Building permits, zoning and land rules in Custer County, Montana

Yes, in the land-use sense: before constructing or expanding a structure in an unincorporated Custer County zoning district, the owner must obtain a zoning conformance permit¹. A qualifying accessory structure under 100 square feet is exempt if it meets the accessory-structure definition and district development standards¹. Montana also has a uniform state building code, enforced by the state where there is no certified local program⁷. The state program exempts farm and ranch buildings, qualifying private garages and storage buildings, and residential buildings with fewer than five dwelling units unless they serve transient guests². A home can therefore be exempt from a state structural building permit while still requiring county zoning conformance approval^1,2. Montana's contractor-licensing exemption lets an owner work on the owner's own property, but it does not itself exempt a project from building, electrical, plumbing, mechanical or inspection requirements⁶. We could not confirm a county owner-builder exemption from zoning approval or a certificate-of-occupancy rule; ask the Zoning Administrator.

The Custer County Zoning Administrator is the first stop for construction on unincorporated land because county zoning conformance approval comes before construction or expansion¹. The official zoning map controls a property's current county zoning status and is kept by the Zoning Administrator and County Clerk and Recorder¹. Reach the Zoning Administrator at 1010 Main Street, Ste. 2, Miles City, Montana, 59301⁸. If a nonexempt project falls under Montana's state building-code jurisdiction, the state process requires plans and an application, then plan approval and fee payment before permit issuance². State-covered work may also need electrical, plumbing and mechanical permits². Custer County's published county process is partial, so ask the Zoning Administrator which county forms accompany the conformance application¹.

Yes. County zoning divides all of Custer County outside Miles City and the Town of Ismay into use districts¹. Miles City and Ismay are outside the county regulations and may have their own municipal rules¹. Dimensional rules depend on the district: R-2, for example, has a 5,500-square-foot minimum lot for a single-family residence, 15-foot front and 5-foot side setbacks, 35% lot coverage, and a 35-foot height limit¹. Other districts differ, so use the official zoning map rather than assuming those figures apply everywhere¹. Montana law also requires county zoning to allow at least one accessory dwelling unit by right on a parcel containing a single-family home, subject to building, fire, sanitation and other health-and-safety rules⁹. The county rules do not confirm a general minimum floor area for site-built homes or a specific tiny-home category; ask the Zoning Administrator how the proposed dwelling is classified.

Custer County Environmental Health, with its Sanitarian and Montana DEQ liaison services, administers onsite wastewater rules for the whole county^3,10. A valid permit is required to construct, alter or increase the load on a system, and replacement systems need one too; no permit is needed where sewage goes to an approved public system³. A permit not yet used for construction stays valid for two years³. Owner-installed systems may not be backfilled until inspected and approved, and a new system may not be used before final inspection and approval³. Off-grid options are narrow: sealed pit privies only where there is no piped water, and holding tanks only for seasonal use under distance and testing conditions³. Montana also requires a local drainfield permit even when a property has a Certificate of Subdivision Approval, and parcels created by subdivision may fall under state DEQ review¹¹. Only 11.6% of Custer County soils are rated without severe limitations for a conventional drain field, so evaluate the site before choosing a house location¹².

Custer County's rules define an RV park as land used for occupied recreational vehicles or tents serving as temporary living quarters for 180 days or less, and RV parks are conditional uses in AG-1 and C-1¹. That rule does not settle long-term occupancy of one RV on a private parcel, so ask the Zoning Administrator about the parcel and proposal before moving in. Tiny-home occupancy is also not specifically confirmed; a tiny home treated as a structure still needs zoning conformance approval before construction or expansion¹. Manufactured homes are expressly allowed as single-family homes in R-1, R-2 and R-3 when placed on a permanent foundation, but the county definition also requires factory construction on or after January 1, 1990, at least 1,000 square feet, a pitched roof, customary siding and applicable HUD compliance¹. Environmental Health should approve the wastewater plan separately³.

1. Ask the Zoning Administrator to confirm the district, dimensional standards and whether the home is permitted outright or needs conditional-use approval¹.

1. Apply for zoning conformance approval before construction or expansion¹.

1. Take the site plan to Custer County Environmental Health and obtain the septic permit before installation; keep the system uncovered until it is inspected^3,11.

1. Check the flood map; inside the Regulated Flood Hazard Area, a floodplain permit application needs a signed application, review fee, scaled plans and elevations, and the Floodplain Administrator decides a complete application within 60 days^5,13.

1. Obtain the county encroachment permit before creating, expanding, altering or relocating an approach in a county right-of-way; the listed minimum fee is $50⁴. A state-highway approach instead goes through the appropriate Montana Department of Transportation district office¹⁴.

1. Confirm whether state building, electrical, plumbing or mechanical permits apply before work starts².

Start with the official zoning map, lot dimensions and intended use because district-specific minimum lots, setbacks, coverage and height limits can change what fits¹. If the sale divides land, Custer County's subdivision rules apply within its jurisdiction, and some proposals near or partly inside a municipality require municipal review or approval too¹⁵. Investigate wastewater before fixing the building site because only 11.6% of county soils are rated without severe limits for a conventional drain field¹². For a new small groundwater development, Montana may allow a permit exception, but a Notice of Intent to DNRC is required before use beginning in 2026¹⁶. We could not confirm a county private-well permit rule, so ask Environmental Health about wells before drilling. Check flood status and legal access, including county or state approach approval as applicable^4,13,14. Custer County averages 14.5 inches of precipitation annually, so verify a dependable water source¹².