How do you find a building's footprint when the signs don't include that information? Measure it! This could be tricky in a No Build area so here is what I did:
I attached a 20x20x0.50 Megaprim to my stomach and walked inside the building. This was good enough but I could have added a grid texture to be fancy. Be careful doing this, though. Megaprims attached to avies can be considered griefing (Web; Web).
I rather liked this small castle from Lapin Feu Prefab Castles & Fish (SLurl). One of this company's gazebos is a centerpiece at my home. Texturing is awesome and there are some clever tricks in the builds.