ADU rules in Howard County, MD — also called accessory dwelling unit regulations or granny flat ordinances — cover setbacks, owner-occupancy, parking, and permit requirements.
Howard County permits an accessory dwelling unit inside an owner-occupied single-family home. The owner must live in the principal dwelling or the ADU, and a permit from the Department of Planning and Zoning is required. Recent Council Bill 3-2026 expanded ADU options countywide.
Under Howard County Zoning Regulations Section 128.0.13, an accessory apartment must be located within an owner-occupied dwelling, and the owner may occupy either the principal dwelling or the accessory apartment. Historically it could occupy no more than one-third of the building's net floor area, up to 1,500 square feet, operating only under a Department of Planning and Zoning permit. Council Bill 3-2026 (ZRA-218), effective April 8, 2026, modernized the rules: an ADU may not exceed 75% of the principal dwelling's exterior footprint, only one detached or attached ADU is allowed per single-family lot, ADUs may not be used as short-term rentals, and off-street parking increases. Because Howard County has no incorporated cities, these county rules apply in Columbia, Ellicott City,
Building or renting an ADU without the required Planning and Zoning permit, or using it as a short-term rental, is a zoning violation subject to enforcement and permit revocation.
Other ordinances people look up for this city. Green dot = verified primary-source excerpt.
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See how Howard County's adu rules rules stack up against other locations.
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