How our extension estimates work
We use published UK building cost guidance to create a broad planning range. The figures are not local builder quotes, quantity-surveyor measurements, or a prediction of your final contract price.
1. Building-cost starting points
The Federation of Master Builders gives broad per-square-metre ranges of about £1,800–£3,000 for single-storey extensions and about £1,000–£2,500 for two-storey work. For this first version, our model uses £1,800/£2,400/£3,000 per m² for low/expected/high single-storey cases and £1,250/£1,900/£2,500 for two storeys. These are initial budgeting assumptions, not prices obtained from individual builders.
2. Project-specific adjustments
We use the floor area, number of storeys, design type, finish, UK region, access and known ground challenges. We make a modest fixed-cost adjustment to smaller jobs. Our regional multipliers and complexity adjustments are indicative judgements, not a verified postcode-by-postcode pricing dataset.
3. What's included
The standard construction allowance models typical site preparation, foundations, structure, roofing, insulation, standard windows and external door, electrics and basic internal finishes. A separate allowance is added only when you select upgraded bifold doors, rooflights, additional structural openings, a new kitchen/bathroom fit-out or drainage diversion. The work categories shown in the report are illustrative allocations of the core amount; they are not measured bills of quantities.
4. Fees, VAT and contingency
We allow 15% of construction expenditure for project design, professional services and approval-related work, subject to a £3,500 minimum. This is a broad allowance, not an exact fee schedule. We assume 20% VAT on construction by a VAT-registered contractor; the professional-fee allowance is treated as inclusive for modelling purposes. We add 10% contingency on construction plus professional fees. Exact VAT treatment, fees and risks should be checked for your project.
5. What could still change
Ground conditions, drainage or sewers, asbestos, service upgrades, structural changes, site access, design, planning constraints, builder workload, London or other local price premiums, inflation and your final specification can materially change prices. Planning and Building Regulations requirements cannot be decided by this calculator.
Sources
- Federation of Master Builders — house extensions
- Federation of Master Builders — single-storey cost guide
- Federation of Master Builders — two-storey cost guide
- HomeOwners Alliance — extension costs, including an explicitly plus-VAT example
- GOV.UK — Building Regulations approvals and fees
- Planning Portal — extension permissions
Model reviewed 29 October 2026. Please compare itemised contractor quotations before committing money.