ADU rules in Polk County

State law makes the county allow at least one ADU, attached or detached, on any lot with a single-family house where a house is an allowed principal use; for unincorporated Polk County that is the AG, AT, ER, RR, LDR, MDR, HDR and MU districts. The county's own text still allows a detached ADU only inside an accessory building such as a detached garage, capped at half of that building's floor area, and bars any standalone structure (a stick-built cottage, manufactured home, tiny home or RV) as an ADU. Iowa Code 331.301(29) defines an ADU as attached or detached, says detached 'includes' part of an accessory structure, and voids conflicting county text, so a freestanding backyard ADU is allowed under state law. A manufactured or mobile home used as an ADU must sit on a permanent foundation and be taxed as real property.

Table 4.1 lists 'Accessory Dwelling Unit' as permitted by right (Y) in the AG, AT, ER, RR, LDR, MDR, HDR and MU districts, the same districts where single-family homes are permitted, and the county text allows an ADU attached to the house. Basement and other internal units fit the ordinance's definition of an ADU attached to the primary dwelling. One ADU per property is allowed. The ordinance covers only unincorporated land; Des Moines, West Des Moines, Ankeny, Urbandale and the other cities zone their own territory.

The county caps an ADU at 1,200 sq ft of gross floor area excluding garage, porch and deck, never larger than the existing house, and an ADU inside an accessory building may take no more than 50 percent of that building. Iowa Code 331.301(29)(a)(2) requires the county to allow an ADU of 1,000 sq ft or half the house's size, whichever is larger, so the 1,200 sq ft county cap governs for houses up to 2,400 sq ft, a larger house earns half its size, and the 'not larger than the house' and 50-percent-of-accessory-building limits are void wherever they cut below that floor. From 7-1-2026 (SF 2369) house size means gross living area excluding garages, decks and unheated porches, and unfinished basements do not count toward the ADU's size. Setbacks and lot coverage apply as they do to the house or an accessory building; county density limits do not apply to ADUs.

The county text still requires the legal owner to live in the house or the ADU (shown by title records and the homestead tax credit) and to record a covenant or deed restriction, with a Board of Adjustment conditional use permit needed for up to 12 months' absence. Iowa Code 331.301(29)(b)(4) bars any requirement that the property owner be a resident, and 331.301(29)(d) voids conflicting county text, so no owner occupancy or owner-occupancy covenant can be required. The county had not amended this provision at extraction.

The county text requires one off-street space for the ADU in addition to one for the house. Iowa Code 331.301(29)(b)(3) bars additional parking beyond that required for a single-family residence, or a fee in lieu, so the extra space is void. The county also allows a separate driveway entrance for the ADU only where it is 150 feet from the existing entrance and meets the entrance permitting rules.

ADUs are permitted by right in every district that allows single-family homes, so an ADU needs a building permit issued by the Zoning Administrator under Article 20, with Environmental Health sign-off on the well and septic system where those serve the lot. Iowa Code 331.301(29)(c) requires approval without discretionary review or hearing on the single-family timeline, with written reasons and a remedy for any denial. The only county hearing tied to ADUs is the Board of Adjustment permit for an owner's temporary absence, which falls away with the void owner-occupancy rule.

The county text says no ADU 'shall be sold, leased, divided or otherwise segregated in ownership' from the house unless zoning and subdivision standards can be met; Iowa Code 331.301(29)(b)(2) bars rental-use rules stricter than 331.301(18) and chapter 562A, so the leasing bar is void and the ADU may be rented long term, while the separate-sale limit survives. No county rental registration or licensing program was found in the county's ordinance listings or the Board of Supervisors record. Under 331.301(18) the county may not regulate short-term rentals (30 days or less) beyond general health, safety, noise and nuisance rules, must treat them as a residential use, and may not require a short-term rental licence or permit fee. The ordinance's bed-and-breakfast use (owner resident, meals, conditional use in LDR, MDR and HDR) is a separate category.