Zoning laws are defaults. By default, you have to provide a certain amount of parking, or are only allowed to put up one additional unit, without additional planning authority. That's an important phrase. If you start building a bunch of houses on your land, you may overload available transportation, utility, and public service capacity. That is why you must first go to a planning commission.
You know what happens 90% of the time when someone with a hundred thousand sqft lot wants to add another house? You fill out some paperwork requesting a zoning variance and get your answer in a few weeks.