Committee weighs bill to extend accessory dwelling unit rules to counties amid sanitation and water‑quality concerns

Senator Forrest Mandeville, sponsor of Senate Bill 532, told the Senate Local Government Committee he modeled the measure on municipal ADU legislation passed last session and inserted it into county (part 2) zoning statutes. The bill would require counties with zoning to allow accessory dwelling units (ADUs) — small, self‑contained living units on a parcel with a single‑family dwelling — and provide an expedited sanitation review process in limited circumstances.

Proponents including Jake Brown (Shelter Whitefish), Tanner Avery (Frontier Institute), Tim Summers (AARP Montana), Adrian Cotton (Montana Building Industry Association) and the Institute for Justice argued the change would broaden housing options, help aging Montanans, support multigenerational households and protect property‑use rights. Samuel Hooper of the Institute for Justice said ADUs “let aging parents live near their children” and provide options for homeowners to add income while preserving privacy.

Opponents raised technical public‑health and environmental concerns. Charles Shane and Beth Norberg (Missoula Public Health and the Montana Environmental Health Association) warned that the bill’s proposed 15‑day expedited sanitation review would be too short for many counties’ staff and could overload limited certified sanitarian capacity. Beth Norberg said many ADU proposals are rewrites to existing approvals and are often technically complex because they involve existing wells, septic systems and constrained lots.

Gallatin County’s contracted DEQ reviewer, Britney Kron, told the committee that 60–75% of local applications she reviews propose an ADU and said her office lacks the staffing to meet a 15‑day turnaround for on‑site wastewater reviews. She suggested an amendment tying expedited review to connections to existing municipal water/sewer systems rather than on‑site wastewater systems.

Officials from the Department of Environmental Quality and several county sanitarians explained that DEQ and counties generally treat an ADU that includes a full kitchen and other living facilities as a separate dwelling unit for sanitation and water‑quality purposes; that treatment can trigger mixing‑zone and water‑quality review under the Montana Water Quality Act. DEQ staff told senators they do not have authority to designate a unit as “seasonal” to avoid full‑time use calculations and cautioned that ADUs can increase wastewater loading in areas with limited assimilative capacity.

Sponsor Mandeville said he worked with DEQ and inserted a proposed amendment placing a four‑year sunset on the DEQ expedited‑review provision, intended to limit administrative burdens while allowing time to gauge impacts. He said the county ADU language mirrors last session’s municipal ADU bill and that he is open to amendments to address sanitation‑review timelines and other technical points.

Committee members asked whether the bill applies only to part 2 zoned areas (it does) and whether it would preclude counties from addressing public‑safety, fire or building‑code matters (those remain under applicable codes where required). Several sanitarians recommended a longer review window (30 days or more) for DEQ/county sanitation reviews. The hearing ended with the sponsor indicating willingness to work on amendments; no final committee vote was recorded.