Jefferson County, IL zoning map, flood zone and property lines by address
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Towns in Jefferson County: Mount Vernon, Ina.
Flood zone by address
ParcelSheet reads FEMA's National Flood Hazard Layer at the lot: the zone, whether it is a high risk area where lenders ask for flood insurance, and the base flood elevation where FEMA lists one. FEMA's countywide study for Jefferson County has 25 FIRM panels, the newest effective September 2010; no coastal V zones are mapped in it.
The official panel and any Letter of Map Amendment are at FEMA's Map Service Center. An elevation certificate for a Jefferson house comes from a licensed surveyor and is often on file with the county or city building office.
Property lines
The parcel outline, parcel number and mapped size come from the Illinois county parcel layers (each county publishes its own), so a Jefferson lot shows the same lines the county supervisor of assessments draws. Jefferson County covers about 365,591 acres of land and 8,042 acres of water.
Mapped lines are approximate. The Jefferson County supervisor of assessments keeps the parcel record (Jefferson County website); a licensed surveyor sets the true corners.
Zoning
Jefferson County zones its unincorporated land, and Mount Vernon, Ina each zone the land inside their limits. ParcelSheet shows the use the county supervisor of assessments lists for the lot and links the county zoning office for the district, the uses it allows and the lot size it needs.
ADU and guest houses
Under Illinois's county zoning law (Counties Code, 55 ILCS 5/5-12001 and following), local governments can allow an accessory dwelling unit on a lot with a house. Whether a Jefferson lot can add one depends on its district; the county zoning office has the rule. ParcelSheet shows the mapped lot size and the ground's slope, the first two things the question turns on.
Septic and soil
ParcelSheet reads the USDA soil survey at the lot: the soil's name, its septic absorption rating and whether it is prime farmland. Jefferson County is soil survey area IL081, Jefferson County, Illinois, last updated August 2026, on Web Soil Survey.
A new or changed septic system in Jefferson County needs a permit from the Jefferson County health department (Illinois Department of Public Health private sewage program; IDPH itself where the county has no health department).
Setbacks
Setbacks in Jefferson County come from the zoning district: the front, side and rear distances a house must keep. ParcelSheet does not draw setback lines; the county zoning office gives them for the lot's district, and Mount Vernon and Ina for land inside their limits.
Hazards that apply
- Wetlands and wildfire. ParcelSheet shows the National Wetlands Inventory near a Jefferson lot and the Forest Service wildfire hazard rating at it.
Questions
Is my house in a flood zone in Jefferson County?
Type the address in the box above and ParcelSheet reads FEMA's flood zone at the lot's center. FEMA's countywide flood map for Jefferson County has 25 panels, the newest effective September 2010. A lender or insurer uses the official FIRM panel; FEMA's Map Service Center has it.
How do I find property lines in Jefferson County?
Open the Illinois map, type the address, and the parcel outline from the Jefferson County supervisor of assessments shows on the map with its parcel number and mapped size. Mapped lines are approximate; only a licensed surveyor sets the true corners.
What is my property zoned in Jefferson County?
Zoning in Jefferson County is set by the county for unincorporated land and by each city (such as Mount Vernon, Ina) inside its own limits. ParcelSheet shows the use code the county supervisor of assessments lists and links the office that sets the zoning; the planning office has the final word.
Can I build an ADU in Jefferson County?
Under Illinois's county zoning law (Counties Code, 55 ILCS 5/5-12001 and following), counties and cities can allow an accessory dwelling unit, and each sets its own rules. In Jefferson County ask the zoning office for the lot's district first; ParcelSheet shows the lot size as mapped so you can see the room you have.
Do I need a septic permit in Jefferson County?
Yes. A new or changed septic system in Jefferson County needs a permit from the Jefferson County health department (Illinois Department of Public Health private sewage program; IDPH itself where the county has no health department). ParcelSheet shows the USDA soil survey's septic rating at the lot, from survey area IL081 (Jefferson County, Illinois).
What can I build on 5 acres in Jefferson County?
Zoning in Jefferson County is set by the county for unincorporated land and each city inside its own limits (such as Mount Vernon, Ina). The zoning district decides the uses, how many homes and the smallest lot it allows on 5 acres. ParcelSheet shows the lot size as mapped and the use code on the parcel record; the planning office has the district and its rules. See the Jefferson County website.
What are the residential setbacks in Jefferson County (2026)?
Setbacks in Jefferson County come from the zoning code of the county, or of the city when the lot is inside city limits: the front, side and rear distances depend on the lot's district. ParcelSheet does not draw setback lines or give a number it did not read; the zoning office gives them for the district. See the Jefferson County website.
Next to Jefferson County: Franklin County, Hamilton County, Marion County, Perry County, Washington County.
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