ADU Information

What is an Accessory Dwelling Unit? 

Common Types of ADUs

ADUs come in many shapes, sizes, and orientation. The most common form of ADU is one that is in or on top of an existing garage with alley access, but that is certainly not the only type that can exist or be permitted in St. Louis.

Detached ADUs

The most common ADU is the Detached ADU. These include dwelling units in new-built structures in a backyard or one built into or on top of an existing garage. Detached ADUs are physically separate from the primary structure on the parcel, but utilities like water and electricity are typically tied into the primary structure.

The official definition in the Zoning code reads: “A subordinate, independent dwelling unit on the same parcel as: a single-family dwelling, two-family dwelling, or semi-detached two-family dwelling, acting as an accessory use, separate from the principal structure. 

Attached ADUs

Attached ADUs are less common in St. Louis, but are also made legal through the recent zoning change. These include dwelling units that are in existing or newly-build structures that are attached to and on the same parcels as an existing residence, but are smaller and have a separate entrance.

The official definition in the Zoning code reads: “A subordinate, independent dwelling unit on the same parcel as: a single-family dwelling, two-family dwelling, or semi-detached two-family dwelling, acting as an accessory use, connected to but not within the principal structure.”

Interior/Basement ADUs

Some cities have permitted for even smaller ADUs that are totally interior to existing residences or structures. These may be called Junior ADUs or Basement ADUs. Currently these have not been defined or made legal in our Zoning Code. Under the existing code, a property owner could establish an additional dwelling unit in their existing structure, but it would be regulated as an additional dwelling unit, not an ADU.

How do I build an ADU?

All ADUs can be legally permitted in all residential zoning districts. The specific use regulations can be found in Chapter 26.20.021 of the Zoning Code.

Building an attached or detached ADU is similar to building any major construction project on your property. You need to seek permits from the Building Division. The Building Division will ensure that all elements of your plans conform with the building codes of the City of St. Louis. 

A review by the Zoning section is a part of the permitting process. If you are concerned whether or not your ADU plans conform with the zoning code, you can call the Zoning Section, or apply for a zoning-only permit.

Once your permit is approved, you can begin construction!

All ADUs must meet the following requirements:

• Parking: no off-street parking is required for an ADU, but if building an ADU removes required parking for the primary house, it must be replaced. This means that if you build an ADU in your garage in the “A” District, you must save or replace the required parking space for your house. If you are building an ADU where no garage previously existed, no need to incorporate a new parking space.

• Height: no ADU can be taller than 25 feet, regardless of how tall the primary house is. This includes ADUs built on top of a garage.

• Area (square footage of the ADUs): The square footage of the ADU can be up to 50% of the square footage of the primary house, or up to 750 sf, whichever is greater – but no ADU can be larger than 1200 sf. This means that smaller houses (less than 1500 sf) can still build an ADU up to 750 sf, but larger houses could not build ADUs larger than 1200 sf. This is the case even if the primary residence is more than 2400 sf and there is room on the lot to accommodate an ADU larger than 1200 sf.  

• Density: each parcel can only have one legally permitted ADU. This means that each parcel, no matter the zoning district, can only establish one ADU, detached or attached.

• Setbacks: no ADU can be built in front of the primary structure. All ADUs must be at least 3 ft from the side lot lines.

Detached ADUs have the additional regulations:

Setbacks: detached ADUs must be 10 ft or more from the primary structure

Nonconforming structures: an ADU can legally be built into, on top of, or on the foundations of an existing non-conforming structure. This means that if your current garage does not comply with the above area requirements, you can still put an ADU in or on it.

Frequently Asked Questions

What are the benefits of ADUs?

ADU’s can provide a number of benefits not only to property owners but other members of the local community as well. For qualifying parcels it can provide a tangible increase in usable square footage without requiring more costly renovation projects. This increase in square footage often leads to an increase in property values as well. ADUs can also provide greater flexibility in housing options and availability. The gentle increase in density that ADUs provide can help to reduce living costs and support local business while not drastically altering neighborhood dynamics. 

I own a duplex. Can I build an ADU in the back yard?

Yes! So long as there is not already an ADU on the parcel, and that you meet the area requirements.

I live in a historic district. Can I still build an ADU?

Yes, but you may need to adhere to specific design or site requirements in your historic district. Visit the Cultural Resources Office webpage to learn more about your historic district.

Can I rent out my ADU?

Yes! Renting out an ADU is a great way to offer affordable housing options and supplement your income.

Can I use my ADU as an AirBnB or Vrbo?

Yes, for now. In 2023, the City passed a zoning amendment to define and regulate Short-Term Rentals (STRs). Some elements of that legislation are tied up in court, so for now there are no regulations on the books. This may change in the future, though, so reach out to the Zoning Section if you have any questions.