House Extension Cost Calculator UK

House Extension Cost Guide UK

Extension Cost Benchmarks 2024

Typical build costs per square metre of new floor area in the UK (2024): budget single-storey: £1,500-2,000/m². Mid-range single-storey: £2,000-2,800/m². Premium single-storey (bifolds, underfloor heating, high-spec finish): £2,800-4,000/m². Double-storey extensions cost approximately 30-40% less per m² than single-storey (foundations, roof, and drainage are shared across more floor area). London and South East commands a 25-35% premium over Midlands/North prices. Loft conversions: £35,000-60,00

Professional Fees to Budget

Professional fees are often overlooked in extension budgets. Architect design and planning drawings: 5-10% of build cost (typically £3,000-8,000). Structural engineer calculations: £1,500-3,500. Party Wall Agreement (if extending within 3m of a neighbour's foundation): £800-2,500 (surveyor fee, both surveyors if neighbour appoints their own). Planning application: £258 for a householder application (if not covered by permitted development). Building Regulations application: £400-900 (full plans

Permitted Development vs Planning Permission

Many single-storey extensions qualify as Permitted Development (PD) and do not require a planning application. PD limits for houses (England): rear extension up to 4m from rear wall (detached) or 3m (attached). Maximum height 4m. No more than half the garden area. Side extensions in PD are more restricted. PD does not apply in Conservation Areas, Areas of Outstanding Natural Beauty, or if previous PD rights have been removed by condition. Always check with your local planning authority before as

Value Added vs Cost

Extensions typically add more value than they cost in many areas. A well-designed kitchen extension adds approximately 5-10% to house value. An extra bedroom adds 8-12% in most markets. However: the return depends heavily on whether the extended house remains proportionate to the street — extending a 2-bed terrace to 4 beds in a 2-bed terrace street creates an over-improved property. The best return comes from extensions that address clear functional gaps: adding a third bedroom to a two-bed hou