Accessory Apartment Design Rules in Columbia Falls

Thinking about adding an accessory apartment to your Columbia Falls property? With Montana’s 2024 ADU law in effect, some local rules you may have heard about have changed. You want clear, current guidance so you can design something that passes inspection and adds real value. In this guide, you’ll learn what state law allows, what the City’s page still says, and the practical steps to design and permit your unit with confidence. Let’s dive in.

What changed statewide in 2024

Montana’s ADU statute makes at least one accessory dwelling unit allowed “by right” on any lot with a single-family home. That means no public hearing or conditional use permit if you meet the objective rules. Key points under Montana Code § 76-2-345:

• ADUs can be internal, attached, or detached.
• Size cap for attached or detached units: the lesser of 75 percent of the primary home’s floor area or 1,000 sq ft.
• Municipalities cannot require extra parking, owner-occupancy, matching exterior materials, impact fees, or stricter setbacks than the primary home.
• Cities can charge a one-time ADU application fee up to 250 dollars and still require standard building permits and inspections.
• Cities may require will-serve letters for municipal water and sewer.

Columbia Falls rules at a glance

The City’s “Accessory Apartment” page lists several standards for accessory apartments, including an 800 sq ft maximum, one off-street parking space, only one unit per lot, detached units placed behind the front of the house, a minimum one-month rental unless permitted for vacation rental, and required building, electrical, and plumbing permits. See the City’s page for details: Accessory Apartment standards.

Where this may conflict with state law

• Parking: The City page lists one extra space. The statute says cities cannot require additional ADU parking. See MCA § 76-2-345.
• Size: The City posts 800 sq ft. State law allows up to 75 percent of the primary home or 1,000 sq ft, whichever is less.
• Permit type: The City page references “conditional use.” The statute requires at least one ADU to be allowed by right.

If local text has not been updated, the state statute controls. For the latest local practice, contact Columbia Falls Planning & Zoning and ask what rules they are applying today.

ADU types and siting

• Internal: Basement or attic apartments within the existing home.
• Attached: A new addition that shares a wall with the house.
• Detached: A separate structure or garage conversion. The City directs detached units to be behind the front of the house. See the City’s Accessory Apartment standards.

State law prevents ADU setbacks, height, or lot coverage from being more restrictive than the primary home’s standards. Confirm your zoning district standards and setbacks with City staff to avoid rework.

Size and layout essentials

Under state law, the maximum size for attached or detached ADUs is the lesser of 75 percent of the main home or 1,000 sq ft. The City page lists an 800 sq ft cap, so confirm which figure applies to your permit. You will need building plans that show cooking, sleeping, and sanitation, along with safe egress, smoke and CO detection, and compliant ceiling heights. Permits and inspections are handled through the City’s building program. Start here: Building & Code Enforcement.

Utilities and access

If you plan to connect to City water or sewer, the City may require will-serve letters. If you are on well and septic, check capacity and approvals with the health department before you build. The statute allows cities to ask for will-serve letters where applicable. See MCA § 76-2-345.

Parking and circulation

Although the City page lists one extra off-street space, state law says cities cannot require additional ADU parking. Even without a mandate, plan practical access and avoid blocking driveways or sidewalks. If your neighborhood has private covenants, check them. Review both the City’s guidance and the statute: Accessory Apartment standards and MCA § 76-2-345.

Rental rules and occupancy

Montana law prohibits cities from requiring owner-occupancy for an ADU or the main home. Columbia Falls’ page states accessory apartments must be rented at least one month unless you obtain a vacation-rental permit under Chapter 18.445. Confirm the current policy with the City before planning short-term use. Start with the City’s Accessory Apartment standards and the statute at MCA § 76-2-345.

Step-by-step permitting checklist

1. Confirm zoning and rule set

• Call or email Planning & Zoning. Ask whether Columbia Falls is applying the state ADU law by right and what size and parking standards they use today.

2. Plan utilities

• If on City systems, request will-serve letters early. If on septic, talk with the health department about capacity and approvals.

3. Choose your ADU type and site

• Internal, attached, or detached. Show the location, access, and setbacks on a simple site plan. Detached units should be behind the front of the house per the City page.

4. Prepare drawings

• Floor plans, elevations, egress, smoke and CO detectors, and code details that building officials expect to see.

5. Apply for permits

• Submit building, electrical, mechanical, and plumbing applications through the City. Check the current fee schedule and the one-time ADU application fee allowance under state law. Start here: Permits & Applications.

6. Schedule inspections

• Follow the inspection schedule and do not occupy the unit until final approvals are issued. See Building & Code Enforcement.

7. Decide on rental use

• Long-term rentals typically need no special permit, but short-term use may require an administrative vacation-rental permit. Confirm with the City.

Common pitfalls to avoid

• Assuming the extra parking space is required when state law forbids it.
• Designing to 1,000 sq ft without checking the 75 percent cap relative to your home.
• Forgetting will-serve letters or septic approvals until late in the process.
• Overlooking HOA covenants or private restrictions that can limit ADUs.
• Starting work before you have issued permits and an inspection plan.

Local context

Montana’s 2023 reform was designed to expand housing options and reduce red tape. Cities across the Flathead Valley have been updating programs since then. For regional context, see local reporting on the state reforms in the Flathead Valley: Legislature cut red tape to increase housing availability. You can also review the bill history at the state site: SB 528 bill page.

Ready to plan your ADU?

If you want a second set of eyes on value, design choices, and rental potential across Northwest Montana, reach out to Erin Gilley for local guidance that blends practical planning with clear next steps.

FAQs

What does “by right” ADU mean in Montana?

• It means you can build one ADU on a single-family lot without a public hearing or discretionary permit if you meet objective rules, per MCA § 76-2-345.

What is the ADU size limit in Columbia Falls?

• The City page lists 800 sq ft, while state law allows up to 75 percent of the home or 1,000 sq ft, whichever is less. Ask Planning which limit they are applying today.

Can Columbia Falls require an extra parking space for an ADU?

• The state statute forbids additional parking requirements specific to ADUs, even though the City page lists one extra space. Confirm current practice with Planning.

Are short-term rentals allowed in ADUs in Columbia Falls?

• The City page requires a one-month minimum stay unless you obtain a vacation-rental permit under Chapter 18.445. Confirm the latest process with the City.

Do I need a will-serve letter for water and sewer?

• The statute allows cities to require will-serve letters for municipal water and sewer. Ask the City early in your process.

Does Montana require owner-occupancy for ADUs?

• No. The statute prohibits municipalities from requiring that the main home or the ADU be owner-occupied, per MCA § 76-2-345.