To comply with strict UK Building Regulations, your existing roof space needs a minimum height of 2.2 metres from the top of the ceiling joists to the underside of the ridge timber. This ensures that once the new independent structural floor steels, floorboards, and ceiling plasterboards are fitted, you are left with a comfortable, legally compliant living headroom of at least 2.0 metres over the main floor area and the new staircase.