The building regulations do not set a single universal ceiling height for every room in a dwelling; height requirements arise mainly through headroom over stairs under Part K, ventilation and habitable room standards, and local authority development plan standards for apartments and certain housing. The applicable figure depends on the room and the guidance in question.
People often expect a single stated minimum ceiling height, but the requirements are distributed across different documents. Technical Guidance Document K sets minimum headroom over stairways and landings. Planning guidance and development plan standards set floor-to-ceiling heights for apartments and for some housing schemes. Habitable room quality is also influenced by ventilation and daylight requirements.
In attic conversions the practical question is usually how much of the floor area has usable height and whether the stair headroom can be achieved. Both should be checked at design stage, because they often determine whether a conversion is feasible at all.