Ever since 1926, zoning has been kosher in America according to the Supreme Court (Village of Euclid v Ambler Realty). Zoning refers to a set of local rules that specify how land can be used. It typically includes rules about "use" (e.g. residential, commercial, industrial, including rules about building dimensions besides height), building height, and may include special areas or "overlays" such as historic preservation areas, where there are extra restrictions and extra review for changing things.
To see a planing and zoning code click here for the City of Los Angeles' Code (or here for a more understandable summary). It's big and complex. Within a use category like Residential, there are subcategories of use and density, like R-1, which only allows things like detached houses and parks, and many others, including zones that permit the construction of row houses and apartment buildings. Similarly for commercial and industrial uses, there are subcategories that are more or less restrictive. What most of these zones have in common is that they are premised on the idea that land uses should be separate. Building a mixed-use project, like apartments over shops, which would allow people to walk to things easily, requires jumping through a lot of hoops, except in a few zones and overlays, which don't constitute much of the City's land.
Some people of urbanist inclination advocate for a so-called "form based code", where the regulation of land uses is entirely given up, and only the dimensions of buildings are regulated. This would allow for maximum mixture of land uses.
I think this is too extreme. The solution to the excessive micromanagement and use segregation caused by traditional zoning codes shouldn't be to abandon all direct control over use. It should be to introduce a simplified code, which separates out the most difficult land uses, but allows for mixtures of all the others, with special review (conditional use permits) for certain uses that may be problematic, and deserve extra scrutiny, but not automatic separation.
Hence next time I will outline a code based on the following five zones. It will be simplistic, but I hope that it will convey the germ of an idea of how we could cut through the current rules, which are holding back vibrant cities and more sustainable suburbs. It won't be politically easy to do something like this, but hey, can't a blogger dream?
MU-1 (mixed-use low density)
MU-2 (mixed-use medium density)
MU-3 (mixed-use high density)
HI (high-impact industrial)
PAC (park, agriculture, and conservation)
+ No parking minimums and conditional use permits for parking above a certain level.
That's right. All land would be in one of these five zones. Stay tuned :)