New Build Cost Calculator UK 2025

The Real Cost of Building in the UK: A Contractor's View

What the m² Rate Actually Means

A headline cost per square metre is useful as a first-pass sanity check, but it tells you almost nothing about the specific cost of a specific building. Two houses with identical floor areas can have build costs that vary by 40% or more depending on their form. A simple rectangular two-storey house with a standard pitched roof and off-the-shelf windows is fundamentally different to construct from an L-shaped single-storey with a flat sedum roof, heavy glazing, and a complex steel structure — even if the gross internal floor area on the drawings is the same. The amount of external envelope (walls, roof, windows) per m² of floor area is one of the biggest drivers of build cost, and that's determined by the shape of the building, not just its size.

The other two major cost levers are specification and procurement. Kitchen and bathroom specification alone can swing the finished cost of a typical house by £30,000–60,000. Timber windows vs PVC, underfloor heating vs radiators, oak internal doors vs MDF — every specification decision adds up. And on procurement: a well-managed competitive tender process with three comparable, experienced contractors and a proper specification document almost always produces a lower price than instructing a single contractor on a handshake and a cost-plus arrangement. The discipline of a proper tender forces clarity on both sides about what is and isn't included.

The Costs That Catch People Out

Build cost is one component of total development cost, and rarely the only surprise. Abnormal ground conditions — made ground, soft clay, high water table, contamination, tree roots — can add tens of thousands to foundation costs before a single metre of wall goes up. A site investigation (boreholes, trial pits, soil analysis) costs £2,000–5,000 and is one of the best investments you can make before committing to a purchase price or a design. Discovering that you need piled foundations rather than standard strip foundations after the design is fixed is an expensive and demoralising problem.

Planning conditions are another source of unexpected cost. Section 106 agreements, Community Infrastructure Levy (CIL), ecological surveys and mitigation, noise impact assessments, highway works, affordable housing contributions — these can add materially to the cost of a scheme and they vary enormously by site and by local authority. Utility connections are consistently underestimated: a new mains water connection, a new electrical supply from a substation, or a gas connection from a remote main can cost £5,000–30,000 depending on the distance and the capacity required. Get indicative connection costs from the utility companies early in the feasibility process.

Controlling Costs and Avoiding Overruns

The most effective cost control tool is a thorough, detailed specification and a properly drawn set of working drawings — prepared before you go to tender, not after. Variations (changes to the scope of work after the contract is agreed) are the primary cause of cost overruns on almost every building project. Every change on site costs 3–5 times more than it would have cost to include it in the original design and specification. Time invested in resolving design and specification decisions before the contract is signed is always returned many times over in reduced variation costs during construction.

On contract form: for most projects, a fixed-price lump-sum contract (JCT Minor Works, JCT Intermediate, or similar) based on a proper specification is the right approach. Cost-plus contracts — where you pay the actual cost of labour and materials plus a management fee — sound transparent but remove the contractor's financial incentive to work efficiently. Fluctuations in material prices, poor subcontractor management, and programme overruns all become your problem under a cost-plus arrangement. Reserve a minimum 10% contingency on the build cost, held separately and released only for genuine unforeseen works — not for specification upgrades. On conversions and refurbishments of older buildings, 15–20% is more realistic.

How Much Does an Extension Cost UK 2025/2026?

Extension costs are one of the most-searched building queries in the UK, and the range is wide because the variables are enormous. As a guide for 2025/2026: a single-storey rear extension in standard specification — brick and block, pitched or flat roof, plastered and decorated, no kitchen or bathroom fit-out — costs approximately £1,800–2,800 per m² of new floor area. A 4m × 3m rear extension (12 m²) therefore typically costs £22,000–34,000 for the build alone, before professional fees, planning, kitchen installation, or decoration. A 4m × 5m kitchen extension (20 m²) in standard specification: £36,000–60,000 build cost. Add £15,000–40,000 for a new fitted kitchen, and £5,000–15,000 for professional fees and planning. Total project cost for a quality kitchen extension: £55,000–115,000 depending on kitchen spec and region.

Double-storey extensions are typically cheaper per m² (£1,600–2,400/m²) because foundations and roof serve twice the floor area — they represent the best value for money where planning allows. A loft conversion with dormer typically costs £40,000–70,000 for a standard 3-bed semi including structural works, dormer or Velux windows, insulation, staircase, and first-fix services. A garage conversion to a habitable room costs £15,000–35,000 — the most cost-effective way to add floor area to a house. "How much does a 4m extension cost UK 2026", "single storey extension cost per m² UK", and "kitchen extension cost UK" attract some of the highest-value advertising in the home improvement sector.