Adding a small rental unit to a property—typically known as a granny flat or an accessory dwelling unit (ADU)—is a very basic way to increase housing options in a community, provide extra income for a homeowner and allow housing for people of diverse ages, especially seniors or young people. If your town makes it challenging or impossible to build something like this, you’re missing out on an important opportunity for housing at a time when many cities are struggling with housing affordability issues.
Furthermore, a municipal policy that prevents such a basic thing like the addition of a small rental unit is indicative of other restrictive policies that mandate empty space instead of productive, well-utilized land. One of Strong Towns' key principles is: "Land is the base resource from which community prosperity is built and sustained. It must not be squandered."