Decks near the Gallatin River: setbacks and floodplain permits

If your lot sits along the Gallatin River or a side creek, the deck you picture may not fit where you picture it. Two sets of rules apply near water on the Gallatin County side of Big Sky: zoning setbacks and floodplain regulations. They come from different documents, and a lot can be subject to one, both, or neither.

The zoning setback

The Gallatin Canyon/Big Sky Zoning Regulation sets watercourse setbacks in Section 30. Section 30.4.a reads:

"All Structures and site improvements shall maintain a minimum Setback of 100 feet from the annual mean high water line of the main branch of the Gallatin River and 50 feet from the annual mean high water line of all other Watercourses."

A few points in that sentence matter for a deck:

• "Structures and site improvements." The rule covers more than houses. Plan as if a deck, its stairs, and a patio all have to stay outside the line, and confirm the details with the Planning Department.
• "Annual mean high water line." The setback is measured from that line, not from the edge of the water on the day you walk the lot, and not from the property line.
• Two distances. 100 feet from the main branch of the Gallatin River, and 50 feet from other watercourses. A small creek crossing a lot still carries a 50-foot setback.

When Section 30 applies, and what it asks for

Section 30.2 applies the watercourse rules to development within 150 feet of a watercourse. If your project falls inside that distance, Section 30.3 calls for a survey, a site plan, and grading and erosion-control plans with the application.

That survey answers the question most river-lot owners have: where can the deck go? A surveyor locates the annual mean high water line and draws the setback lines on the site plan. Without it, any deck layout near the water is a guess.

The county's Land Use Permit instructions already require site plans to label outdoor amenities such as decks, porches, and hot tubs, along with setbacks. On a river lot, those lines carry the most weight.

Replacing a deck that is already too close

Older homes along the canyon may have decks inside today's setback. If you plan to replace or enlarge one, ask the Gallatin County Planning Department how the regulation treats it before you design anything. The answer for a same-size replacement can differ from the answer for an expansion, and this guide does not try to predict it.

Floodplain permits

Floodplain rules are separate from zoning. Gallatin County's Floodplain Hazard Management Regulations, Ordinance 2023-004, took effect May 4, 2023. The county's floodplain page says permits are needed for "grading, excavation, placement of fill, bank stabilization work" and for "construction or placement of structures" within the regulatory floodplain.

The regulatory floodplain comes from FEMA's Flood Insurance Study and rate maps for the county, effective April 21, 2021. You can look up a parcel on FEMA's Flood Map Service Center. The county has also posted channel migration studies for major rivers, which show where a river channel may move over time.

This guide did not review the ordinance's deck-specific language. If any part of your lot is in a mapped floodplain, call the Gallatin County Planning Department at 406-582-3130 before design starts, and ask whether the deck, its footings, and any grading need a floodplain permit.

On the Madison County side, the current rules are Ordinance 1-2024, the county's Floodplain Development Regulations, which replaced the 2017 ordinance. Madison County's floodplain page links to them.

Wildfire rules still apply

River corridors are forested, and the same zoning regulation covers wildfire. Section 23.10 requires at least five feet of fire-resistant ground cover around the exterior walls of all structures. Plan the ground beside the deck with that strip in mind.

A sequence for river lots

1. Check FEMA's flood map and the county's channel migration studies for your parcel.
2. Have a surveyor locate the annual mean high water line and draw the 100-foot and 50-foot setback lines.
3. Fit the deck, stairs, and any patio outside those lines.
4. Ask the Planning Department about the Land Use Permit and any floodplain permit, with the survey in hand.
5. Then move to structure: snow load and footing depth.

For the full list of approvals, see who approves a deck around Big Sky. When you have a survey, mention it in your deck request.