5 county-level rules, plus city-specific rules for 1 city in Johnson County, Iowa.
Verified from official government sources
Correcting a common belief: since July 1, 2025 Iowa has a statewide ADU mandate. Senate File 592 requires Johnson County and every city to allow at least one accessory dwelling unit on a single-family lot, with no owner-occupancy condition.
Iowa Code §331.301(27) / §364.3(20) (accessory dwelling units, Senate File 592, 2025)
An accessory dwelling unit shall not exceed one thousand square feet or fifty percent of the size of the single family residence, whichever is larger.
In unincorporated Johnson County a detached shed of 200 square feet or less needs no building permit, but it must still sit in the rear yard behind the house and meet the zoning setbacks.
Johnson County Planning, Development & Sustainability, Building Permits FAQ (2024 International Residential Code exemption)
A permit is not required to construct a detached building that is 200 square feet or less. This permit exemption is only applicable to a one-story accessory structure used as a tool shed, storage shed, playhouse or similar use.
Converting a garage into living space in Johnson County is a change of occupancy that needs a building permit. Inspectors check egress, insulation, and smoke and carbon-monoxide alarms.
Johnson County requires a building permit for a carport. It counts as an accessory structure, must meet setbacks and lot coverage, and has to carry Iowa snow and wind loads.
How Johnson County treats a tiny home depends on its foundation. Built on a permanent foundation it is a dwelling under the building code; on wheels it is generally an RV or manufactured home.
1 cities in Johnson County have their own accessory structures rules. Each link goes to that city's dedicated page with code citations.
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Johnson County Ordinance Hub →