How much does a single storey extension cost per m² in 2026?

In 2026, a single storey extension typically costs between £1,800 and £2,500 per m² depending on location, specification and ground conditions. London and the South East sit at the upper end, while northern England and Wales are generally lower. These figures include foundations, structure, roof, electrics, plumbing and finishing but exclude professional fees and VAT.

Why is a two storey extension cheaper per m² than single storey?

A two storey extension shares the same foundation and roof as a single storey, so those fixed costs are spread over double the floor area. The upper floor adds walls, a floor structure and staircase but avoids duplicating the most expensive elements. Typical two storey costs run £1,400–£2,000/m² compared to £1,800–£2,500/m² for single storey.

What is included in the material breakdown?

The breakdown covers foundations (concrete, excavation, reinforcement), structure (blockwork, brickwork, cavity insulation, DPC), roof (tiles or flat roof membrane, insulation, fascia), windows and doors, first and second fix electrics, plumbing and heating, and internal finishing (plastering, flooring, decoration). Professional fees, planning and Building Regulations fees are not included.

How much contingency should I allow for an extension?

A contingency of 10–15% is standard for domestic extension projects. This covers unforeseen issues such as poor ground conditions requiring deeper foundations, discovering asbestos, hidden drainage runs, or changes to the specification during the build. For older properties or complex sites, 15–20% is prudent.

Do extension costs vary by region in the UK?

Yes, significantly. London and the South East can be 20–30% more expensive than the national average due to higher labour rates and material delivery costs. Northern England, Wales and Scotland tend to be 5–15% below the national average. This calculator applies regional multipliers to give location-adjusted estimates.

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Extension Cost Calculator — UK Build Cost Estimator

Estimate UK extension build costs including foundations, structure, roof, electrics and finishing with regional pricing adjustments.

Extension type

Ground floor area

Finish level

Location

Number of windows

Number of external doors

How We Calculate This

This calculator estimates total extension build costs using base rates per m² adjusted for extension type, finish level and UK region.

Base cost per m²

Single storey: £2,150/m² (baseline)
Two storey: £1,700/m² (shared foundation/roof)
Wrap-around: £2,300/m² (complex junction details)
Side return: £2,400/m² (structural glazing, narrow access)

Breakdown percentages

Costs are split into foundations (12–15%), structure (25%), roof (10–15%), windows/doors (10%), electrics (8%), plumbing (7%) and finishing (the remainder). Regional multipliers adjust the total for local labour and material rates.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.