Building permits, zoning and land rules in Mercer County, Illinois

Unincorporated Mercer County requires a building permit from Mercer County Zoning; ask the office which projects and exemptions apply^1,3. Inside Aledo, the city handles building permits, including an all-inclusive permit, construction drawings and a survey for new homes⁷. Septic permits remain with the Mercer County Health Department countywide under state regulation^4,5. County building-permit applications must show lot dimensions and proposed structure locations³. The county building-permit fee is $25 plus $5 per $1,000 of improvement cost, with a separate $25-per-foot tower or turbine charge³. Agricultural uses are exempt from the county zoning ordinance's general permit and certificate provisions, but applicable septic and state rules still apply².

Mercer County Zoning handles county building-permit and zoning applications at 100 SE 3rd St, Aledo, IL 61231³. Call 309-525-3799 or email zoning@mercercountyil.org; the part-time office asks callers to allow up to 48 hours on weekdays for a response³. Applications may be emailed, but fees must be mailed, and the office mails the building permit after payment arrives³. Illinois statewide baseline building-code requirements took effect in 2025; ask the permitting jurisdiction which local amendments and code apply⁸. The Illinois Energy Conservation Code covers new buildings with county or municipal building-permit applications, subject to statutory exemptions; additions and repairs are covered only in the affected portions⁹. We could not confirm Mercer County's inspection schedule or full certificate-of-occupancy requirements; ask Mercer County Zoning³.

Mercer County zoning districts apply outside incorporated municipalities; for land inside municipal limits, contact the city or village hall^2,3. Matherville has its own village zoning regulations¹⁰. Private septic systems inside Matherville still go through the Mercer County Health Department⁵. Complete county zoning applications must reach the zoning officer at least 30 days before the zoning-board meeting; zoning changes involve notice, zoning-board review and County Board action³. The first land split may be any acreage without a subdivision plat; after that, parcels under 5 acres require a plat, with a $300 platting-officer fee³. Ask Mercer County Zoning about rezoning separately from platting; the county publishes a separate acreage-based rezoning statement³. Obtain the parcel's district-specific lot, setback and height rules before designing a home¹¹.

The Mercer County Health Department handles septic permits countywide under Illinois Department of Public Health regulation⁴. Construction or repair of an onsite wastewater system requires a septic permit⁵. Arrange an onsite soil evaluation with at least 3 borings by a certified soil classifier, protect the system area from compaction, and contact a licensed Mercer County private sewage disposal contractor unless homeowner installation applies⁵. Submit the application, soil report and fee to the health department; contractor installation costs $200 and homeowner installation costs $600, and the septic permit lasts 1 year⁵. Surface-discharging systems also require an Illinois EPA NPDES discharge permit⁵. About 41.4% of county soils rate as free of severe limitations for a conventional drain field, so complete the soil evaluation before fixing the house site¹².

Mercer County's R-3 Seasonal Home District allows travel trailers and camping trailers only as a conditional campground use, with occupancy limited to 30 days in a calendar year; mobile homes are excluded from that campground use². That narrow rule does not establish that residential RV living is allowed on an ordinary private parcel, so ask Mercer County Zoning about the parcel's district, proposed use and length of stay³. We could not confirm a rule written specifically for tiny homes; ask Mercer County Zoning about the proposed dwelling³. Manufactured homes follow district-specific rules: R-3 seasonal homes and R-4 homes require individual building permits and at least 600 square feet, while specified AG-1 farm dwellings and hardship placements require conditional-use permits and at least 825 square feet².

1. Establish whether the parcel is unincorporated or inside municipal limits, then ask Mercer County Zoning or the city or village hall for the applicable district^2,3.
2. Before buying a split parcel, ask Mercer County Zoning about subdivision platting and any separate rezoning requirement³.
3. Prepare lot dimensions and structure locations for a county building-permit application; zoning applications need at least 30 days before board review³.
4. Arrange soil evaluation, then submit the septic application, soil report and fee to the Mercer County Health Department⁵.
5. Apply for the building permit through the governing jurisdiction and confirm applicable codes, including energy requirements^1,3,9.
6. Obtain an IDOT driveway permit before access work in a state-maintained road right-of-way¹³.

Before buying in Mercer County, establish the governing jurisdiction and zoning district with Mercer County Zoning or the applicable city or village hall^2,3. Ask the zoning office about land-split platting and separate rezoning requirements rather than treating acreage alone as approval to build³. Order the septic soil evaluation early: about 41.4% of county soils rate as free of severe limitations for a conventional drain field¹². A new, modified or sealed private well needs a permit before work begins⁶. Mercer County's effective property-tax rate is about 1.9%, with a median annual bill near $2,710¹².