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With accessory dwelling units (ADUs), your home becomes a smart place to provide more living space. In addition, it also adds significant value to your home. We will discuss the 5 most common types of ADU’s. We will also talk about what they cost on average, and how they can enhance your space in the long run.

ADUs are often designed as long-term rentals. Sometimes, they function as accommodations for additional family members. So, an ADU can be an ideal option whether you’re looking to generate extra revenue or need more square footage for extra living space.


Basement secondary housing unit
Basement secondary housing unit (source Accessory Dwellings)

It’s important to know that a refinished basement and a basement ADU are different. Like most cities, Portland has very specific rules about what constitutes an ADU. For example, if the unit is to be classified as one, a basement ADU requires plumbing and ventilation for a separate kitchen and bath. Furthermore, in Portland there shouldn’t be having a street-facing door to the ADU. As the city wants to avoid homes having multiple front doors the main access needs to be a side door.

The head height is another consideration to keep in mind when converting a basement to an ADU. Many unfinished basements have a low head height. In order to meet minimum ceiling height requirements, we may need to dig down into the existing floor. That will require a larger expense upfront. It will contribute to the length of the overall conversion project too.

You’ll also want to think about how to maximize natural light in the basement. Sometimes we tend to lift homes in order to increase basement ceiling height. This will give space for the installation of larger windows, thus much more light will enter the basement space. Even if you don’t have plans to lift your home, you can consider installing larger windows in the unit so let that natural light in.


Basement ADU
Basement ADU (Source

The basement ADU’s:-

  • are cool in the summer,
  • can offer more privacy than street level housing,
  • generally, result good resale value if you decide to sell your home.

Average cost: $120,000-$220,000



This type of ADU has the feel of a house. It is basically a detached living space on your property, just on a smaller scale. It should be equipped with its own sewer, water, electrical and other components. This is a great ADU option as it’s totally separate from the main living space.

You can also cordon off an area of the yard where the ADU is built. That is in order to create more separation as well as dedicated access to whoever is occupying the unit.

This type of ADU considers appearance and location. In certain areas like Portland, the unit still needs to match the exterior aesthetic of the main home. The reason behind is that the  city wants to avoid having clashing design styles popping up in various neighborhoods.

You can place the ADU right up against the lot line. There may be height restrictions preventing the construction of a two-story ADU. On the plus side, being able to build right at the lot line allows for more separation and flexibility. This is particularly in smaller lots.


Backyard cottage secondary housing unit
Backyard cottage secondary housing unit (Source Sightline Institute)

A backyard cottage is a great option for a rental unit. Meaning of added separation is that both you and your renter have additional privacy. The typically larger size of this type of ADU may result in higher rental incomes too. Other than this, it can make a great living space for extended family members.

Average cost: $200,000-$400,000 (single story)/$300,000-$550,000 (two story)


Garage ADUs
ADU with garage underneath (Source

Another common type of ADUs is one with living space on top and a garage underneath. The garage is with extra storage for a vehicle or other items.

Adding a living above a typical two-car garage is a very sensible option. The layout of the garage lends itself to ample living quarters above. You can create a separate entrance either inside the garage or outside of it. This helps create separation from the main living space. It will provide privacy all around.

Click here to read about more Types of accessory dwelling units:

Average cost: $300,000-$500,000


Attached ADU
Attached ADU (Source

Attached ADUs can be an addition to your existing home. It can also be a space inside your home that you convert into an ADU, like an attic.

Access is the biggest consideration with this type of ADU. It’s almost like an apartment or townhome. It needs to have a completely separate entryway from the main living unit. A solution is to create separate access points such as a common front door/mudroom area that forks off into two entrances. One of these can fork off to the main living space, while the other to the attic ADU.

In most instances, with an attached ADU, you will need to create fire and sound separation between the two areas. The exception is if the area is already finished and you’re converting its use, like putting in a kitchen and an exterior door to turn it into an ADU.


These types of ADUs

  • are great for smaller or compact lots where space is at a premium.
  • there is no need to be concerned about some land to allow for separation between the structures.

Average cost: $200,000-$380,000

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