Johnson County, IA Building Setbacks & Zoning: Lot Coverage Limits (2026)
Key Facts
- Lot coverage definition
- 8:1.4(131), all structures on the lot
- Accessory structure cap
- 30% of the yard occupied, all districts
- ADU rule
- Cannot exceed zone's lot coverage requirement
- Stormwater trigger
- 5,000 sq ft new impervious rooftop
- Site Plan exception cap
- 25% footprint / 1,200 sq ft impervious
Summary
Johnson County's Unified Development Ordinance defines lot coverage at Section 8:1.4(131) as the portion of a lot covered by all structures, and enforces it through a district-by-district accessory structure cap rather than a single countywide percentage. In the AR, A, SF, and every other unincorporated residential and commercial district, the equivalent Accessory Structure Regulations subsection caps accessory buildings at 30 percent of the yard they occupy.
131. Lot Coverage. The portion of a lot that is covered by all structures on the lot. ... 3. Accessory structures shall not occupy more than thirty (30) percent of the yard it occupies. ... The auxiliary dwelling unit shall meet the setback requirements of the primary dwelling unit and shall not cause the lot coverage requirement of the zone to be exceeded.
Official source re-checked September 7, 2026: the cited page had not changed since it was quoted.
Full Breakdown
4(131) that 'Lot Coverage' means the portion of a lot that is covered by all structures on the lot. Rather than setting one percentage ceiling for total lot coverage countywide, the ordinance applies coverage limits at the accessory-structure level inside each zoning district's own Accessory Structure Regulations subsection. 7(E)(3) caps accessory structures at 'thirty (30) percent of the yard it occupies,' and the identical 30 percent cap repeats verbatim in the equivalent accessory-structure subsections for the A, SF, RS, RM, RC, RMH, RR, C, CH, AG-T, C-AG, ML, MH, and P districts, covering every unincorporated zoning category.
24(E)(3) requires that an auxiliary dwelling unit 'shall not cause the lot coverage requirement of the zone to be exceeded,' tying the ADU's footprint back to whatever coverage standard already governs the parcel. 26(A)(5) caps footprint expansions processed without a new Site Plan at 25 percent of existing building footprint and 1,200 square feet of new impervious area. None of the district bulk regulation tables list a separate 'lot coverage' column; setbacks, lot width, and height are the tools used instead, with the 30 percent accessory-structure rule serving as the closest analog to a coverage cap.
Violations & Fines
Building an accessory structure beyond the 30 percent yard-coverage cap, or an auxiliary dwelling unit that pushes a lot past its zone's coverage limit, is a County Infraction under Section 8:8.3, carrying a civil penalty up to $750 for a first offense and up to $1,000 for a repeat violation, plus possible criminal penalties up to $625 and 30 days in jail under Section 8:8.4.
Frequently Asked Questions
Does Johnson County set one lot coverage percentage for the whole county?
How does an accessory dwelling unit affect lot coverage?
Is there a separate impervious surface limit?
Sources & Official References
Other rules in Johnson County
Compare Johnson County to another location·View the Iowa building setbacks & zoning overview
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