Ames HPC adopts code changes to align accessory dwelling unit rules with new state law

The Ames Historic Preservation Commission voted unanimously on June 9 to adopt Alternative 1, approving revisions to local rules governing accessory dwelling units so the city can comply with new state legislation that takes effect July 1.

The change was driven by state law that, according to city staff, prohibits local regulations that treat accessory dwelling units differently than single-family homes for matters such as height, setbacks and density. “The city last year adopted new accessory dwelling unit regulations,” said Ray, a city staff member, “and earlier this year, the state passed new legislation that preempts or overrode our requirements. We are now required to change everything by July 1.”

Commissioners were told the state law sets a maximum size for ADUs of 1,000 square feet or 50% of the primary residence; removes local ability to require owner-occupancy or additional on-site parking; and prevents cities from requiring ADUs to match exterior design elements such as roof pitch or finish materials. Ray said historic-design review may still apply: “They can't be required to match exterior design, roof pitch, or finishing materials, but they can still be required to comply with historic guidelines and regulations.”

Under the proposed local revisions, the commission and staff will revise Chapters 13 (rental housing), 29 (zoning) and leave Chapter 31 (historic preservation districts) unchanged for now. The city code previously limited ADUs to 900 square feet; the state cap is 1,000 square feet. Staff explained that setbacks for ADUs will change because ADUs must be treated the same as primary dwellings: rear setbacks would increase to match the primary dwelling (20 feet), and side setbacks will depend on building height (commonly 6 to 10 feet).

The commission and staff clarified several scope items during discussion: ADUs will continue to be limited to one bedroom under the proposed local rule; ADUs will be allowed on lots with duplexes but not on lots with apartment buildings; the code will remove the local requirement that an owner live on the property to rent an ADU; and ADUs will be exempted from certain rear-yard lot-coverage limitations for simplicity. Ray said staff did not believe these changes would “materially impact” the commission’s design-review workload because ADU proposals in historic districts would still be subject to the normal review for additions or standalone structures.

A motion to adopt Alternative 1 was made and approved unanimously. The motion text on the record was: “I propose that we adopt alternative 1 as recommended.” Meeting minutes record the outcome as adopted; no individual roll-call vote was recorded in the transcript.

Commissioners asked several follow-up questions that staff said they would clarify in later code edits, including whether ADUs on noncontributing historic properties must meet preservation guidelines; staff said they would research that and return with clarifications if needed.

The commission’s action updates local regulations to conform to state law and directs staff to finish the code amendments before the state law takes effect on July 1.