As far as having a space to rent out as a rental or Airbnb, I always suggest making sure it has its own separate entrance and, if possible, backyard area that is separate or can be separated from the main backyard. If there isn’t already an ADU, and if there is interest in having one for the future, get the plans done and permitted now while it’s still legal; then you will be grandfathered in and can build it at your leisure.