About ExtensionCosts.co.uk
ExtensionCosts.co.uk helps UK homeowners understand what an extension, loft conversion or garage conversion really costs – before they speak to a builder.
Why we built this site
Most cost figures you find online quote only the building work, leaving out design fees, structural calculations, building control, planning, party wall surveyors and VAT. Those extras commonly add 25–40% to the final bill. We built ExtensionCosts.co.uk to give homeowners an honest, all-in figure so they can plan a realistic budget and spot quotes that look too high or too low.
What we do
- Free cost calculator – instant estimates for nine types of extension and conversion, adjusted for size, finish and region.
- In-depth cost guides – researched from published 2026 UK cost data, with sources listed on every page.
- Planning guides – plain-English explanations of permitted development, prior approval and building regulations.
- Free quotes – we can put you in touch with local builders and loft conversion specialists.
How we make money
Our calculator and guides are free to use. When you ask for quotes, we pass your project details to a carefully selected local builder, who pays us a fee for the introduction. This never costs you anything, and you are under no obligation to hire anyone. We may also show adverts on some pages.
Our editorial standards
Every cost figure on this site is based on published data, which we cite at the bottom of each guide. We explain our methodology in full on our how we calculate page and review figures regularly. Our content is general information, not professional advice – always get detailed quotes and check planning requirements with your local council.
Questions or corrections? Get in touch.