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Story County, IA Accessory Structures: ADU Rules (2026)

Some Restrictions

Compiled from the official code textEditor Martyn O'NeillLast verified September 2026

ADU rules in Story County, IA, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.

Key Facts

Zoning permit: Required before ADU construction or conversion
Max size: 1,200 square feet gross floor area
Per parcel: Only one ADU allowed
Distance limit: Generally within 200 feet of main dwelling
Septic permit: Required from Environmental Health before zoning permit
Land division: Splitting ADU onto separate parcel is prohibited

ADU Eligibility Quick-Check

An Accessory Dwelling Unit (ADU) is a smaller, independent living unit on the same lot as a single-family home: converted garages, detached cottages, basement or attic conversions, and attached additions all qualify. Most states have streamlined ADU permitting since 2020, but local rules still control size, height, and setback.

Is your lot zoned single-family or multi-family residential?
Does your lot have at least 1,200 sq ft of buildable space?
Is your existing home owner-occupied (in most states this is not required, but some require it for ADU permits)?
Will the ADU stay under your state's size cap (commonly 850-1,200 sq ft for detached)?
Can you provide one off-street parking space (waived in many states within 0.5 mi of transit)?

If you answered 'yes' to most of these, you likely qualify in Iowa under recent ADU streamlining laws. The Story County, IA rules below cover local size, height, and setback requirements.

Summary

An accessory dwelling unit on unincorporated land in Story County needs its own zoning permit, its own 911 address, and its own approved septic permit before that zoning permit will issue. Section 86.04(3)(L) caps a converted or new ADU at 1,200 square feet, limits a parcel to one ADU, and requires it sit within 200 feet of the existing single-family home.

These county ordinances apply to unincorporated areas of Story County. Incorporated cities within the county may have their own rules that take precedence over county-level regulations.

L. An accessory dwelling unit conforming to the requirements specified herein: (1) Zoning Permit Required. A zoning permit shall be submitted for any accessory dwelling unit, including when an existing single-family dwelling or accessory structure is modified to create an accessory dwelling unit therein. (2) Wastewater and Water facilities. The accessory dwelling unit shall be provided with wastewater and water facilities. A septic permit application shall be required to be submitted to the Environmental Health Department for all accessory dwelling units ... prior to issuance of the zoning permit for an accessory dwelling unit. (3) 911 Address Required. The accessory dwelling unit shall have an established 911 address that is separate from the 911 address of the existing single-family dwelling. ... (4) Size and Other Design Standards. a. The accessory dwelling unit shall not exceed 1,200 square feet in gross floor area. This does not include finished or unfinished basements. ... ... c. There shall not be more than one accessory dwelling unit per parcel, tract, or lot.

View official code

Full Breakdown

04(3)(L) treats an accessory dwelling unit as a conforming accessory use in the A-1 Agricultural District, but only once several conditions are met. A zoning permit is required for any ADU, including one created by converting an existing single-family dwelling or accessory structure, per subsection (1). Before that zoning permit can issue, subsection (2) requires a septic permit application to the Story County Environmental Health Department covering wastewater and water facilities, and subsection (3) requires a separate 911 address, distinct from the main house's address, also established before the zoning permit issues. 7 square feet. Subsection (6) separately blocks dividing the land to place the ADU on its own parcel apart from the primary dwelling.

Violations & Fines

Building or converting an ADU without the required zoning permit, septic approval, or 911 address, or exceeding the 1,200 square foot cap or the one-ADU-per-parcel limit, is enforced under Chapter 93 like any Land Development Regulations violation: a written Notice of Violation gives 30 days to correct, and an uncorrected violation becomes a County infraction under Section 93.03 carrying a civil penalty of up to $750 for a first offense and up to $1,000 for each repeat offense.

Frequently Asked Questions

How big can an accessory dwelling unit be in unincorporated Story County?

Section 86.04(3)(L)(4) caps an accessory dwelling unit at 1,200 square feet of gross floor area, not counting a finished or unfinished basement, and limits any separate garage built just for the unit to 600 square feet, while requiring paved or gravel access to its parking unless it shares parking with the main house.

Can I build more than one ADU on my property?

No. Section 86.04(3)(L)(4)(c) allows no more than one accessory dwelling unit per parcel, tract, or lot, and Section 86.04(3)(L)(6) separately bars dividing the land to put the ADU on its own parcel apart from the main single-family dwelling.

Do I need a septic permit before I can get a zoning permit for an ADU?

Yes. Section 86.04(3)(L)(2) requires a septic permit application to the Story County Environmental Health Department for every accessory dwelling unit, including one created by converting an existing structure, and that approval must come before the county issues the zoning permit.

How far can an ADU sit from the main house?

Section 86.04(3)(L)(4)(e) generally caps the distance at 200 feet from the existing single-family dwelling, though the county allows a greater distance when needed to avoid the septic system and its setbacks or to protect critical natural resource areas on the C2C Future Land Use Map.

Sources & Official References

[1]Story County Code of Ordinances (American Legal Publishing)

Other rules in Story County

Fence Regulations: Height LimitsSome Restrictions Shed & Carport RulesSome Restrictions Garage ConversionsSome Restrictions Noise Ordinances: Amplified Music & Outdoor SoundSome Restrictions Noise Ordinances: Barking DogsSome Restrictions Parking Rules: Abandoned VehiclesSome Restrictions Fence Regulations: Retaining WallsSome Restrictions Animal Ordinances: Pet LimitsSome Restrictions

All Story County rules

How Story County compares: Easiest Cities to Build an ADU·Compare Story County to another location·View the Iowa accessory structures overview

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