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, with no special review, no conditional-use permit, and 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, so the city interpretation is binding). A building permit is required before construction; there is no separate ADU application, only 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, so verify applicability for ADUs.
source · reviewed 2026-06-22
Billings ADU permit questions
- Do I need a permit to build an ADU in Billings?
- Yes. An accessory dwelling unit (ADU) in Billings requires a building permit. ArchPermit tracks 13 cited requirements for this jurisdiction (11 required, the rest conditional). Always confirm the exact forms, fees, and code edition with the local permitting authority that issues the permit.
- What does an ADU permit package require in Billings?
- Core required items include: Beam Calculations; Building Permit Application; Cross-Sections (Sectionals); Energy Code Compliance (2021 IECC); Floor Plans; Foundation Plan Details, plus 5 more. Each requirement links to its authoritative source, and nothing is permit-ready until an independent licensed professional reviews and seals it.
- How are ADU permits submitted in Billings?
- Billings accepts permit submittals via CityView Portal (online submission required). ArchPermit prepares a draft package you can submit there after a licensed architect reviews and seals it.