How Much Does a House Extension Cost in the UK?

A house extension costs between £1,500 and £3,500 per m² of floor area in the UK. The total project cost depends on the size, type, specification, and location — and can range from £20,000 for a modest kitchen extension to £200,000 for a large two-storey addition.

Extension Type	Typical Cost Range
Small single-storey (up to 20m²)	£20,000 – £45,000
Standard single-storey (20–40m²)	£45,000 – £90,000
Large single-storey (40m²+)	£90,000 – £150,000
Two-storey extension	£60,000 – £150,000
Side return extension	£35,000 – £65,000
Wrap-around extension	£70,000 – £130,000

All figures are indicative and exclude VAT (20%). Costs increase significantly in London and the South East.

Cost Per m² for House Extensions in the UK

The most reliable way to estimate an extension is by cost per m²:

Specification Level	Cost Per m² (exc. VAT)
Basic (standard fittings, no special features)	£1,500 – £1,900
Mid-range (quality kitchen, bifolds, tiled bathroom)	£1,900 – £2,600
High specification (bespoke finishes, underfloor heating, premium materials)	£2,600 – £3,500
London premium (inner London, all specs)	Add 25–35%

Note: cost per m² typically decreases as the extension gets larger — fixed costs (foundations, scaffold, planning) are spread over more floor area.

What Drives Extension Costs?

Ground conditions and foundations

Poor ground conditions — clay soil (common in London), made-up ground, or tree roots — can add £3,000–£15,000 in additional foundation costs. A standard strip foundation on solid ground is the base assumption; engineer specifications may change this significantly.

Structural openings into the existing house

Creating a wide opening between the extension and the original property requires steelwork — typically an RSJ beam. A standard opening costs £1,500–£4,000 including the beam, padstones, and temporary support. Multiple openings or large spans cost more.

Roof design

A flat roof is typically the least expensive option (EPDM rubber or Bauder felt). A pitched roof matching the existing property adds 15–25% to the roof cost but often looks better and adds more value.

Glazing specification

Standard UPVC double glazed units are included in base costs. Aluminium bifold or sliding doors (a popular choice for rear extensions) add £3,000–£8,000 per set. Roof lanterns add £2,500–£6,000 each.

Services

Running new electrics, plumbing, and heating to the extension adds cost. A rear extension typically needs extended heating (radiators or underfloor), new circuits, and potentially drainage for a utility room or bathroom.

Regional Cost Variation

Region	Cost Per m² (mid-spec)
Inner London	£2,800 – £4,200
Outer London / Home Counties	£2,200 – £3,200
South East (outside M25)	£2,000 – £2,800
South West	£1,900 – £2,600
East of England	£1,900 – £2,600
Midlands	£1,700 – £2,400
North West / Yorkshire	£1,600 – £2,200
North East	£1,500 – £2,100
Scotland	£1,700 – £2,400
Wales	£1,600 – £2,200

Typical Cost Breakdown: 25m² Rear Extension (Mid-Spec)

Trade / Cost Category	Typical Cost
Demolition and enabling works	£1,500 – £2,500
Foundations (strip, standard ground)	£4,000 – £7,000
External walls (brick/block cavity)	£5,000 – £9,000
Structural steel (single opening)	£2,000 – £3,500
Roof structure (flat)	£3,500 – £5,500
Flat roof covering (EPDM)	£2,000 – £3,500
External windows and doors (UPVC)	£3,000 – £6,000
Internal insulation and plasterboard	£2,500 – £4,000
Plastering	£1,500 – £2,500
Electrical (first + second fix)	£2,500 – £4,500
Plumbing and heating	£2,000 – £4,000
Floor finishes (tiled)	£2,000 – £3,500
Decoration	£1,500 – £2,500
Scaffold	£1,500 – £2,500
Waste disposal	£500 – £900
TOTAL (approx)	£39,000 – £62,000

What’s Included and What Isn’t

Typically included in main contractor quotes:

All groundwork and foundations
External walls, roof structure and covering
External doors and windows
Internal plastered and painted walls and ceilings
Electrical first and second fix
Central heating extension
Basic floor screed

Typically not included (check your quote):

Kitchen fitting and appliances
Bathroom suite and tiling
Premium floor finishes (hardwood, high-spec tile)
Bifold/sliding doors (often a significant extra)
Underfloor heating
Landscaping and external works
Planning and Building Regulations fees
Party wall surveyor costs
VAT (20%)

How to Get an Accurate Extension Quote

The traditional approach — visiting the site, measuring up, phoning suppliers, building a spreadsheet — takes experienced estimators 4–8 hours per job. And the prices used are often out of date before the quote is even sent.

PricingPro generates a fully itemised extension estimate in minutes. Describe the job in plain English (“25m² rear extension, brick/block walls, flat roof, bifolds, East Midlands”) or upload your drawings, and the AI produces a line-by-line estimate using live prices from 100+ UK suppliers.

Generate an extension estimate →

FAQs

Do I need planning permission for a house extension? Most single-storey rear extensions fall under permitted development and don’t need planning permission — provided they meet size and siting criteria. Extensions over 4m deep (detached houses) or 3m deep (semi/terrace), extensions to the side elevation, and two-storey extensions usually require planning permission. Always check before starting.

How long does an extension take? A standard 20–30m² single-storey extension takes 12–20 weeks from groundworks to handover. Complex projects, bad weather, or planning delays can extend this significantly.

Will an extension add value to my home? A well-designed extension typically adds 10–20% to property value. A large kitchen extension or additional bedroom can add more. The return varies by property type and location — it’s usually strongest in areas where the existing house is under-valued relative to the land.

What is the 45-degree rule for extensions? The 45-degree rule is a planning guideline used by many councils to assess whether an extension would significantly overshadow neighbouring properties. In simple terms, the extension shouldn’t cross a 45-degree line drawn from the nearest window of the neighbour’s habitable rooms.

House Extension Cost UK 2026: Pricing Guide for Every Extension Type