Our extension cost calculator. Free for architects.

Log in to the platform here and enter the project you’d like to receive a cost estimate for. Once you have done that, a task will appear allowing you to request your free cost calculation.

Easy and simple to use, our extension cost calculator will deliver your project estimate within 24 hours – straight to your inbox.

Our extension cost calculator has been designed by a RICS Chartered Quantity Surveyor to give you or your client an estimate for the cost of a house renovation. We use the RICS NRM 1 construction standard in our methodology, which specifies a set of measurement rules that can be understood by anyone involved in a construction project.

We use published schedules of rates for building projects from RICS’s Building Cost Information Service BCIS and SPONS Price Book. Using their rates, we are able to estimate the cost of an extension based on recent real world data from similar projects in comparable locations.

The cost estimates excludes fit-out items (kitchens, external works, bespoke glazing, bespoke joinery and bathrooms) as these depend on the quality of finish. They can take up 5-25% of your overall costs depending on choice.

Ultimately, the only way to get an accurate build cost for exact work you want done is to go to tender with detailed information for builders to price on. Once you start obtaining at least 3 real quotes from proper contractors, such as on Weaver’s platform, you will be able to get a precise market cost specific to your designs.