Property in a flood zone
Floodplain designation affects insurance, buildability and price. It should affect assessment.
A parcel in a mapped floodplain carries costs and restrictions that a parcel outside it does not.
What it does to value
Mandatory flood insurance for a mortgaged property, restrictions on building and substantial improvement, difficulty at resale, and in floodway areas potentially no development at all. All of that shows up in sale prices.
Evidence
The FEMA flood map panel showing your parcel. An elevation certificate. Insurance premium documentation. Photographs of actual flooding, dated. And most persuasively, sales of comparable properties inside and outside the zone.
Partial-parcel floodplain
Common and frequently mis-assessed. If a third of your lot is in the floodway and unbuildable, that third is not worth the same per square foot as the buildable remainder. A survey overlaid with the flood map makes the argument visually.
After a flood
Report damage to the assessor immediately with photographs. Do not wait for the notice. Illinois also provides a natural disaster homestead exemption for rebuilt residences in defined circumstances — ask the Supervisor of Assessments.