Conversely, people who used to be able to live in a flat are now unable to live in the area at all because we've outlawed building flats. Similarly, regarding:

Conversely, people who used to be able to live in a flat are now unable to live in the area at all because we've outlawed building flats. Similarly, regarding: "unviable for anyone to ever afford a 4 bed house" - it's not like areas that ban conversions to flats have cheap 4 bed houses. They have expensive 4 bed houses, often filled with 4-8 roommates who likely would prefer their own flat.