An in-law apartment is a small apartment installed in extra space in a private residence that allows an elderly relative to live with family but with some independence and privacy. Before building such a space, local building and zoning requirements must be checked. There may be restrictions against fully appointed kitchens due to fire hazard or requiring the tenant to be a relative. Adding an in-law apartment may increase your taxes, decrease the value of your home, or be illegal in some areas. Check with your town municipal offices and a realtor for more information for your particular area. A duplex is dwelling with two fully appointed apartments. One unit may be slightly larger if the dwelling was built for owner occupation. The units are not affected by the restrictions that may affect in-law apartments.