How Much Does it Cost to Build an ADU?

There are many reasons why you might want to build an Accessory Dwelling Unit (ADU) on your property.  Some common motivations are:

  • Generation of rental income.
  • Providing close-but-separate housing for a family member.
  • A retirement plan: using the “empty nest” of the larger home for income by moving into the ADU.

This illustration summarizes the flexibility of ADUs nicely:

Accessory Dwelling Units | Hammer & Hand

Illustration by Ryan Sullivan of Paste In Place.

Regardless of the reason for building an ADU, everyone has the same first question: how much will it cost? The answer depends on a few factors…

Attached or detached? Conversion or new structure?

One factor that will affect cost is whether it’s an attached (a basement unit or addition) or detached (“DADU”) structure. Building a detached ADU is a lot like building a new home, complete with foundation, four exterior walls, and roof, so they can be more expensive than building an attached ADU.


Don’t let their diminutive size fool you; even small structures require design work from an architect, particularly given the design puzzles that limited space can present. Figure in 8-15% of construction cost for your ADU design, depending on level of design services required and your choice of designer.

Construction Costs

Construction costs vary quite a bit depending on the specifics of a project. We’re often asked, “What does an ADU cost to build per square foot?” But when building a tiny house, we are essentially squeezing all of the elements of a larger house onto a smaller footprint, so per square foot cost falls short as a useful measure. Rather, overall cost is more helpful. Generally speaking, the ADUs we build (particularly the stand-alone variety) start at a couple hundred thousand dollars in construction cost, with that cost increasing depending on design choices about finishes, fixtures, and building components.

Other Fees

In Portland, the City levies System Development Charges, or SDCs, on construction projects to cover infrastructure costs for things like transportation, water, parks, etc. But the City has temporarily suspended SDCs until July 31, 2018 provided the ADU is constructed and receives final inspection and certificate of occupancy by June 30, 2019. This can easily save you over $10,000 when building an ADU in Portland. Learn more from the City of Portland’s ADU Program Guide.

In Seattle, the Emerald City charges sewer, electrical, and construction fees based on the square footage of the structure being built. These fees often total $4,000 – $6,000 depending on the size of the property and specific needs. Learn more from the City of Seattle’s zoning page.


More Information

For more info, check out:

ADU 101

FAQ – Portland ADUs & Backyard Cottages

FAQ – Seattle ADUs & Backyard Cottages


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