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St. Louis, MO Accessory Structures: ADU Rules (2026)

Some Restrictions

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

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

Key Facts

Max height: 25 feet
Max size: 1,200 sq ft
Units per parcel: one (attached or detached)
Side setback: 3 feet minimum
Off-street parking: not required for ADU

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 Missouri under recent ADU streamlining laws. The St. Louis, MO rules below cover local size, height, and setback requirements.

Summary

St. Louis allows one attached or detached accessory dwelling unit per residential parcel, capped at 25 feet in height and 1,200 square feet, and set back at least 3 feet from side lot lines under the zoning code's 2025 ADU standards.

…2.Height.a.The height shall not exceed twenty-five (25) feet.3.Area.a.An accessory dwelling unit may be up to fifty (50) percent of the area of the largest primary dwelling unit on the parcel or seven hundred fifty (750) square feet, whichever is greater.b.An accessory dwelling unit shall be no more than one thousand two hundred (1,200) square feet.4.Density of population.a.Only one (1) accessory dwelling unit shall be permitted, per parcel.

Source: St. Louis Planning & Urban Design AgencyView official code

Official source re-checked September 7, 2026: no newer edition of the code had been published (publisher’s edition: Code of Ordinances: Supplement 11).

Full Breakdown

Ordinance No. 72036 (adopted July 11, 2025) added accessory dwelling units to the "A" Single-Family Dwelling District use list and set site requirements at St. Louis Revised Code § 26.20.021. An ADU can't exceed 25 feet in height or 1,200 square feet, and its footprint is capped at 50% of the largest primary dwelling on the parcel or 750 square feet, whichever is greater. Only one ADU is allowed per parcel, it can't be built beyond the average front yard line, and it must stay at least 3 feet from side lot lines. A detached ADU also needs at least 10 feet of separation from the rear wall of the primary dwelling. No extra off-street parking is required for the ADU itself, but it can't remove parking required for the primary unit.

Violations & Fines

Building an ADU that doesn't meet these standards is a zoning-code infraction punishable under Chapter 1.12's general penalty (§ 26.100.010); the City Counselor can also sue to force compliance under § 26.100.020.

Frequently Asked Questions

Can I build a detached ADU behind my house in St. Louis?

Yes, if it meets § 26.20.021: under 25 feet tall, within the size caps, at least 3 feet from side lines, and at least 10 feet from the primary dwelling's rear wall.

Do I need to add parking for a new ADU?

No. § 26.20.021(A)(1) says off-street parking isn't required for an ADU, but you can't remove parking already required for the primary dwelling.

How many ADUs can one lot have in St. Louis?

Only one, per § 26.20.021(A)(4): adding it doesn't change the density regulations for the primary use.

Sources & Official References

[1]St. Louis Revised Code (Municode)

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How St. Louis compares: Easiest Cities to Build an ADU·Compare St. Louis to another location·View the Missouri accessory structures overview

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