From the excellent SideFX webinar series. The asset is still very much a WIP, but i'm right at the bleeding edge of my Houdini skills - some of the logic of clamping / linking the values was making my brain hurt!
Also, the builder can accept any spline shape to create the building. I had this set up to demo in the video but forgot to show it....duh.
Thanks to SideFX and Ari Danesh for providing such amazing tools and content for free.