Help me calculate floor space in my loft

[DB_SamX]

^Yup, Building Regs approval is probably going to be required for such a change, not to mention structural survey.

Not quite as small as when people put in a sofa and coffee table and projector.

 

[Fido McNasty]

Just for the record, the other items that will encroach upon that usable floor area may include any loft water tanks and pipework, access (stair well, and landing area if you're planning multiple rooms), chimney stack. Bear in mind also that additional and probably nescessary insulation will bring the rafters height inwards by a few more cm.

Muck about with www.floorplanner.com to begin to get a feel for things, although it cannot give you section detailing yet.