How the rules, costs and demand figures on this site are produced
The three kinds of fact on this site come from three different places. This page explains each one and its limits.
Planning and building rules
Statements about permitted development, planning permission and Building Regulations are taken from the current legislation and the official guidance that explains it: the General Permitted Development Order as amended, the Planning Portal’s interactive guidance, the approved documents, and GOV.UK. We quote the limits as they stand on the date shown on the page. Where a limit depends on your circumstances — a terraced house, a conservation area, a flat, a house that has already been extended — the page sets out the condition rather than giving one number that is only true for some readers.
We do not interpret borderline cases. If your project sits near a limit, the page will tell you that a lawful development certificate or a pre-application enquiry is the way to find out, because the only answer that protects you is one in writing from the authority.
Costs
Cost ranges are taken from the regional cost model maintained on BestBuilders.co.uk, a site run by the same company, which combines national cost guides with a regional labour index. We apply the model’s ranges for the extension type and region and we show them as ranges. They assume a straightforward site with normal access and a standard finish; listed buildings, poor ground, difficult access and high specifications all sit above them. A range on this site is a sanity check for the quotes you receive, not a quote.
Demand data
Where a page reports what homeowners are asking — the share still at the research stage, the share without drawings, the most common extension type — the figures come from enquiries submitted through our own quote tool over the period stated on the page. They describe the people who used this site, not the country, and we present them as that. Individual enquiries are never published.
Review cycle
Regulatory pages are re-checked whenever the underlying guidance changes and at least once a year; cost pages when the cost model is refreshed; demand pages when a new period’s data is compiled. Every page carries its last-updated and next-review dates.