to the rear or side: a kitchen-diner below with a double bedroom, en-suite or bathroom above. It is the way a three-bed becomes a four-bed without touching the loft.
2026 range: £75,000–£165,000 design and build inclusive, kitchen and bathroom fit-out on top. A 3 m deep full-width two-storey rear is £75,000–£100,000; 4 m deep £95,000–£130,000; a two-storey side or wraparound £120,000–£165,000.
Planning: usually a planning application. Permitted development allows a two-storey rear to 3 m only if it stays 7 m from the rear boundary, and most London gardens are too short.
Build time: 16–24 weeks on site, with 12–16 weeks of design and approvals before that.
Best for: growing families who need another bedroom as well as a bigger kitchen, on a plot where a loft conversion alone will not do it.
Two storey extension service · House extension cost guide

Side extension on a semi-detached or detached house

What it is: building into the passage or driveway beside a semi or detached house, in one or two storeys. Single storey it gives a utility, downstairs WC or a wider kitchen; two storeys it adds a bedroom and bathroom above.
2026 range: from £35,000 for a single-storey side extension (£35,000–£78,000); £60,000–£128,000 for side and rear together, priced as a wraparound; £75,000–£165,000 for two storeys.
Planning: a single-storey side extension is usually permitted development (up to half the width of the house and 4 m high). Two-storey side extensions always need a planning application.
Build time: 10–14 weeks single storey; 14–18 weeks with a rear extension; 16–24 weeks for two storeys.
Best for: 1930s semis and detached houses with a side passage or garage that earns its keep better as living space.
Side extension service · House extension cost guide

How to choose between them

Start with the plot, not the budget. A terrace with a side return and a short garden points to a side return or wraparound; a semi with a long garden points to a rear or two-storey rear; a semi with a wide side passage points to a side extension. The shape the house offers usually decides more than the price list does.

Then decide what the ground floor has to do. One big kitchen-diner is a rear extension. A brighter, wider kitchen that keeps the garden is a side return. Kitchen, dining and living in one room is a wraparound. Another bedroom as well is two storeys.

Check the planning route before you fall in love with a drawing. Rear and side return extensions are usually permitted development with a Lawful Development Certificate; wraparounds and anything on two storeys usually need a planning application, which adds time before work starts. Our house extension planning permission guide and planning rules by council pages cover the detail for your borough.

Compare like with like. Our ranges are design and build inclusive: architectural design, structural engineering, building control and the build under one contract. When you compare quotes, add those items to any builder-only figure before you line them up.

Frequently asked questions

Which house extension type is cheapest?
A single-storey rear or side return extension, both from £35,000 in our 2026 ranges. The side return is usually the cheapest way to transform a terraced kitchen because the footprint is small.

Which extension adds the most space?
A two-storey extension (£75,000–£165,000) adds the most floor area; a wraparound (£60,000–£128,000) adds the most ground-floor space.

Do I need planning permission for a house extension?
Often not for a single-storey rear or side return extension, which are usually permitted development. Wraparounds and two-storey extensions usually need a planning application. We check your address and the planning history before we advise.

Can I combine an extension with a loft conversion?
Yes, and doing both under one contract is common. See loft conversion and extension and, for loft options, loft conversion types.

What is the next step?
Call 020 8037 7332, book a free 30-minute call, or get an instant estimate for your postcode. See finished work on our projects page and read client reviews.