House extension cost in 2026 — full UK price guide
A house extension is the most popular renovation project in the UK — single-storey rear and side-return extensions alone account for roughly half of all permitted-development planning notifications. This 2026 cost guide breaks down what every type of extension actually costs to build, what drives the variation between £40k and £180k, and how the price varies across London and the South East. The numbers below come from RCB Design & Build delivery costs across the last 18 months — not industry averages.
House extension cost by type (London/M25 corridor, 2026).
Project type
From
To
Notes
Small single-storey rear (≤20 m²)
£40,000
£60,000
Tight scope. Standard glazing.
Standard single-storey rear (20–30 m²)
£55,000
£85,000
Includes kitchen relocation in most cases.
Large single-storey rear (30–45 m²)
£80,000
£120,000
Roof lantern or structural slot glazing standard.
Side-return extension (10–18 m²)
£35,000
£60,000
Iconic Victorian terrace upgrade.
Wrap-around extension (30–45 m²)
£75,000
£140,000
Side + rear combined.
Two-storey rear extension (25–40 m² per floor)
£85,000
£160,000
New bedroom and bathroom upstairs.
Side extension (single storey, 10–20 m²)
£40,000
£75,000
On semi-detached / detached homes.
Garage conversion
£18,000
£35,000
Lowest-cost route to extra liveable space.
Regional cost variation
How regional cost varies (m² rates for the typical build).
Region
From
To
Notes
Prime central London (W1, SW1, SW3, NW1)
£144,000
£174,000
Premium finishes and listed-building consent standard.
Inner London (SE/SW/E/NW/N inside South Circular)
£120,000
£150,000
Strong Article 4 / conservation-area constraints.
Outer London (Bromley, Bexley, Croydon, Sutton, Havering)
£40,000
£120,000
Mid-band labour rates, PD typically intact.
Kent / Surrey / Essex commuter belt
£38,000
£132,000
Affluent areas (Sevenoaks, Tunbridge Wells, Weybridge) sit near top.
Outer Kent / Essex / Hertfordshire
£34,000
£114,000
Below-London labour costs; longer logistics.
What drives the cost
Glazing
Glazing is typically 15–25% of total extension cost. Aluminium slim-frame slider doors: £6k–£12k. Structural slot glazing: £8k–£18k. Roof lantern: £4k–£8k. Bifolds (now less fashionable): £4k–£7k. A glass box extension can hit 40% glass cost.
Foundations
Standard strip foundations on competent ground are typically 8–10% of cost. If the soil is clay near a mature tree, you may need a piled raft costing 15–20% more. A condition survey establishes this in 1 week.
Steel work
A standard knock-through with a single steel beam adds £4k–£8k. Multiple beams (e.g., side-return + rear) add £8k–£18k. Goalpost frames replacing two perpendicular walls run £12k–£25k.
Roof type
Flat warm-roof construction with single-ply membrane: £140–£180/m² roofing only. Pitched tiled roof: £180–£240/m². Glass roof lantern: £700–£1,200/m². A roof in three different planes (e.g., over a wrap-around) carries a 20% complexity premium.
Kitchen relocation
A kitchen extension where the existing kitchen position moves needs new M&E runs (gas, water, soil, electrics, extract). Allow £4k–£8k of extra first-fix work versus an extension that doesn't touch the kitchen footprint.
Planning route
PD route (Larger Home application) adds 8 weeks. Full planning adds 10–12 weeks. Listed building or conservation area consent can add 16+ weeks.
Engineer designs steels and foundations; we submit Building Control.
Party Wall Awards
4–8 weeks
Notices, schedule of condition, award.
Strip-out + groundworks
2–3 weeks
Existing rear wall demolished, foundations dug.
Superstructure + roof
4–6 weeks
Brickwork up, steels in, roof on, watertight.
First fix + glazing
3–4 weeks
M&E first fix, doors and windows installed.
Plaster + second fix
3–4 weeks
Plaster, skirtings, doors, sockets, lights.
Snagging + handover
1 week
Defects, certificates, BC sign-off.
