Estimate the build cost of a single or double-storey house extension in the UK by floor area and finish quality.
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How This Works
Extension build costs are typically quoted per m² of floor area. Single-storey: £1,500–2,500/m². Double-storey: £1,400–2,200/m² (more cost-efficient as foundations and roof are shared). Garage conversion: £800–1,200/m² (no new foundations or roof). Professional fees (architect, structural engineer, building control) add ~15%. Budget ±20% from any estimate.
Many extensions fall under Permitted Development (PD) rights, meaning no planning permission is needed. PD limits for rear single-storey extensions: 4m for detached houses, 3m for other houses (can be extended to 6m/8m via Prior Approval). PD does not apply in conservation areas, AONB, listed buildings, or if your PD rights were removed by planning condition. Always check with your local planning authority and submit a Lawful Development Certificate application to get written confirmation.