I have a building in a 3D perspective view, but we recently allowed the user to add indoor walls. Now, the user wants to see the layout of the building (assume all floors have the same indoor layout and size) on the roof as an image (or texture in this case). This prompts to ask me if there is some tried and tested method for implementing the same.
You draw a polygon or polygons with the floor plan texture on it. This is a very very basic question.
It the roof is pitched then perhaps you should remove it for this viewing mode as it would not be very useful unless the roof were flat.
If you do not have that floor plan texture then you can use render to texture to create this but if you’re at the level of having to ask this question then this might be a steep learning curve for you. There are tutorials available on how to perform RTT.