House Extension Cost UK: Prices per m² & Budget Checklist

The Federation of Master Builders’ published guide cites £1,800–£3,000 per square metre for a single-storey extension. It explains that location and specification affect the price. This is a cited guide figure, not a rate measured or guaranteed by JASOTO.

The headline rate does not specify its VAT basis or provide a complete schedule of inclusions. Ask a designer or builder what an early allowance covers before multiplying it by the proposed area. Do not apply this single-storey reference automatically to a two-storey extension, basement or a different scope of work.

Start with the space and uses you need, then ask for a design and written specification that a builder can price. Confirm whether the stated area is measured internally or externally and use the same measurement throughout your comparisons.

Ask the project team to identify the proposed foundations, structural alterations, roof, windows, doors and service changes. Request an explanation of site assumptions and anything that still needs investigation. A lower area rate is not enough to compare a basic structure with a room that includes a fitted kitchen or bathroom.

Create one project list and mark each item as included, separately priced, provisional or excluded. Ask the relevant professional or contractor to confirm who supplies each item and whether labour, materials and applicable VAT are included.

Keep costs outside the builder’s package visible. For example, a kitchen supplier’s quote might include cabinetry but leave installation or connections to another contractor. Resolve the responsibility and price for each task before treating the budget as complete.

RICS recommends at least three detailed quotations and a fully itemised, costed schedule of work. Its consumer guide also recommends a written contract, a completion date and an agreed method for handling changes.

Send each contractor the same information and ask them to explain deviations. When a quotation includes an allowance, request the amount, what it covers and how the final charge will be agreed. Compare the payable totals only after you have identified work that one offer excludes.

Ask for the final payable amount including applicable VAT on each quotation. Do not add VAT again to an amount already described as inclusive. If a figure excludes tax, request the inclusive equivalent from the person providing it.

Keep a separate record of provisional amounts and the assumptions behind them. Agree how the price and timetable will change if inspection or design work alters the scope. Set any contingency reserve with your project adviser based on the unresolved risks; a reserve is not an approved instruction to spend it.

Before additional work starts, ask for the reason, revised cost and effect on the programme in writing. Update the budget when you approve a change so that an early figure is not mistaken for the current project total.

Planning Portal’s England guidance explains that permitted development can allow some house extensions without a planning application, subject to limits and conditions. Previous extensions, the property type and restrictions affecting the site matter; a neighbour’s extension does not establish permission for yours.

Confirm the planning route with the local planning authority or your adviser before committing to construction. This linked guidance concerns England. For a project elsewhere in the UK, use the relevant national and local guidance. Agree who handles checks, applications and fees.

GOV.UK states that building regulations approval is different from planning permission and that you might need both. Its guidance covers the construction and extension of buildings and advises checking with a building control body if you are unsure.

Ask the designer or contractor to identify the building control arrangements, required inspections and completion documentation for your project. Record these responsibilities in the scope and budget. Do not assume that a planning decision settles the building regulations position.

Give contractors a brief that identifies your intended room, dimensions, drawings and finish level. Ask for exclusions and provisional amounts alongside the price, plus start and completion arrangements. Keep the agreed scope with any revised quotation.

The calculator below adds the amounts you enter; it does not generate a market estimate. Enter one booking for a complete extension package and add only separately quoted extras not already included. Use tax-inclusive figures consistently. Keep unresolved allowances and a contingency reserve clearly identified outside the confirmed total.