Modular homes are like manufactured (HUD code) homes, but are built to a state code, rather than the federal HUD can accessory dwelling unit (ADU) is a legal and regulatory term for a secondary house or apartment that shares the building lot of a larger, primary home. The unit cannot be bought or sold separately, but they are often used to provide additional income through rent or to house a family member. For example, an elderly parent could live in a small unit and avoid having to move to an assisted living facility.