What is an Accessory Dwelling Unit or DADU?

Accessory dwelling units (ADU) are known by many names. A detached additional dwelling unit (DADU) is a separate structure from the main house. It is typically located in the rear yard of the property, above or within a converted garage, and often referred to as a backyard cottage, laneway house, alley house, carriage house, or backyard studio. Accessory dwelling units may also be attached to or inside of the main home, in which case it’s often called a mother-in-law, granny flat, in-law suite, or garden apartment.

We love this “Granny Pad” by Best Practice Architecture

We love this “Granny Pad” by Best Practice Architecture

Whatever you call them, ADUs and DADUs offer many benefits for homeowners, families, neighborhoods, the local economy, and the environment: 

  • Additional income from rent 

  • Ability to house guests or family members

  • Housing flexibility supports aging in place and multi-generational living

  • Employs local workforce during construction

  • New residents help sustain neighborhood businesses and thriving districts lead to increased property values 

  • Smaller homes use less resources to build and less energy to heat & cool

  • Compact development near jobs reduces commutes and vehicle dependence 

 Sound like a great idea? Here are some resources for anyone contemplating an ADU project. 


 Can I build an ADU on my property?

ADUs are allowed in most residential zones in Tacoma and Seattle, but specific requirements vary by jurisdiction. Already allowed in Seattle, legislation to allow DADUs in all single-family zones in Tacoma is expected to be adopted by City Council soon. The actual feasibility of any ADU project depends on many factors including dimensions and area of the property, size of existing home and its location relative to the property boundary, plus any accessory structures already on the site such as garages or sheds. 

A visit to the land use planner on duty at your local building permit department should get you a Yes or No, or WC STUDIO can do this legwork for you as part of a Feasibility Study for an Accessory Dwelling Unit or DADU.

What does it cost to build an ADU?

Most ADUs are less than 1,000 square feet, but they still need all the basics of a home—kitchen, bathroom, living, and sleeping spaces—and a freestanding ADU (aka, DADU) is essentially a small house which requires all of the trades needed to build any new home. Here is a breakdown of the costs for the Georgetown DADU a two story, 800 square foot Detached Additional Dwelling Unit we designed recently. Our experience indicates construction costs in Tacoma to be comparable with south Seattle.


ADU Budget.jpg

Garage, Attic + Basement Conversions

Creating a separate apartment entirely within an existing home or garage will likely cost less than building a backyard cottage or an addition. Oversized garages, especially garages with attic or loft spaces, are often good candidates for ADU conversion. Converting a basement or attic is another option. Adequate ceiling height, the ability to create separate entrances, and required egress windows that are at least 5 feet away from adjacent properties will be the primary feasibility concerns for these kinds of ADUs.


Should I hire an architect for my ADU project?

With the many stock plans and pre-fabricated options for DADUs on the market, a lot of people may wonder if they need to hire an architect for this type of project. Even if you feel comfortable going it alone through the building permit process, architects bring great value to any project.

Here is why we recommend working with an architect on your (D)ADU project:

  • You are making a significant investment in your home. For a fraction of the construction cost (usually 8-12%), an architect will create a custom-tailored design to meet your specific needs and preferences.

  • Working with an architect to create a healthier environment through natural light and a connection to the outdoors is money well spent whether you plan to occupy the additional unit or rent it out. A well-designed space that is open and airy commands higher rent.

  • ADUs are small. An architect will help you maximize the money you are spending by making the best use of every available square foot.

  • Architects consider the energy consumption and ongoing lifecycle costs of a building as part of a wholistic design approach, saving you money over the long term.

  • Since a small house or DADU has more surface area per cubic foot of interior volume, creating a well-insulated shell is extra important. Done correctly, very little energy will be required to condition the home. An architect will detail the building envelope and observe the construction to ensure the best sustainable building practices are implemented.

  • Some jurisdictions require that you generally match or complement the exterior design of the existing main home. A stock design may not meet this requirement.

  • The building orientation and window placement of a stock plan or prefabricated ADU may be less than optimal for your site’s shape and views from and into neighboring homes.

  • Making modifications to stock plans often ends up costing as much as an original, custom design.

Ready to get started designing an ADU or DADU for your property? Get in touch!