Where I live (older neighborhood adjacent to a university campus), the trend is to finish off basements and rent them out as apartments. Or for a group of students to rent a home, trying to as many people into the home as the zoning laws will allow (or more). Or for an organization that does drug rehab to buy the homes and use them as temporary housing for several clients at a time. I believe this cost is paid by health insurance.
I don’t see backyard homes. They use too much energy to build and maintain. I see apartments and homes being used by non-related groups of people. In some cases, there may be separate apartments. In others, each person (or two people) may rent a bedroom, and the group may share and kitchen living room (if zoning laws will permit this). Zoning laws try to limit this, as much as they can.