Model on the basis of the floor plans (scale from the room dimensions). Heights and roof estimated from photos – approximate.
Walkthrough
PC: click in the image → W A S D / arrow keys to walk, mouse to look, Shift faster, Esc to release. Phone/tablet: drag left = walk, drag right = look.
Stairs: just walk on them. Thumbnails at the bottom right = real photos of the room you're in.