Your question is very vague.

Are you looking at surveying existing buildings? E.g. using methods or technologies such as photogrammetry, total stations, laserscanning, 3D cameras?

Do you want to have an exact "as-is"? Or are you looking at 3D modelling tools to create the 3D geometry?

Do you require complex, freeform geometry? Or is a more rigid, mostly orthogonal geometry fine?

Is it meant for 3D visualisation? Tools such as SketchUp, 3ds Max, Cinema4D, Rhinoceros, AutoCAD..., but also realtime systems (Game Engines) or dedicated  rendering systems...

Or rather meant for construction documentation? E.g. using ArchiCAD, Revit, AutoCAD, MicroStation, VectorWorks?

Or still something completely different?

Google Sketchup is the best free 3D geometry creation. It can be integreated with EnergyPlus for Building energy simulation. Hope this help