How extension cost is calculated
Single-storey extensions in London typically price out at £2,400–£3,400/m² of new internal floor in 2026 — slightly less for simple shapes, more for complex glazing. A 30 m² rear extension prices at £72,000–£102,000 on this basis, before second-fix kitchen and flooring. Two-storey extensions are cheaper per square metre (£2,100–£2,900/m²) because foundations and roof are shared across two floors. Wrap-arounds carry a 10–15% complexity premium because the roof has more junctions and the design needs to fit two existing walls.
Single-storey vs two-storey extension
If the planning route allows, two-storey extensions deliver substantially more square footage per pound — £2,100–£2,900/m² vs £2,400–£3,400/m² for single-storey. They also add a bedroom and bathroom upstairs, which is usually more valuable for resale than extra kitchen space. The downside is much stricter planning rules: most LPAs require neighbour daylight studies and refuse two-storey extensions that fail the 25/45 rule.
How to control extension cost
Three biggest cost-control levers: (1) keep the kitchen in roughly its existing footprint to avoid £4k–£8k of M&E rerouting; (2) specify aluminium-frame doors rather than steel-frame Crittall replicas, which cost 2–3× more; (3) supply your own kitchen, flooring and tiles rather than buying through the builder — this saves the typical 10–15% trade markup, on a £25k kitchen that's £2.5k–£3.75k saved. The fourth lever is the building shape: rectangles are cheap, L-shapes 10% more, anything with three or more building junctions is significantly more expensive.
Planning and party walls
Single-storey rear extensions up to 6 m on detached and 4 m on terraces typically sit inside Permitted Development via a Larger Home application. Anything beyond that, or in a conservation area or Article 4 zone, needs full planning. Party Wall Awards are mandatory for any excavation within 3 m of a shared wall — surveyor cost is £1,200–£2,500 per neighbour. Schedule of Condition is taken before works start. We coordinate the whole process in-house.
ROI on house extensions
Single-storey rear extensions typically deliver 50–70% of build cost back at resale on average London terraced housing. Two-storey extensions can hit 70–100% ROI when they add a bedroom. Wrap-around and double-storey kitchens in prime central London can exceed 100% — the kitchen-diner is the most-valued room for buyers in this market. Build quality matters: a poorly finished extension can be a net negative.
Frequently asked questions
How much does a house extension cost in 2026?
A standard single-storey rear extension costs £55,000–£85,000 for a 20–30 m² scheme in London, before second-fix kitchen and flooring. Side-returns run £35,000–£60,000. Wrap-arounds £75,000–£140,000. Two-storey extensions £85,000–£160,000.
How much does a side return extension cost?
A typical Victorian terrace side-return extension costs £35,000–£60,000 in London for a 10–18 m² scheme. That includes the foundations, brick build-up, steel beam, structural glazing and first-fix M&E. Kitchen units and flooring are extra.
Do I need planning permission for a house extension?
Single-storey rear extensions up to 6 m on detached and 4 m on terraces sit inside Permitted Development via a Larger Home Extension application (Neighbour Consultation Scheme). Conservation areas, Article 4 zones and any first-floor extension need full planning.
How long does a house extension take?
Plan on 6–10 weeks of design and planning, then 12–18 weeks on site for a typical single-storey rear extension. Two-storey and wrap-around extensions add 4–6 weeks.
Can I extend my house without planning permission?
Yes, in most cases. The Larger Home Extension scheme allows up to 6 m on detached and 4 m on terraces. Side extensions (≤ half the original width, single storey) sit inside PD. Always get a Lawful Development Certificate to lock the planning status in.
What's included in an extension price?
Our quotation includes foundations, structural steel, brickwork, roofing, glazing, first-fix M&E, insulation, plaster, Building Control fees and Party Wall coordination. Kitchen units, flooring, sanitaryware and final decoration are client-supplied for cost control.
How much does an extension add to house value?
A well-executed single-storey rear extension typically adds 50–70% of build cost in resale value. Two-storey extensions that add a bedroom add 70–100%. Wrap-arounds in prime central London can exceed 100% ROI.
The numbers above are real averages — but every project is different. Use our 60-second instant quote, configure your project in detail, or book a free site visit for a fixed written quotation in 5 working days.