How much does it cost to build a DADU in Seattle in 2024?

Seattle construction cost trends

Construction costs remain relatively flat in the third quarter of 2023. An analysis from Mortensen Construction* found that construction costs increased 2.7% over the last year in Seattle and 2.3% nationally. This is a welcome departure from the double digit cost increases over the last number of years. The study shows a cumulative 193% increase in construction cost from 2009, which coincidentally, is the year that DADUs were first legalized throughout Seattle.

How much does it cost to build a DADU?

Not surprisingly the average cost to build a DADU has increased in pace with these larger trends. In addition to increases in material and labor costs the size and complexity of DADUs have increased as well. ADU code changes meant to encourage the construction of more ADUs, have increased the allowable size of ADUs. Many of the DADUs built today are larger than they were in 2009. The DADU pictured above contains 1,440 sq. ft. where a DADU with a similar program built prior to recent code changes could have only been 800 sq. ft. in size. The increasing sale of DADUs as condos has led to more speculative builders entering the market. ADUs built for sale are further pushing the size and amenities offered in DADUs which now compete with modest size single family homes in our tight real estate market.

What will it cost to build a DADU in 2024?

The cost to build an ADU will likely decrease this year based on a number of predicting factors. Data from the Mortensen Construction Cost Index shows that in Seattle material costs are flat and number of people employed in construction as roughly the same as in 2019 and 5% lower than in 2022. Meanwhile, the number of residential building projects completed in Seattle has increased 22% in the last year. Conversely, the number of new residential building permit applications, an indicator of future demand for construction, has dropped 38% in the last year. That indicates a rapid decline in future residential construction activity in the coming year. This is happening at the same time as commercial construction is also slowing amid rising vacancy rates and higher borrowing costs. These trends bode well for lower material, labor and sub contractor costs in the near future.

What will it cost to build your DADU?

The first questions we usually answer for people planning to build a DADU are about feasibility, cost, and design which are all interrelated. A preliminary site assessment, which can be done via phone or email, is the first part.  At that time, we can answer questions and we can discuss budget ranges for recently completed DADUs.

*Mortensen Construction compiles data on a wide range of projects throughout the country in their annual Construction Cost Index.