The measurement that decides everything
28 August 2026
Not the floor area. The height from the new floor to the ridge, measured over where the stair will land.
Every loft conversion enquiry starts with a floor plan in somebody’s head. The room is already furnished in there. What decides whether that room exists is a single vertical measurement, and it is almost never the one people arrive with.
A new floor structure sits above the existing ceiling joists, not on them, which immediately costs you around 150 to 200 millimetres of the height you measured standing in the loft. What is left has to clear 2.2 metres at the point the staircase arrives, because that is where a person stands up.
If it does not clear it, the options are a dormer that raises the roof locally, lowering the ceilings in the rooms below, or accepting that the space is a study rather than a bedroom. All three are legitimate. Finding out which one applies after you have paid for drawings is not.
It takes ten minutes with a tape measure and it is the first thing we do.
Permitted development, in plain terms
Forty cubic metres on a terrace, fifty on a semi or detached, and a short list of things that void it entirely.
Why drawings and build should be the same firm
Because the gap between a drawing and a buildable thing is where every stalled job lives, and split contracts mean nobody owns it.