There is no such thing as "change of use" for domestic planning regs. And you won't need PP for this in any case
However, if you put in a fixed staircase, then this will become controlled works under b/regs and you will require this permission. It will be considered a habitable room and will need to comply particularly in terms of insulation and structure of the loft area
However, if as you say, this room was formed as part of the initial build, and it is a room - ie plastered walls and ceiling, then you would argue that the room is "as built" and you would only need b/regs permission to install the stairs