Mostly a combination of single-family zoning (illegal to build a duplex, let alone an apartment/condo building, in huge areas of many cities), minimum square fo

Mostly a combination of single-family zoning (illegal to build a duplex, let alone an apartment/condo building, in huge areas of many cities), minimum square footage requirements for new buildings and lots, and parking minimums (new development, often including renovations of existing buildings, required to have X number of parking spaces per Y residents/occupants).