Plan Then Build exists because checking whether a home extension needs planning permission and working out what it will cost usually means switching between separate tools: a government planning portal, a builder's cost calculator, and a council website for local fees. This site puts a permitted development verdict and a cost estimate for the same project in one place.
Permitted development verdicts are computed from the General Permitted Development Order (GPDO) 2015, Schedule 2, Part 1 (Classes A-E for houses). Postcode lookups use postcodes.io and planning.data.gov.uk to check conservation area and Article 4 status. Council fees, decision times and named Article 4 areas are sourced from each council's own published data, cited on the relevant council page.
Cost estimates come from published 2026 UK build-cost guides and regional multipliers, refreshed quarterly - they're a ballpark range for budgeting, not a quote.
Coverage is England only for now, since permitted development rules differ by nation.
Every result on this site is guidance, not legal advice. The only way to get a legally binding answer is a Lawful Development Certificate from your local planning authority.
Questions, corrections, or spotted an error in a council's data? Get in touch.