Extension Cost Estimator

This calculator gives an early-stage cost range for the four most common UK home extensions — single-storey, double-storey, loft conversion and basement dig — based on size, structure type and finish level. It's not a quote; it's the figure you use to decide whether to keep going with the idea.

It matters because extensions are the most variable category of home build cost. A single-storey rear extension can land anywhere between £2,000/m² and £3,500/m² depending on glazing, structure, ground conditions and finish. The decision to extend versus to move is usually made on a back-of-envelope number — and a wrong one early can cost six months and £10,000 in fees before anyone notices.

It's most useful at the "should we?" stage and at the start of architect conversations. It's less useful once you have detailed planning drawings — by then a quantity surveyor or a build-cost estimator with the actual specification can give you a number that's accurate to within 5%.

A worked example

A 20m² single-storey rear extension to standard finish typically lands around £44,000 base build. With premium glazing, bespoke joinery and an integrated kitchen, the finishes uplift adds 30–40%, bringing the total to roughly £60,000. A 25m² loft conversion runs cheaper per m² because much of the shell already exists — perhaps £45,000–£55,000 with an en-suite. A basement dig of the same size starts at £80,000+ because of excavation, waterproofing and the structural work required underneath an existing house.

Why this matters

Extending is rarely a financial decision in isolation — it's a comparison against the cost of moving (deposit reset, stamp duty, estate agent fees, removals) for the equivalent extra space. For most homes that comparison is closer than people expect: a £60,000 extension competes squarely with the £30,000–£50,000 cost of trading up. A realistic build figure is what makes that comparison honest.

Common mistakes

• Treating the build cost as the total — professional fees (10–15%), VAT on professional services, and planning/building control fees sit on top.
• Choosing the cheapest contractor without checking they're insured for the scope and structurally competent for the work.
• Underestimating ground conditions: clay soils, high water tables and proximity to drains can push foundation costs sharply higher.
• Forgetting party-wall agreements for terraced and semi-detached extensions — surveyor fees on both sides typically add £1,500–£3,500.
• Specifying the kitchen and finishes after planning approval is granted; design changes after that point are expensive.

Beyond the numbers

A well-budgeted extension splits roughly: 60% build shell, 20% finishes (kitchen, bathroom, joinery, flooring), 10–15% professional fees, and the remainder on planning, building control, party-wall, VAT and contingency. Project delays are the most reliable hidden cost — most extensions overrun the original timeline by 4–8 weeks, and a slipping date can mean extra rented accommodation, storage, or simply living without a working kitchen for longer. The two decisions that move the final number most are the structural design (single span vs steel-supported open-plan) and the glazing specification (standard double-glazed windows vs full-width sliding doors with structural openings). Both decide cost more than finishes do. Pair this figure with the Renovation Budget to plan the surrounding works.

Related tools: Renovation Budget · Stamp Duty · Mortgage Calculator · Insulation Savings