Here is a list of addons that contain spawnable building models.
You can find houses, facilities, sheds, buildings, etc.

For optimal performance, spawn these buildings as prop_dynamic entities, using Entity:SetSolid( SOLID_VPHYSICS )[wiki.garrysmod.com] before calling Entity:Spawn()[wiki.garrysmod.com].

Please complete the list by adding comments with addons links!