Boise, ID Environmental Rules: Flood Elevation Requirements (2026)
Key Facts
- Freeboard
- 2 feet above base flood elevation
- Applies in
- Floodway Fringe & shallow-flood zones
- Certification
- Licensed engineer or land surveyor
- Permit needed
- Floodplain Development Permit
- Enforced by
- Planning Director / Building Official
Summary
Boise requires new and substantially improved homes in the Floodway Fringe to elevate their lowest floor, including basements and crawl spaces, two feet above the base flood elevation.
New construction and substantial improvement of any residential structure (including but not limited to Manufactured Homes) located in the Floodway Fringe shall have the lowest floor including basements and crawl spaces, elevated two feet above the base flood elevation.
B. New construction and substantial improvement of any residential structure located in the Area of Shallow Flooding (AO Flood Zone) shall have the lowest floor, including basements and crawl spaces, elevated one foot above the flood depth specified on the FIRM measured at highest adjacent grade, or at least two feet above highest adjacent grade when a flood depth is not identified. The height of the adjacent grade and the lowest floor including basement and/or the first floor shall be certified by a licensed professional engineer or registered land surveyor.
C. Fully enclosed areas below the lowest floor that are subject to flooding shall be designed to automatically equalize hydrostatic flood forces on exterior walls by allowing for the entry and exit of floodwaters. Designs for meeting this requirement shall either be certified by a registered professional engineer or shall meet or exceed the following minimum criteria:
(i) A minimum of two openings having a total net area of not less than one square inch for every square foot of enclosed area subject to flooding shall be provided.
Official source re-checked September 7, 2026: no newer edition of the code had been published (publisher’s edition: 2026 S-31: Code current through: Ord. 20-26, passed 6-9-2026).
Full Breakdown
Boise's FP-O Flood Protection Overlay, part of the Development Code's zoning standards in Chapter 11-02, sets stricter elevation rules than FEMA's baseline. Within the mapped Floodway Fringe, any new residential structure -- including manufactured homes -- must have its lowest floor, basements and crawl spaces included, built two feet above the base flood elevation shown on the FEMA Flood Insurance Rate Map for Ada County. Nonresidential buildings get the same two-foot freeboard or must be flood-proofed to that height. A licensed professional engineer or registered land surveyor must certify the elevation, and the Planning Director enforces compliance through the floodplain development permit process.
Violations & Fines
Building without the required 2-foot freeboard is a Code violation enforced by the Planning Director; violations are misdemeanors under BCC § 1-4-1, with each day of noncompliance a separate offense.
Frequently Asked Questions
How much freeboard does Boise require above base flood elevation?
Does the rule apply to manufactured homes?
Sources & Official References
Other rules in Boise
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