We're looking into providing our son with a separate part of our house
to use as his home (he currently rents a flat locally). It all looks
fairly easily do-able but one issue is the need for a new staircase.

I'm pretty sure there's enough space but looking at staircases from
Screwfix and such it's very difficult to workout how much space the
actually need.

hole in the celing, square of plywood, some rope and a few pulleys...

a human powered lift, it'll keep him fit using it, and take up less room
than a stair case