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 have images and ill try to upload them later…