First I removed all the inner parts to leave only the casing. This one was a suitable model as the cover is in two parts and leaving out the other half of the cover leaves the inside exposed.
I spent an evening disassembling the power supply and trying to fit in the power supply cover and hard disk mount as rooms inside the casing. Finally I got them in an order I liked. The power supply cover on the left in the middle floor is attached with mirror mounting tape. The right half of the floor is a metal plate from the casing, the rest is pieces of thick cardboard. The two upper floor rooms are the CD/DVD drive mount and the power supply mount. The red thing on the left is a Coca-Cola vending machine fridge magnet and it's there to give an idea of the scale.
The ground floor will have a kitchen, bathroom and hall. I'm not quite sure of the other floors yet, but I'm thinking of some kind of a studio or research lab or something like that. The idea is that this is a converted factory building and lot of the grey metal will remain visible. Also, much of the furniture will be made of metal. I'm even planning to use the parts I removed from the power supply to build some sort of "machine" that was left over when the factory was closed down.
The two rooms in the upper floor will be covered by that half of the casing cover that will be attached in its place when the house is finished. That is why I'm using the large holes at both ends to make big picture windows for them. That way you can still see inside with the cover half attached.
Next: Bathroom