Billings ADU permit requirements
Last reviewed: June 22, 2026
Quick answer
Permitting an accessory dwelling unit (ADU) in Billings, Montana means pulling a building permit from the local authority that issues it. ArchPermit tracks 13 source-cited requirements (11 required, the rest conditional) for this jurisdiction. Submittals go through CityView Portal (online submission required). These requirements were last reviewed June 22, 2026; confirm the exact forms and fees with the local permitting authority.
A source-cited checklist of what an ADU (accessory dwelling unit) permit package needs in City of Billings. Every item links to its authoritative source; items that vary are flagged to confirm locally.
ADUs are allowed by-right in the N1, N2, and N3 zone districts per an official Zoning Code Interpretation (Mar 11 2025, §27-1617) implementing Montana state law MCA 76-2-345 — no special review, no conditional-use permit, no public hearing. The interpretation voids prior local standards that conflict with state law, including special review, owner-occupancy mandates, deed restrictions, design standards different from the primary dwelling, and any additional off-street parking. Size: a detached ADU ≤75% of the primary dwelling's gross floor area or 1,000 sf (whichever less); attached ≤80% or 1,000 sf (the city interpretation grants 80% for attached, though MCA 76-2-345 states 75% for both — the city interpretation is binding). A building permit is required before construction; there is no separate ADU application — it is a residential building permit.
- Beam Calculationsrequired
Structural beam calculations.
source · reviewed 2026-06-22 - Building Permit Applicationrequired
City of Billings Building Division application: job address, legal description, owner/contractor/engineer, project type, class of work, dwelling-unit count, valuation, and signed owner/agent agreement.
source · reviewed 2026-06-22 - Cross-Sections (Sectionals)required
Height clearances, exterior materials, insulation requirements, ice-and-water barrier, grade requirements, dwelling separation.
source · reviewed 2026-06-22 - Energy Code Compliance (2021 IECC)requiredverify locally
Energy compliance for new dwelling construction (Billings adopted the 2021 IECC); confirm the exact form (e.g. REScheck/COMcheck) with the Building Division.
source · reviewed 2026-06-22 - Floor Plansrequired
Egress windows/doors, smoke & CO detectors, dryer-duct limit, kitchen/bath vents, combustion air/flue, mechanical locations, attic/crawl access.
source · reviewed 2026-06-22 - Foundation Plan Detailsrequired
Footing/pad sizes, rebar size & spacing, min frost depth, drain tile, wood-to-grade clearance, anchor-bolt spacing, damp-proofing, Ufer ground, crawl-space vents, mono-slab insulation.
source · reviewed 2026-06-22 - Framing Plan Detailsrequired
Header/beam qualification, roof/floor layout, girder/truss bearing, cantilevers, fire blocking, draft stopping, wall bracing (R602.10), stud size/height/spacing, deck & pier details.
source · reviewed 2026-06-22 - Full Plan Set Including Site Planrequired
One complete plan set; the site plan must show setbacks, top of concrete & top of wall, floodplain, and drainage (structures within 5 ft of a property line require a 1-hr rating).
source · reviewed 2026-06-22 - Manual J (HVAC load calculation)required
HVAC load calculation (Manual J) for mechanical sizing.
source · reviewed 2026-06-22 - Stair Detailsrequired
Rise/run/nosings, height/width/headroom, handrails/guardrails, illumination, landings.
source · reviewed 2026-06-22 - Truss & Joist Layouts and Profilesrequired
Roof truss and floor joist layout/profile drawings.
source · reviewed 2026-06-22 - Soils Reportconditional
Geotechnical/soils report — required 'if not on file' for the site.
source · reviewed 2026-06-22 - Will-Serve Letters (municipal water + sewer)conditionalverify locally
The city MAY require will-serve letters from municipal water and sewer (allowed under MCA 76-2-345(7)); not on the residential checklist — verify applicability for ADUs.
source · reviewed 2026-06-22