Billings ADU Permits & Requirements
Fees $1,000–$4,000 · 4–6 week review · Up to ~1,000 sq ft
Quick Answer
Billings ADUs: $150K–$260K
Permits: $1,000–$4,000 · Updated April 2026
Permit Fees Breakdown
| Fee Type | Cost |
|---|---|
| Building Permit | $800–$3,000 |
| Plan Review | $300–$1,200 |
| ADU Application / Review | $0–$250 |
| Water & Sewer Taps | $3,000–$10,000 |
Montana SB 528 bans ADU-specific impact fees. Figures are estimates — verify the current schedule with the City of Billings at billingsmt.gov or via the CityView fee estimator.
Permit Process Timeline
Zoning & Eligibility Verification
1–2 weeksConfirm the parcel allows an ADU under SB 528 and the local residential district
Plan Preparation
2–4 weeksPlans built to Montana amendments (36-inch frost, 115 mph wind, 30 psf snow)
Building Permit Review
4–6 weeksCity of Billings Building Division review via the CityView portal
Construction
4–7 monthsBuild phase with inspections; foundations are often scheduled around the freeze
Final Inspection & CO
1–2 weeksFinal inspections and Certificate of Occupancy
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Zoning & Requirements
Max ADU Size
~1,000 sq ft
Height Limit
Per primary zone
Setbacks
Per underlying zone
Parking
None required
Key Regulations
- Montana SB 528 / MCA 76-2-345 requires at least one ADU per single-family lot statewide (2023; upheld by the Montana Supreme Court in 2026)
- No ADU-specific impact fees; the separate ADU application/review fee is capped at $250 by state law
- No owner-occupancy requirement — you may rent both the primary home and the ADU
- No additional off-street parking may be required as a condition of ADU approval
- ADU setbacks, height, and coverage cannot be more restrictive than the rules for the primary house in that zone
- Build to Montana amendments: 36-inch frost depth, 115 mph wind design, and 30 psf ground snow load
Climate & Construction Notes
Billings sits on Montana's high plains: footings must reach the locally enforced 36-inch frost depth, the envelope is built for a 115 mph design wind and a 30 psf snow load, and hard freeze-thaw cycles demand quality flatwork. Lots on the Rimrock benches can hit shallow Eagle Sandstone bedrock (rock excavation), while much of the valley floor has expansive clay soils that drain poorly — so drainage and foundation design matter even in a semi-arid climate.
Frequently Asked Questions
How long does it take to get an ADU permit in Billings?
Building permit review for a complete submittal is typically 4–6 weeks. Total project time including design and construction is usually 6–12 months. Verify current timelines with the City of Billings.
What types of ADUs are allowed in Billings?
Billings allows detached ADUs, attached ADUs, and garage or basement conversions in its single-family residential districts, generally up to about 1,000 sq ft (or the size of the main house, whichever is less).
Do I need to live on the property to build an ADU in Billings?
No. Montana SB 528 eliminated owner-occupancy requirements, so you can rent both the primary home and the ADU. Confirm the current local implementation with Billings Planning & Community Services.
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