Who actually measured it
Usually nobody. The statement that a building is under 11 metres generally comes from the estate agent, the seller, the managing agent or a housing association, and is passed on with that attribution. Your conveyancer will not have visited the building, and will say so.
That is worth knowing because the threshold is binary and the building may be close to it. Five storeys or 11 metres, whichever is reached first, is a test with a right answer — and a confident assurance from somebody with no measuring equipment is not the same as that answer.