Every range on this site is calculated from one validated data file, built on published national 2026 cost guides. Here is exactly where the numbers come from, how they are adjusted, and what they can and cannot tell you.
The UK ranges are validated against public national cost guides, most recently in August 2026:
Where a guide quotes prices excluding VAT, we convert to including 20% VAT and say so in the data. The US planning ranges are validated separately against US cost guides, with metro factors on top; the planning rules follow Planning Portal (planningportal.co.uk) and the HomeOwners Alliance permitted development guide, checked 3 August 2026.
An estimate here is a planning range, not a quote. It cannot see your property, your access, your ground or your taste. Use it to set a realistic budget, to sanity-check quotes and to compare options; then let itemised quotes from local trades decide. Nothing you enter leaves your browser, and no one pays to influence the numbers.
Start with the cost calculator, or put a quote you already have through the quote check.
Because a single number would be a guess dressed up as certainty. Real quotes vary per trade, property and specification; an honest band with a clear mid-range tells you more than false precision. Budget 10 to 15 percent contingency on top and always get at least three itemised quotes.
Every figure carries its verification date, and the whole set was last checked in August 2026 against the public guides listed above. When a source moves, we update the data file and every page follows automatically.
No. There are no leads sold, no affiliate links to trade platforms and no email capture. The estimates are free and the data stays independent; that independence is the whole point.