O'Fallon, IL Zoning, Building Permits & Septic Rules

Inside O'Fallon city limits in St. Clair County, Illinois, the city's Community Development Department issues building permits¹ and enforces the city's own zoning ordinance, Chapter 157 of the Code of Ordinances². Septic permits are a separate matter: the St. Clair County Health Department administers private sewage permits countywide³. Whether a given O'Fallon lot may use septic at all, we could not confirm.

At a glance

Building permits: Issued by the City of O'Fallon Community Development Department for new homes, additions, decks and sheds over 100 sq ft¹
Zoning: City zoning ordinance (Chapter 157) with districts such as A, RR, SR-1, MH-1 and MR-1²⁴
Lot size, SR-1: 10,000 sq ft minimum, 75 ft wide⁵
Lot size, RR: 1 acre minimum⁶
Septic: St. Clair County Health Department permit, $300 fee; city sewer rules not confirmed⁷⁸
Wells: County health office permit, $200 fee, where the county program applies⁹
RV, tiny home, ADU: Not confirmed; ask the city's Planning & Zoning Division¹⁰

Who to call

Building permits

City of O'Fallon Community Development Department, Building & Inspections Division

Building permits, inspections, occupancy permits and floodplain questions inside O'Fallon city limits

618-624-4500

255 South Lincoln, O'Fallon, IL 62269

Online permits →

Zoning

City of O'Fallon Planning & Zoning Division

Zoning questions, zoning map lookups, site plans, subdivisions and home occupations inside O'Fallon

618-624-4500

255 S Lincoln Avenue 2nd Floor O'Fallon, IL 62269

Septic permits

St. Clair County Health Department – Water & Private Sewage

Private sewage (septic) permits and well permits for St. Clair County

618-233-7769

scchdinfo@co.st-clair.il.us

19 Public Square, Suite 150, Belleville, IL 62220

Contact details are copied from each office’s official page and were checked against it on October 24, 2026.

Off-grid gear

Going off-grid in O'Fallon means bringing your own water and power. The kit we’d start with:

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Does O'Fallon, Illinois have zoning, and what does it mean for lot size and RVs?

Yes. Inside O'Fallon city limits, the city's own zoning ordinance applies, and districts include A, RR, SR-1, MH-1, MR-1, MR-2, O-1, B-1, B-2, I-1 and I-2²⁴. The city explains that its zoning ordinance regulates what types of uses are permitted and how a property can be developed within a particular district¹⁰. Rules depend on the district, so find your lot on the city zoning map first.

In the SR-1 single-family district, the schedule lists a 10,000 sq ft minimum lot, 75 feet of width and a 35-foot height limit⁵¹¹. In the RR rural residential district, the minimum lot is 1 acre with 100 feet of width⁶. Lot coverage appears to be capped at 30% in SR-1 and 25% in RR, but we could not fully confirm that reading of the table¹²¹³.

We could not confirm O'Fallon's rules on living in an RV, tiny homes or accessory dwelling units. Ask the Planning & Zoning Division at 618-624-4500 before you buy¹⁰.

Do you need a building permit in O'Fallon, and who issues it?

Yes for most real construction. The City of O'Fallon, not St. Clair County, issues building permits inside city limits, through the Community Development Department's Building & Inspections Division¹. The city lists new one- and two-family homes, basement finishes, room additions, remodeling, decks, pools and sheds among the work that needs a building permit¹. The city says a building permit is required for sheds and accessory structures over 200 square feet, a zoning permit covers sheds of 101 to 199 square feet, and structures of 100 square feet or less need no permit but must still meet building codes and setbacks¹.

Property owners submit the building permit application to the planning and zoning department so the plans can be checked against zoning¹⁰. We could not confirm which building code editions the city has adopted, its permit fees or its owner-builder rules; ask the Building & Inspections Division.

How do septic permits work inside O'Fallon?

Septic permits do not come from the city. The St. Clair County Health Department administers the private sewage program for the county³, and a permit must be obtained from its office before constructing or repairing a private sewage system in St. Clair County⁸. The initial permit fee is $300⁷.

The Health Department must refuse a septic permit where a sanitary sewer is available, which includes a practical connection within 300 feet for a single-family home⁷. Because much of O'Fallon is built up, a lot near a city sewer main may have to connect rather than install septic. We could not confirm the city's sewer-connection rules, so ask the city's Public Works office and the Health Department before counting on a septic system.

If a lot does use private sewage, the county code calls for one acre of usable space for a residential structure, and the Health Department may require more¹⁴.

Building a home in O'Fallon: the steps, in order

Look up your lot's zoning district and confirm the use, lot size and height limits with the Planning & Zoning Division¹⁰.
Ask the city whether the lot has sewer and water available, and call the St. Clair County Health Department if it does not⁸.
Submit the residential construction building permit application to the city, which is used for new home construction¹.
Get a county well permit if you plan a private well, where the county program applies⁹.
Schedule inspections and the final occupancy approval through the Building & Inspections Division¹.

What stays with St. Clair County, and what to check before you buy

The county's building and zoning rules cover unincorporated land only; inside O'Fallon, the city handles building permits and zoning¹². Septic and well permits stay with the county Health Department³⁹. For land outside city limits, read the St. Clair County page.

Before you buy, confirm the zoning district, sewer availability, and any floodplain issue; the city's Building & Inspections Division handles floodplain management questions¹. St. Clair County's effective property tax rate is 2.1%¹⁵.

Watch-outs

We could not confirm O'Fallon's building code editions, permit fees, owner-builder rules or RV and tiny-home rules; call the Building & Inspections Division at 618-624-4500¹.
Setback and lot-coverage figures for SR-1 and RR come from a table whose columns we could not fully verify, so check them with the Planning & Zoning Division before you design¹⁶.
A septic permit can be refused where a sanitary sewer is available within 300 feet of a single-family home⁷.

Sources

Written October 24, 2026 from the rules quoted below, each taken from an official source and re-checked by an independent reviewer. This is research, not legal advice: rules change, so confirm with the office before you buy or build.

Unanswered questions are usually a sign that O'Fallon publishes no rule on the point, not that none exists. St. Clair County’s page lists the county offices to call.

Zoning, building permits and septic rules in O'Fallon, Illinois