Accessory Dwelling Units (ADUs)

ADU types

How to Apply

Before applying, please review the ADU FAQs below.

To apply for an ADU, please complete the following documents:

Please email the completed documents to

The Planning & Zoning Department will review the application and will contact the applicant regarding next steps. Any ADU will require a building permit, and anyone who wishes to rent an ADU must also obtain a Millcreek business license.



The City Council adopted an Accessory Dwelling Unit (ADU) ordinance on September 27, 2021 that became effective November 15, 2021. You can view the adopted ordinance HERE.


What is an ADU?

An ADU is a second residence on the same lot as a single-family home. There are three types of ADU:

Internal – an internal ADU is a residence located within the existing footprint of a home. Traditionally, these ADUs have been called mother-in-law apartments.

Attached – an attached ADU is a residence constructed as an addition to an existing home.

Detached – a detached ADU is a residence in a separate structure on the same lot as the existing home. A detached ADU can be a new building, or an existing building could be converted into an ADU in certain circumstances.

Why are we allowing ADUs?

Over the course of nearly two years of public engagement, we found that many residents want ADUs, and many others are concerned about the impact ADUs might have in our neighborhoods. Our General Plan calls on us to “strive for a variety of housing choices in types, styles, and costs of housing throughout Millcreek.” We are in the midst of a housing crisis, and it is becoming increasingly expensive to live in Millcreek. ADUs are a part of a multi-pronged strategy to promote more opportunities for affordable housing in Millcreek.

Where are ADUs allowed? How big can they be?

ADUs are allowed only in a R-1 (single-family residential) or A-1 (agriculture) zone. In addition, the following requirements apply:

Lot Size – an internal or attached ADU is only allowed on lots that are at least 6,000 square feet in area. A detached ADU requires a lot that is at least 8,000 square feet in area.

Location – A detached ADU must be in a rear yard. An exterior door to an internal or attached ADU must be on the side or rear of the residence.

ADU Size – A detached ADU can only be up to 50 percent of the size of the existing residence.

ADU Occupancy – Attached and detached ADUs can house no more than two adults, and any number of children.

How close can an ADU be to my house?

Attached ADUs are subject to the side and rear yard requirements of any single-family home. A new detached ADU must be at least 5 feet from the side or rear property line of the property it’s on. An ADU conversion of an existing detached structure is possible, provided that the setbacks of the existing structure do not change, and the structure meets all building and fire codes.

How tall can a detached ADU be?

A detached ADU may be as tall as any detached accessory structure. In most cases, the maximum height of an ADU is 24 feet. Structures with a height greater than 14 feet must be further set back. If someone converts an existing nonconforming structure into an ADU, they cannot increase the height of the existing structure.

What are the design standards? Where can windows and doors be located?

Our ADU regulations are designed to allow ADUs in a manner that is least impactful as possible to surrounding residents. Transparent windows, doors, exterior lights, and balconies must face away from a neighboring residential property. In addition, ADUs are prohibited on slopes greater than 30%. Finally, ADUs are subject to the building envelope and lot coverage requirements in the City’s Residential Compatibility Overlay Zone (RCOZ). While there are options for exceptions to RCOZ for primary dwellings, the new ADU ordinance does not allow exceptions to RCOZ to accommodate an ADU.

Where will everyone park?

All ADUs are required to have at least one off-street parking stall, and no ADU parking is allowed on the street. If someone wishes to convert their garage into an ADU, that is only possible if the lot can accommodate at least one additional parking space for the ADU, and they must replace their garage parking with at least one covered stall.

What about utilities?

Because ADUs are accessory to the main single-family homes, ADUs are not allowed to have separate utility meters. All utilities must be serviced from the main single-family home.

Do I have to live in the home I own in order to have an ADU on my property?

Yes. ADUs are only eligible on lots that have owner-occupied dwellings. Upon approval, the city will record a notice on the property title stating that the eligibility for an ADU is contingent upon owner occupancy.

Can I use an ADU as a short-term rental?

No. Short-term rentals in ADUs are prohibited. ADUs are designed to promote housing flexibility in Millcreek neighborhoods.

What are the permit and license requirements?

Any ADU will require a building permit. As part of a building permit, the City Engineer, a Planner, and the Fire Marshal will review the project for compliance with all applicable building, fire, zoning, and engineering requirements.  Anyone who wishes to rent an ADU must also obtain a Millcreek Business License. If an existing building doesn’t comply with current setback, height, or coverage requirements, an attached ADU or an ADU conversion may require a permit from the Land Use Hearing Officer.

Where can I read the rules?

Click HERE for a copy of the adopted ADU ordinance.