Illinois Statewide Rule

Illinois Floodplain Construction Permit Statewide Standards

Heavy RestrictionsApplies statewide across Illinois (2026)

Key Facts

Statute: 615 ILCS 5/
Rule: 17 Ill. Adm. Code 3700-3708
Permit agency: IDNR Office of Water Resources
NE IL elevation: 1 foot above BFE
Floodway: State permit required

Last verified: September 5, 2026Source: Illinois General Assembly

Summary

Illinois law requires state-issued permits for construction within regulatory floodways and mandates municipal participation in NFIP minimum standards across all designated flood hazard areas.

Sec. 18f. (a) The Department of Natural Resources shall define 100-year floodplains within the State of Illinois on a township by township basis and may issue permits for any construction within such 100-year floodplains on or after the effective date of this amendatory Act of 1971. The Department shall publish and distribute suitable reports, together with mapping and hydrologic exhibits pertaining to 100-year floodplains defined and established under this Act. In defining applicable 100-year floodplains, the Department shall cooperate with, and shall consider planning and zoning requirements of, regional planning agencies created by statute, counties, municipalities and other units of government. A period of thirty days shall be allowed for any agency to submit written comments to the Department regarding any proposed 100-year floodplain area. If such agency fails to return comments to the Department within the specified time period the Department may proceed with the publication and institution of the 100-year floodplain permit procedure.

Source: Illinois General AssemblyView official code

Full Breakdown

Under the Rivers, Lakes, and Streams Act (615 ILCS 5/) and 17 Ill. Adm. Code 3700-3708, IDNR Office of Water Resources requires permits for construction in regulatory floodways statewide. NFIP-participating communities must enforce minimum FEMA standards, including elevating residential structures one foot above base flood elevation in northeastern Illinois counties. The Urban Flood Awareness Act and statewide statute establish floor elevation, anchoring, and floodway encroachment rules that local floodplain ordinances cannot weaken.

Violations & Penalties

Building in a regulatory floodway without an IDNR permit, placing residential structures below required base flood elevation, or filling floodways without compensatory storage.

Frequently Asked Questions

Do I need a state permit to build in an Illinois floodway?

Yes. The IDNR Office of Water Resources requires a permit under 17 Ill. Adm. Code 3708 for construction, fill, or modifications within regulatory floodways anywhere in the state.

How high must homes be built in Illinois flood zones?

NFIP minimums require the lowest floor at or above base flood elevation. In northeastern Illinois counties under 17 Ill. Adm. Code 3708, residential structures must be elevated at least one foot above BFE.

Can a city allow building in a floodway in Illinois?

No. Local governments cannot waive IDNR floodway permits or NFIP minimum standards. Municipalities may adopt stricter rules but must enforce state and federal floodplain requirements.

Sources

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