Legal duplex versus Accessory Dwelling Unit (ADU) – What is the difference?

When shopping for duplexes (a home with two dwelling units) in the Seattle area, you will come across a few different variations. There are legal duplexes and Accessory Dwelling Units (ADU). There are important distinctions between the two that you need to understand before making a purchase.

Legal Duplex

two entry doorsFor a single home with two living units to be legal, one of two things must be true. If the duplex has been built within the last ~40 years, the zoning must allow for multi-family homes. Most commonly, these are in areas zoned Lowrise 1 (LR1), Lowrise 2(LR2) or Lowrise 3(LR3). Provided that the lot coverage and parking situation is correct, investor-owned duplexes are legal in these areas and you can rent out both units to tenants.

Earlier in the 20th century, Seattle did not have the comprehensive zoning laws in existence today. There were a variety of duplexes built in areas that are now only zoned for single-family homes. Zoning laws today would not allow such a structure to be built or rented, but if the home was a duplex before the zoning laws were in effect, then the home is grandfathered as a legal duplex. Note that the burden is on you to document when the home was made into a duplex. If there is a duplex building permit in 1920, you are OK. If there is a permit evidence that it was converted to a duplex after the zoning laws are in effect, it is not a legal duplex and you can’t rent out both units unless you get it authorized as an ADU. Permit research can be conducted at the City of Seattle’s Department of Planning & Development.

Accessory Dwelling Unit (ADU)

An ADU is similar to a duplex in that it includes separate dwelling areas. Unlike duplexes, ADUs are allowed in single-family zones, but there are restrictions. The key difference is that a legal ADU must have one of its units occupied by the property owner as a permanent and principal residence. If the owner moves out, they may NOT rent out both units to tenants.

Establishing or building an ADU in Seattle requires that the use be permitted and the owner must sign and record an owner-occupancy covenant. The City of Seattle has two helpful memos available to understand the process for Attached ADU and Detached ADU (backyard cottage houses). The city can shut down ADUs that are not authorized.

Who’s going to know if I rent out both units?

Many novice investors may decide to purchase a duplex and rent it out, regardless of its status as a legal duplex. How is anyone going to find out, right? Well, you don’t want to be on the receiving end of an enforcement action from the city. If you are discovered to be renting out a unit in an illegal duplex, the city will force you to evict your tenants and remove the second kitchen. If you don’t comply, there are stiff fines that can be levied.

While city housing inspectors aren’t going door-to-door looking for illegal duplexes, they do have a complaint system in place that ensures quick compliance actions. All it takes is one anonymous neighbor who doesn’t want a rental property in their neighborhood to complain and your use as a rental property will come to a swift and untimely end.

Do your homework before buying a duplex

Before purchasing a duplex in Seattle, make sure that you can legally rent out both units. Start by confirming the current zoning for the property and then check out the permit history on file at the city. Do NOT go forward based on promises from the seller or real estate agents, as they may or may not have done this research.

If you intend to operate an ADU in your principal residence, make sure that you have the use authorized. You are also required to disclose this information to future buyers of the home.

The home’s status as a legal duplex or ADU will have a significant impact on your ability to resell the home in the future. A legal duplex has a much broader audience of investors who may want to purchase the home, compared to an ADU which requires it to be an owner-occupied residence.