Using the self build cost chart

We’ve put together a breakdown of benchmark costs for popular types of extension project. The prices in the table are indicative of what you can expect to pay to engage a main contractor to complete the works for each of our five extension scenarios. It is possible to reduce costs by undertaking some of the project management or building works yourself – check out the box (above) for a quick guide.

We’ve assumed a standard quality and performance level for a bespoke home project (better than Building Regulations, but not high-end). The costs given here are based on cavity wall masonry construction, but the online version of our calculator allows you to select from a range of other common construction options, including timber frame, insulating concrete formwork and oak frame .

These figures include allowances for all the infrastructure required for each project type – including electric circuits, hot and cold water supplies and installing the central heating system. They also factor in a basic level of internal fit-out, such as second fix electrics (switches, sockets etc), plasterboard and final decoration.

A number of big-ticket fit-out items are omitted from the table, including floor finishes, sanitaryware, kitchen fit-out and landscaping. This is simply because they represent massive variables depending on your design tastes and specification choices. For instance, self builders could spend anything from £5,000 to £50,000+ on a new kitchen. Other possible cost uplifts include clearing the site, complex foundations and any requirement to move drainage or other services.

Design fees would be in addition to the build costs quoted below. These depend on the nature of your project and what level of service you opt for with your designer. Other fees that may apply include surveys, structural engineering and project insurance.

Visit www.self-build.co.uk/calculator for a fully interactive version of our self build cost calculator.