How Much Does It Cost to Build?

A house extension is one of the most popular ways to add space and value to a property without moving. Costs vary enormously depending on the type, size and specification — a small single storey rear extension is very different financially from a large double storey wrap-around.

The figures below are construction costs per m² for the Midlands. Add 15–30% for London and the South East, 5% for the South West, and subtract 8–15% for the North, Wales and Northern Ireland.

A loft conversion is typically the most cost-effective way to add a bedroom — you're using existing roof space without extending the building footprint. The type of conversion matters enormously: a simple Velux conversion barely alters the roof structure, while a mansard conversion essentially rebuilds it.

Most conversions require building regulations approval but fall within permitted development rights, meaning no planning permission unless you're in a conservation area or flat conversion.

Building a new house from scratch gives you full control over the layout, specification and energy performance. Construction costs (excluding land) typically run from £1,500/m² for a straightforward build to over £3,500/m² for premium or complex projects.

VAT is usually zero-rated on new residential construction, which is a significant saving. Don't forget to budget for land, utility connections, architect fees, planning and building control — these can add 20–30% on top of the build cost.

Basement projects fall into three categories: converting an existing cellar into habitable space (cheapest), lowering an existing cellar floor with underpinning, or excavating a brand new basement under an existing property (most expensive and complex).

Basements are high-risk projects — ground conditions, the water table, proximity to neighbouring foundations and access for excavation machinery all drive significant cost variation. Allow a 15–20% contingency.

Garage conversions offer outstanding value for money — the structure already exists, so you're paying for the internal fit-out rather than new construction. A single garage (around 15–18 m²) converted to a bedroom, home office or playroom is one of the most affordable ways to add a usable room.

Most garage conversions don't need planning permission (they fall under permitted development) but do require building regulations sign-off for insulation, ventilation, fire safety and structural checks.

Garden rooms range from insulated timber-frame garden offices to fully self-contained annexes with their own kitchen and bathroom. A simple garden office or gym is typically delivered as a modular structure and installed within days; a brick-built annexe is a proper construction project with foundations, utilities and planning permission.

A self-contained annexe for a dependent relative usually needs planning permission (it's a separate dwelling). A garden room/office used as an extension of the home normally falls under permitted development if it doesn't exceed 50% of the garden area.

A conservatory uses a largely glazed structure (roof and walls), while an orangery has solid brick or masonry walls with a glazed roof section or lantern — making it feel more like a proper room extension. Both are typically installed under permitted development without planning permission for most houses.

Modern thermally efficient aluminium or timber conservatories and orangeries are significantly more liveable year-round than older uPVC polycarbonate-roofed structures. Budget accordingly — the cheapest options tend to be unusable in summer or winter.

Barn conversions create characterful rural homes but are notoriously difficult to cost accurately — the existing structure's condition can vary from sound to near-derelict. A barn in good structural condition with solid walls and a sound roof will be much cheaper to convert than one requiring significant structural repair.

Remote rural locations add cost: longer supply chains, contractor travel time, difficulty getting machinery to site, and often the need for new septic tanks, boreholes and off-grid utilities. Planning consent under Class Q permitted development rights has specific conditions — always check with the local authority.

Residential apartment blocks are developer projects with complex M&E (mechanical and electrical) requirements, fire compliance, accessibility, and often a concrete or steel frame. Costs per m² are higher than houses of equivalent spec due to corridors, lifts, communal areas, and building safety compliance (especially post-Grenfell).

Height thresholds drive cost. In England, new blocks of flats over 11 m need sprinklers, and buildings 18 m and over face a ban on combustible external wall materials and (for new schemes) a second staircase requirement. Lift requirements depend on accessibility standards and local plan policy. Wales and Scotland have their own rules — get fire and accessibility strategy advice early.

Commercial fit-out costs vary enormously by use type. A warehouse needs very little internal fit-out, while a restaurant requires an industrial kitchen, extract ventilation, wet areas and often bespoke joinery. The shell and core condition of the existing building is also critical — a new-build shell needs full M&E, while a recently vacated Cat A office may already have services in place.

Cat A is the landlord's base fit-out (ceilings, raised floors, basic M&E); Cat B is the tenant's full fit-out (partitions, kitchen, meeting rooms, branding). Most commercial projects are Cat B.