Home Extension Costs UK 2025: Single-Storey, Two-Storey and Wrap-Around

Home extensions remain one of the most popular ways for UK homeowners to add space without moving — and in London and the South East, they consistently add more value than they cost. In slower markets, the calculation is tighter. This guide sets out real costs by extension type, regional benchmarks per square metre, what drives prices up, and the professional fees that are frequently omitted from early estimates.

Home Extension Costs by Type

Costs vary substantially by extension type and specification. The table below uses two tiers: basic (standard blockwork, rendered finish, simple roof, uPVC windows) and premium (bifold or sliding doors, lantern or vaulted roof, underfloor heating, quality internal fit-out).

Extension Type	Basic Spec	Premium Spec
Single-storey rear	£20,000–£45,000	£45,000–£80,000
Two-storey rear	£35,000–£65,000	£65,000–£120,000+
Side return (terraced/semi)	£25,000–£45,000	£45,000–£55,000
Wrap-around (rear + side return)	£50,000–£80,000	£80,000–£100,000+
Garden room / outbuilding	£10,000–£25,000	£25,000–£40,000

Side return extensions are specific to terraced and semi-detached houses, making use of the narrow passage alongside the house. They are typically combined with a rear extension to create a large open-plan kitchen-diner — this combined approach is what most people mean by "wrap-around."

These figures are for build cost only. Professional fees, planning, and party wall costs are covered separately below.

Cost Per m² — Regional Benchmarks

Build cost per square metre varies significantly by location — primarily reflecting local labour rates, material delivery costs, and contractor demand.

Region	Build Cost Per m²
London	£2,500–£4,500/m²
South East	£2,000–£3,500/m²
South West	£1,800–£3,000/m²
Midlands	£1,500–£2,500/m²
North England / Wales	£1,400–£2,200/m²
Scotland	£1,500–£2,500/m²

These are build-only costs. Add professional fees (typically 10–15% of build cost) and VAT at 20% (standard rate for extensions to existing dwellings — extensions are not zero-rated).

A 20m² single-storey rear extension in the South East at £2,500/m² = £50,000 build cost. Add £7,500 in professional fees, £11,500 VAT, plus planning, structural, and party wall costs: total project cost approaches £75,000.

What Drives Costs Up

These are the most common items that push extension costs above initial quotes:

Bifold or sliding doors: +£5,000–£15,000 over standard French doors. Aluminium bifolds are significantly more expensive than uPVC but more durable and better-performing thermally. A 4-panel 3.6m aluminium bifolding door set typically costs £3,000–£6,000 supply-only; installation adds £500–£1,000.
Underfloor heating (wet system): +£80–£120/m² installed. Wet (hydronic) UFH has lower running costs than electric mat systems but requires a manifold, pump, and boiler connection. Electric mat systems cost less to install (+£50–£80/m²) but more to run. Insulation board below the heating layer is essential — without it, you heat the ground, not the room.
Vaulted or glazed lantern roof: +£3,000–£8,000 over a standard flat or pitched roof. Roof lanterns introduce natural light dramatically but require structural upstands and weatherproofing details that add labour time.
Structural steel (RSJ beams): +£2,000–£5,000 per beam installed. Required when spanning large openings, removing load-bearing walls to open up the ground floor, or when existing structural walls are altered. Each beam requires structural engineer calculations.
Difficult ground conditions: clay subsoil, high water table, or a sloping garden can add 10–20% to foundation costs. Stepped foundations, deeper strip footings, or pile-and-beam foundations are all more expensive than a standard strip in stable ground. If you're on clay soil, also be aware of subsidence risk — disturbing ground near an existing structure should always be assessed by a structural engineer first.
High-specification kitchen: if the extension's purpose is a kitchen-diner, the kitchen fit-out itself can add £10,000–£30,000+ depending on specification. This is separate from the build cost.

Professional and Regulatory Costs

These are consistently underestimated by homeowners and sometimes omitted entirely from early builder quotes. They are not optional.

Cost Item	Typical Range
Architect or architectural designer drawings	£2,000–£8,000
Structural engineer calculations	£800–£2,000
Planning application fee (England, householder)	£258
Building Regulations application (full plans)	£400–£1,500
Party Wall surveyor fee (per neighbour)	£700–£1,500

A project requiring architect drawings, structural engineer, planning permission, building regs, and party wall for one neighbour can accumulate £5,500–£13,000 in professional fees before any build work begins.

Building Regulations approval is always required for an extension, regardless of whether planning permission is needed. There are no exemptions. For a full breakdown of notifiable works and what the process involves, see our building regulations approval guide.

Planning Permission vs Permitted Development

Many single-storey rear extensions can be built under Permitted Development (PD) rights without applying for planning permission, subject to size limits. For the full breakdown of what you can build without planning permission, see our permitted development rights UK guide.

Key limits:

Single-storey rear — standard PD: up to 4m projection from rear wall (detached houses), up to 3m (semi-detached and terraced). Maximum 4m height.
Single-storey rear — Larger Home Extension scheme (Prior Approval): up to 8m (detached) or 6m (semi/terraced). Must notify the council; neighbours can object during a 42-day window.
Two-storey extensions: full planning permission required in almost all cases. Must be at least 7m from the rear boundary.
Wrap-around extensions: almost always require full planning permission due to combined scale and impact.

Even for PD-compliant work, always obtain a Lawful Development Certificate (£206, 8 weeks). Without one, you have no formal record of compliance, which can delay or block a future sale.

Party Wall Act — When It Applies

The Party Wall etc. Act 1996 applies if your extension:

Is built on or up to the boundary line (a "party fence wall" notice is required)
Requires excavation within 3 metres of a neighbour's structure to a depth lower than their foundations

Process: serve written notice 1–2 months before works begin. If the neighbour consents in writing, no surveyor is needed. If they dissent or fail to respond within 14 days, both parties must appoint surveyors to agree a Party Wall Award covering working hours, protection measures, and schedule of condition.

The building owner (you) pays both surveyors' fees: £700–£1,500 per surveyor. An agreed surveyor (acting for both parties) costs less — typically £700–£1,200 total — but both parties must consent to the arrangement.

Return on Investment

Extension ROI varies by location, property type, and specification:

Single-storey kitchen-diner extension: typically adds 5–8% to property value in South East England. On a £500,000 house, that's £25,000–£40,000 added value against an average cost of £50,000–£70,000 including fees. The gap narrows as house prices rise.
Two-storey rear extension: can add 10–12% in value but requires full planning permission and a higher build cost. Most likely to achieve positive ROI in London and the Home Counties.
Garden room: adds £10,000–£25,000 in perceived value but rarely achieves a 1:1 return on the build cost.
Worst ROI: overimproving a property beyond the ceiling price of the street. Extending a two-bed terrace in a road where no house sells for more than £300,000 rarely recovers extension costs.

Rule of thumb: do not spend more than 15% of your current property value on an extension in a market where prices are broadly flat.