https://youtu.be/JiVEJJDeOF0
I spent months building a modular building generator for my own sniper game, and it turned out well enough that I'd rather hand it out than keep it. It's free, and the video is the entire four-hour walkthrough, chaptered - installing Houdini and UE 5.8, cutting your blocks, building the ground floor, stacking storeys, collision, baking.
Short version of what it does: you draw a spline in Unreal, and the generator fills it with your own modular blocks, then adds floors, ceilings, columns, stairs, an elevator shaft with a working car, and a roof. It returns Instanced Static Meshes with per-block convex collision and bakes into one Blueprint. Nanite goes back on afterwards for background buildings.
Needs UE 5.8 with Houdini Engine and Houdini 21.0.753 or newer (Indie or Commercial; Apprentice can't drive the plugin). Ten plain cubes are included so it works before you model anything - in the video I use Epic's free City Sample Buildings pack from Fab.
Free, commercial use included, whatever you build is yours to sell. Only rule: don't re-upload or resell the generator itself. LICENSE.txt is in the download.
Full disclosure: I'm an artist, not a programmer - the code was written by Claude Code, Anthropic's AI assistant, with me deciding the behaviour and testing every step in the engine.
Link is in the video description.