- You can only build single-family units (no apartments, no duplexes, etc.) - Each lot must be at least a half acre - You must have at least one (or two) parking spots per lot. - You must have multiple stairwells and an elevator for any multi-floor shared units.

And on. And on. And on. Zoning and building restrictions are an absolute gauntlet in many places. For an extreme example, check this out: https://reason.com/video/2018/12/27/san-francisco-mission-ho... (He did eventually succeed, but the cost and effort was tremendous.)