In-law apartments take that two generation large house and make it work for other stages of the life cycle, including old age. They can reduce the isolation of elderly households from the simple services that family or close neighbours can provide more effectively than institutions. Almost all of us are or will be in many of those groups during our lives. Having an in-law apartment may be a good idea, but be prepared to adjust to privacy needs and the different habits of others. Making an In-Law Apartment Happen Zoning may be a question in your community.
Most places require that an in-law apartment be for relatives only.
Contact the local municipal offices and ask for the planning or building department. This by itself may be a barrier to building and in-law apartment, however it is only part of the process. Professional help may be needed. Contact a realtor for assistance in determining the impact this will have on the market value of the home. It may be the wrong move for you home.