Allows ADUs to provide additional housing choice for its residents. This code focuses on moderate income housing and requires the owner of the primary dwelling and ADU reside in one of the units, but does not specify the requisite time period. The ADU may not exceed 900 square feet or 35% of the primary dwelling unit size. Biddeford sets moderately strict requirements for stairways, balconies, parking, entrances, exterior materials, lot orientation, and number of occupants.