What are the tiny house laws in Montana?

Montana counties can allow tiny dwellings through zoning districts¹. County commissioners decide whether to adopt those districts and set their rules; Montana law does not approve every small house statewide¹. Check your parcel's district rules before treating the statute as permission to build¹.

Montana's county-zoning definition covers a dwelling of 350–750 square feet on a permanent foundation¹. Single-family use must last at least 45 days¹. You still need to check existing covenants, historic-district conditions and local design-review standards¹. Ask the county planning office whether your proposed home fits the district before you order plans¹.

Montana's 350–750-square-foot range defines a tiny dwelling under the county-zoning statute¹. That range does not confirm a statewide minimum size for every house¹. A general statewide minimum dwelling size remains unconfirmed; ask the Building Codes Program about your design⁴.

Certified local programs enforce Montana's building codes where those programs operate⁵. The state program covers the remaining jurisdictions, with state exemptions⁵. Ask which office will review your home and which code edition applies before you pay for detailed drawings⁵. You can get state permit applications from the Building Codes Program's website⁴.

You can pursue an accessory dwelling if your Montana parcel already has a single-family home². Counties using zoning powers under Montana's county ADU statute must allow at least one accessory dwelling by right². Your unit can be attached, detached or internal, but building, fire and health rules still apply².

Montana municipalities using zoning powers must also allow an ADU on a lot with a single-family dwelling⁶. An attached or detached municipal ADU is limited to 75% of the primary home's floor area or 1,000 square feet, whichever is less⁶. County and municipal ADU application-review fees can reach $250, plus usual building fees^2,6. Ask the zoning office which ADU rule covers your address^2,6.

You need a local health department permit for a Montana drainfield even if the parcel has a Certificate of Subdivision Approval³. Ask that department for the parcel's sanitation approval and any sanitary restrictions³. Ask whether your proposed dwelling fits the approved use before you choose its location³.

Check Montana's building-permit exemptions separately⁷. In state jurisdiction, residential buildings with fewer than five dwelling units are exempt from state building permits unless they serve transient guests⁷. Certified local jurisdictions can set their own building-permit requirements⁷. Ask the building office about separate electrical, plumbing and mechanical permits⁷. If you plan to rent your tiny house to visitors, tell the office when you ask about the exemption⁷.