Garage Apartment and ADU Builders in Bozeman, MT
A dwelling above the garage. Montana rewrote the rules for accessory dwelling units in 2023, and Bozeman replaced its whole development code in 2026, so this is the build where published advice ages fastest.
No obligation. Every estimate starts with a walkthrough of the lot, the access and where the building can actually sit.
Gallatin County, MTThe build
The moment it becomes housing
Sleeping, cooking and sanitation in one space makes a dwelling unit. That single line changes the entire regulatory picture.
A garage apartment looks like a garage with a loft and is regulated like a house. Once the space above has a place to sleep, a place to cook and a bathroom, it is a dwelling unit, and the building has to meet residential code for egress, fire separation between the dwelling and the garage below, insulation, ventilation and stair geometry.
The permitting also changes. In unincorporated Gallatin County, the state building code exemption that covers a private garage does not cover a dwelling. A building that would have needed no state building permit as a garage needs a different answer once someone lives in it.
Inside a city the picture is more favourable than it used to be. Montana's 2023 statute made at least one accessory dwelling unit a matter of right on lots with a single-family home in any municipality that exercises zoning powers, and it stripped out several of the conditions cities used to apply.
What has not changed is that this is the most expensive thing you can put above a garage. It is a small house on a second floor, with all of a house's systems.
Included
What a garage ADU build includes
Two-storey foundation and floor
Footings to 48 inches inside Bozeman, a framed floor rather than an attic truss in almost every case, and a stair designed as the primary means of egress for a dwelling.
Fire separation and egress
Rated separation between the garage below and the dwelling above, egress windows sized to code, smoke and carbon monoxide detection, and ventilation designed for a habitable space.
Water, waste, heat and power
Plumbing supply and waste, a heating system sized for the space, an electrical service that accounts for a second dwelling, and on unsewered parcels a septic capacity question answered before design.
The law
What Montana settled in 2023
Montana settled the baseline in 2023. MCA 76-2-345 requires every city or town that exercises zoning powers to allow at least one accessory dwelling unit by right on a lot with a single-family home, attached, detached or internal. The unit can be up to 75 percent of the primary home's floor area or 1,000 square feet, whichever is smaller.
The statute also lists what a municipality cannot require: extra off-street parking, owner occupancy of either unit, matching roof pitch or exterior materials, or standards tighter than those applied to the main house. Some city guidance published before 2023 still lists those conditions. When a published FAQ and the statute disagree, ask the planner which one they are applying now.
Statutory parameters from MCA 76-2-345. The statute sets a floor that municipalities must meet; individual city standards may still differ on matters the statute does not address. Confirm with the planning department for your parcel before you design around any of it. This is general information, not legal advice.
Process
How a garage ADU project runs
Confirm it is allowed here
Municipal lot inside a zoning city, or unincorporated ground. The answer changes everything downstream.
Answer the waste question
City sewer capacity, or on septic, whether the system can take another dwelling. This can stop a project.
Size it to the cap
Up to 75 percent of the primary home or 1,000 square feet, whichever is smaller.
Design as housing
Egress, fire separation, insulation, ventilation, stair, and the deeper foundation for two storeys.
Permit and review
Building permit, plus a Certificate of Appropriateness where the property sits in the conservation overlay.
Build and inspect
Concrete, structure, envelope, then the mechanical, electrical and plumbing inspections a dwelling requires.
Before you commit
The three things that decide whether this is viable
Waste water. On city sewer it is a connection. On septic it is a capacity calculation, and if the existing system cannot take another dwelling, the project either includes a system upgrade or it does not happen. Find this out first. Source: Gallatin City-County Health Department.
The size cap. The unit can be up to 75 percent of the primary home's floor area or 1,000 square feet, whichever is smaller. On a modest house, the 75 percent figure binds well before the 1,000 square feet does, and people are often surprised by how small the permitted unit is.
Which rules are actually current. Bozeman guidance published before 2023 still lists conditions the state statute now prohibits, such as required off-street parking and owner occupancy. Bozeman also replaced its entire development code effective 1 February 2026. We ask the planner what they are applying rather than relying on a published page, and we recommend you do the same before making financial commitments.
None of this is legal advice. For a project of this size, a conversation with the planning department and, where money is involved, with an attorney, is worth the hour.
Cost
Start with whether it is possible on your lot
For an ADU the first question is not cost, it is whether the waste system and the lot support a second dwelling. We can usually tell you quickly.
Questions
About garage apartment (adu)
Can I build an apartment above my garage in Bozeman?
Montana law requires municipalities that exercise zoning powers to allow at least one accessory dwelling unit by right on a lot with a single-family home, and that includes detached units above a garage. The unit is capped at 75 percent of the primary home's floor area or 1,000 square feet, whichever is smaller.
Bozeman's specific standards sit inside a development code that was replaced on 1 February 2026, so confirm current requirements with Community Development rather than relying on any published summary, including this one. Sources: MCA 76-2-345, City of Bozeman, Unified Development Code adoption.
Does the city require an extra parking space for the ADU?
Under MCA 76-2-345, a municipality cannot require additional off-street parking for an accessory dwelling unit, or a fee in lieu of it. Some city material published before the statute still says otherwise.
If you are told a parking space is required, it is a fair question to ask which provision it is being required under.
What does a garage apartment cost?
Price the garage on its own band, then the dwelling portion at $190 to $330 per square foot. A 700 square foot unit above a two-car garage is roughly $133,000 to $231,000 for the apartment alone, plus the garage below.
It is the most expensive thing you can put above a garage because it is a small house with all of a house's systems, on a second floor.
Do I need to live on the property?
Montana law says a municipality cannot require owner occupancy of either unit as a condition of allowing an accessory dwelling unit. That is a change from what many local ordinances used to require.
Financing is a separate matter and your lender may have its own conditions.
Can I do this on unincorporated county land?
It is a different analysis. The state building code exemption for a private garage does not extend to a dwelling, and the 2023 accessory dwelling statute is written around municipalities that exercise zoning powers rather than unincorporated county ground.
Where a county zoning district applies, its own accessory dwelling provisions govern. Some districts around Bozeman have specific and fairly restrictive accessory dwelling standards. Source: Gallatin County Planning & Community Development.
How does septic capacity affect this?
It is often the deciding factor on unsewered land. Adding a dwelling adds fixtures and design flow, and the existing system either has the capacity and the drainfield area or it does not.
This is a Gallatin City-County Health Department question and it should be answered before design, not after. A system upgrade is a significant cost and it may also constrain where the building can sit.
Related
Other buildings people compare this with
Garage With Loft
Storage or bonus space over the bays. A different roof, a different stair and a different structure from a single-storey build.
What is involved →Detached Garages
Two, three and four-car detached garages built on their own footings, sized to the lot and the setbacks rather than to a catalogue.
What is involved →Garage Slabs & Concrete
Footings, frost walls, slabs and aprons. The part of the build that is impossible to fix later.
What is involved →Next step
Send us the site, not just the size
An address and a rough footprint is enough to start. What the ground and the jurisdiction allow is usually the part that decides the project.