Keep in mind that if you use a Window Select, everything that is in line with the window is selected. For example, if you select geometries from the front of your building, you might, inadvertently, select geometries from the back of it.
It might be a good idea to double check before deleting that only what you want to delete is indeed selected. Use X-Ray Mode and some Orbit/Pan/Zoom to do that. After deleting, go out of X-Ray Mode and check the model to make sure that you didn’t erase useful geometries. At this stage you can still Undo to repair any damages.
As pontier out by others, using groups and components is a good way to minimize such problems. Personally I always use components. Then, if I inadvertently delete one I can get it back from the In Model Components List.