The RICS Code of Measuring Practice defines Gross Internal Area, Net Internal Area, Gross External Area and a range of related concepts. The point is consistency — everyone using the same definitions means valuations, lease terms and cost benchmarks can be compared apples-to-apples.

For domestic clients the most common practical question is conservatories and outbuildings. The Code distinguishes properly-integrated additions from detached and semi-detached structures, and what you include depends on what you’re measuring for. Pricing a build? Include them with appropriate rates. Valuing for sale? Different convention.

Asking your QS or valuer which Code definition they’re using is a small question that prevents large misunderstandings.