The 25% glass check

Lots of glass? Check it against the 25% rule.

For an extension in England, windows, doors and rooflights together are normally limited to 25% of the new floor area, plus the openings you take out of the old wall. Over that, Building Control will usually ask for an energy calculation before sign off. Check yours in a minute.

A quick guide based on Approved Document L 2021, Volume 1 (extensions to existing homes in England). It uses the opening sizes you enter. Your building control body has the final word, and a designer can often show compliance another way, for example with better glass and insulation elsewhere.