This is a paid asset, but now you can download Buildings Generator for FREE, but please remember this package is provided only for learning purposes or to be able to test before buying the product, NOT FOR COMMERCIAL PURPOSES.
Detail this asset from Unity Store: Original Link
Buildings Generator v1.0
A useful and easy to use window allowing you to create procedurals buildings in one click !
It allows you to generate a lot of buildings from samples models in few seconds. Stop wasting your time modeling each little building of your city !
You can set a specific value or a range for a random value for the size, the number of floors, the number of doors, the number of elements on top of the roof, the number of windows, … and just click a button to procedurally generate a building ! If you like the generated building, you can just click another button to save it as a prefab in the assets !
Source code included.