--- title: Extension Cost Calculator UK | Self Build Mentor description: Extension cost calculator built on real quantity surveyor rates. Single and two storey, over-garage, loft conversions, fees and your VAT position. --- SELF BUILD **MENTOR** EXTENSION COST CALCULATOR Running total — Excluding VAT and the abnormals Midlands & Wales rates, Q3 2026 Free tool · no sign-up to use it # What will an extension cost? Priced by a quantity surveyor in September 2026, with the size band chosen for you — because a small extension costs more per square metre than a large one and nobody volunteers for the expensive bracket. **Where these numbers come from** A quantity surveyor priced everything marked QS in September 2026. Anything marked SBM is our own judgement, used where he left the question open. Every line tells you which it is. 1 ### The existing house Even if you are only extending, the age and the protection of what is already there changes the price of the work that touches it. Existing internal floor area All floors added together, measured inside the external walls. A three-bed semi is usually 85–110 m². If you only know it in square feet, divide by 10.76. m² When was it built? Is it protected? 2 ### How much of the existing house are you doing? Pick the band that matches the heaviest work you intend across the house. If half of it is band 2 and half is band 3, pick band 3 — the trades still have to mobilise. 3 ### Are you adding new floor area? New floor area only. We pick the size band for you from the area you enter, because a small extension costs more per square metre than a large one and nobody volunteers for the expensive bracket. New floor area Total across all new floors. Leave at zero if you are not extending. m² What kind of extension? 4 ### Converting space you already have? A loft, a garage or a cellar. This is floor area you gain without the building getting any bigger — usually the cheapest square metres in the whole project. Type of conversion Usable floor area gained Only the part you can stand up in. In a loft, measure where the headroom is over 1.5 m — it is usually far less than the footprint of the house. m² 5 ### Anything in the garden? A garden room, an annexe or a garage. Priced on its own — the age and protection of the main house do not apply to a new building in the garden. Type Floor area m² 6 ### Where it is, and how hard it is to work there Region Our rates were captured at Midlands and Wales prices. Everything else is an adjustment on top. Working conditions — tick everything that applies These compound. A terrace with no rear access that you stay in throughout is all three, and a builder prices it that way. 7 ### Professional fees Our QS did not price fees, so these are our own figures. Percentages are of construction cost, which is the convention rather than an endorsement of it. Design and drawings Other consultants and statutory costs The first two are ticked because on a renovation they are close to certain. 8 ### Your VAT position Renovation VAT is not always 20%, and the difference is routinely worth more than the fee for the advice that finds it. Pick the one that describes your project. Nothing to price yet ### Add a floor area and your estimate appears here You can set every other option first if you like — none of them do anything until there is an area to apply them to. Estimated construction and project cost — Cautious — Most likely — If it goes against you — ### Where the money goes | Item | Cautious | Most likely | If it goes against you | | --- | --- | --- | --- | Save this breakdown as a CSV Print or save as PDF ### VAT ### What this figure does not include This is the honest part, and it is the part that sinks renovation budgets. Read it before you rely on the number above. Open it Take it with you ### Get this estimate as a spreadsheet Every figure above, with the rates it was built from, so you can change an assumption and see what happens. We will also send you the rest of our free tools. Skip this and just show me the print view → On its way ### Check your inbox The spreadsheet is in your email. If it has not arrived in a couple of minutes it will be in your junk folder — move it out, or you will miss everything else we send. Print or save as PDF [Talk it through with Allan](https://calendly.com/allan-selfbuildmentor/self-build-strategy-session) The honest next step ### A number is not a budget This tool gets you to the right order of magnitude, which is enough to decide whether a project is worth pursuing and enough to know when a builder's price is strange. It is not enough to buy a house on, or to borrow against. For that you need someone to look at the building. A Strategy Session is an hour on your project specifically — your house, your drawings if you have them, your numbers. You come away knowing what the real risks are and what to do about them in what order. [Book a Strategy Session](https://calendly.com/allan-selfbuildmentor/self-build-strategy-session) [More free tools](https://www.selfbuildmentor.co.uk) Self Build Mentor · rates basis Q3 2026 · review September 2027