Ok I’ve figured out a way to make rooms inside buildings, its a little bit convoluted but it means that you can have doors and everything!!!
First design the main room of the building and include doors to where you want the rooms to be the roof tool can be used but I think making the roof flat out of slabs far easier. - Build this!
Second once the main building is completed use the design tool to set the floor of a room and use the individual wall tool to grow the walls around the edge of the floor. Then add a flat roof on top! Then build this!
This can be repeated through as many rooms as you want so long as they operate more as extensions to the original building.
I only started playing recently and I did not have problems with multi-room building, though I tend to not use it often because of occlusion. (imagine if you have a room totally surrounded by other rooms, and have another floor above, which cannot be made invisible like roof. it will be very hard to access it. for players that is, not hearthlings). thus, no complex rooms or many-storey buildings for me until the visibility system can be improved… (e.g. like that of the Sims, where the position of the cursor determines the primary room to prioritize visibility, and any other rooms in front of it will have all walls collapsed)
its 2016 now you know