Nice model for just starting out.
Ok so the dimensions are 19.267m x 10.5 m x 7.02m.
The big question is when you add the windows, are you going to just cut a hole out of the wall, or just push/pull them to the inside a little. This is if you want to be able to see the interior. If this is so, you might want to create some wall thickness. Which you will want to do to save some cash.
The cost for all non ceramic materials is price per cubic cm. That is material volume. So start hollowing before you get deeper into the project.
SU is good for insetting windows, doorways. Remember when you cut anything, and want to push/pull, select the cut/right click,under entity select bounding edges,then push pull. In some cases it will create a unfilled area if you do not.
The longer section running perpendicular to the taller section is not connected. This causes it to not print correctly. The floors you walk on 0n the second and third floor need to be removed, will cause problems with printing.
The idea is to create 1 manifold surface. Sorta like a balloon where the material of the balloon is the printing material.If you want, I can send you an example, and help since its for yer mum.
Keith
What format you want it in to import. I cannot figure how to export as a skp