Ames, IA Accessory Structures: ADU Rules (2026)

Sec. 29.409 of the Ames Zoning Ordinance permits an Accessory Dwelling Unit on any lot holding only a single-family residence, and it may be attached to the home, detached in a freestanding structure, or created by converting an existing accessory structure so long as that structure meets all ADU requirements, including setbacks; no ADU may be carved out of an existing single-family residence itself. The unit must function independently, with its own bathroom and kitchen connected to public water and sanitary sewer, and be placed on a permanent foundation whether built on site or premanufactured.

Under Sec. 29.409(3), the ADU follows the same maximum height, minimum setbacks, minimum lot size, minimum building frontage, maximum lot coverage, density, and architectural standards as the single-family residence on the property, with setbacks based on height calculated individually for each structure; ADUs are exempt from the accessory-structure standards of Sec. 29.408 and from that section's rear yard coverage restrictions. Size is capped under Sec. 29.409(4) at 1,000 square feet or 50 percent of the single-family residence's Gross Floor Area as defined in Sec. 29.201, whichever is larger, and the unit is limited to one bedroom, with any additional room meeting the ordinance's bedroom definition prohibited.

No additional on-site parking is required for the ADU beyond what the single-family residence already requires, though a sidewalk must connect the ADU's primary entrance to the parking area, the residence, or the street. Properties in overlay districts such as the Single-Family Conservation or Historic overlay may face added design standards, and the more restrictive rule controls in a conflict.