Montana ADU Law Explained: Build a Rental Unit in Your Backyard (2026)
Montana now requires cities and counties to allow ADUs on single-family lots. Here's the size limits, local variations, construction costs, rental income potential, and step-by-step process to build one.
What Is an ADU?
An Accessory Dwelling Unit (ADU) is a self-contained living space on the same lot as your primary home. Think: a backyard cottage, a converted garage apartment, a basement suite with its own entrance, or a detached studio with a kitchen and bathroom.
ADUs are also called mother-in-law suites, granny flats, casitas, or backyard homes. Whatever you call them — in Montana, you now have the legal right to build one.
Montana's ADU Law: What Changed
Montana became one of the most ADU-friendly states in the country through a series of housing reform bills:
Montana ADU Legislative History
Required municipalities to allow at least one ADU by right on any lot with a single-family dwelling. No special hearing or variance needed.
Revised zoning laws to strengthen ADU rights in municipalities. Part of Gov. Gianfortes pro-housing package. Took effect immediately.
Extended ADU requirements to counties with zoning — not just cities. Counties can no longer prohibit ADUs.
Cities and counties cannot prohibit ADUs on single-family lots, cannot require owner-occupancy, and cannot impose excessive parking requirements.
The bottom line: Your local government cannot say no to an ADU on your single-family lot. They can set size and design standards, but they must allow it.
Size and Design Requirements
Montana law and most local codes set these parameters:
| Requirement | Standard |
|---|---|
| Maximum size | 1,000 sq ft OR 75% of primary home (whichever is smaller) |
| Minimum features | Own kitchen, bedroom, and bathroom |
| Building codes | Must meet residential building, fire, and health codes |
| Parking | Local government cannot impose excessive parking requirements |
| Owner occupancy | Cannot be required (you don't have to live on the property) |
| Setbacks | Subject to local setback requirements |
| Height | Subject to local height limits (SB 528 allows taller in urban areas) |
Source: Zook Cabins — Montana ADU Regulations, MCA 76-2-345
Types of ADUs
Detached ADU (Backyard Cottage)
A new, separate structure built in your yard. Most expensive option but highest rental value and privacy for both you and tenants.
Cost range: $100,000–$250,000+ depending on size, finishes, and site conditions.
Attached ADU (Addition)
An addition to your existing home with a separate entrance, kitchen, and bathroom. Can be a side addition or above-garage build.
Cost range: $80,000–$180,000
Conversion ADU (Garage/Basement)
Converting existing space — a garage, basement, or attic — into a separate dwelling. Cheapest option if the space already exists.
Cost range: $40,000–$120,000
Internal ADU (Subdivision)
Dividing an existing larger home into two separate units. Requires separate entrances, plumbing, and kitchen facilities.
Cost range: $30,000–$80,000
Rental Income Potential
By City (Estimated Monthly Rent for 600-800 sq ft ADU)
| City | Estimated Rent | Notes |
|---|---|---|
| Bozeman | $1,200–$1,600 | Highest demand, student/worker market |
| Missoula | $1,100–$1,400 | Strong university demand |
| Billings | $900–$1,200 | Lower cost market, steady demand |
| Helena | $900–$1,100 | Government workers, stable |
| Great Falls | $800–$1,000 | Lowest market, but cheapest to build |
| Kalispell | $1,000–$1,300 | Growing market, seasonal demand |
Investment Math Example
A 700 sq ft detached ADU in Missoula:
- Construction cost: ~$150,000
- Monthly rent: $1,200
- Annual gross income: $14,400
- Annual expenses (taxes, insurance, maintenance): ~$3,500
- Annual net income: ~$10,900
- Cash-on-cash return: 7.3%
- Payback period: ~14 years (without appreciation)
If the ADU also increases your property value by $100,000+ (common in tight markets), your actual return is significantly better.
Property Tax Implications
Good news for ADU landlords: If you rent your ADU long-term (28+ day leases, 7+ months per year), it qualifies for Montana's reduced tiered property tax rate — the same as your primary residence.
If you use it as a short-term rental (Airbnb-style), the ADU portion would be taxed at the flat 1.90% rate.
See: Montana Property Tax: 2026 Tiered Rates
Local Variations to Watch
While the state mandates ADU allowance, cities can still control:
| City | Key Local Rule |
|---|---|
| Bozeman | Occupancy cap of 2 people in ADUs |
| Great Falls | May prohibit rental of accessory living spaces in certain zones |
| Missoula | Allows in all zones, relatively permissive |
| Whitefish | Resort area — check HOA/covenant restrictions |
Critical: Check your specific lot's zoning, HOA covenants, and any deed restrictions before starting. The state law prevents local government from banning ADUs, but private covenants may still restrict construction or rental use.
Step-by-Step: How to Build an ADU in Montana
- Check zoning and setbacks — Contact your city/county planning department for lot-specific requirements
- Review HOA/deed restrictions — Private covenants can restrict what the government cannot
- Design within limits — 1,000 sq ft max or 75% of primary home
- Hire an architect or use pre-approved plans — Many companies offer ADU plan sets ($2,000–$5,000)
- Apply for building permit — Submit plans to your local building department
- Secure financing — Home equity loan, construction loan, or cash
- Build — Hire a licensed contractor; typical timeline is 4–8 months
- Final inspection — Pass building, electrical, plumbing, and fire inspections
- Set up as rental — Lease, insurance, property management software
- Start collecting rent — List it, screen tenants, and generate income
Financing Options
| Method | Pros | Cons |
|---|---|---|
| Home equity loan (HELOC) | Low rates, tax-deductible interest | Uses your home as collateral |
| Cash-out refinance | One payment, fixed rate | Resets your mortgage |
| Construction loan | Purpose-built for building | Higher rates, shorter term |
| Personal savings | No debt, no interest | Ties up liquid capital |
| FHA 203(k) | Rolls into mortgage | Complex process, FHA limits |
Freddie Mac and Fannie Mae now offer renovation loans that specifically include ADU construction — ask your lender about current programs.
Source: Freddie Mac ADU Fact Sheet
Related Reading
- Montana Property Tax: 2026 Rates — How ADU rental income affects your tax rate
- Montana Rental Property LLC: $35 to Form — Should you put your ADU in an LLC?
- Montana Rental Regulations by City — City-specific zoning rules
- Airbnb vs Long-Term Rental — STR vs LTR math for your ADU