ADU Construction in Washington: What You Need to Know

Accessory dwelling units, commonly known as ADUs, have become one of the fastest-growing segments of residential construction in Washington State. Whether you want to create a separate living space for aging parents, generate rental income, or simply add value to your property, an ADU can be an excellent investment.

Washington State passed legislation in recent years that makes it easier to build ADUs in most residential zones. Many cities on the Eastside, including Redmond, Bellevue, Kirkland, and Sammamish, now allow both attached and detached ADUs on single-family lots. Some jurisdictions have eliminated minimum lot size requirements and reduced parking mandates, further lowering barriers to construction.

The cost of building an ADU on the Eastside typically ranges from $150,000 to $400,000 or more, depending on the size, complexity, and finishes. A basic 400-square-foot studio unit with standard finishes will land on the lower end, while a full 1,000-square-foot unit with a kitchen, bathroom, and premium materials will be on the higher end. Site preparation, utility connections, and foundation work are often the largest variable costs.

Permitting is one of the most important steps. Your contractor should handle the entire process, including site plans, structural engineering, energy compliance, and building department submissions. In most Eastside cities, expect the permitting process to take 4 to 8 weeks. Construction itself typically takes 4 to 6 months for a detached ADU.

One of the most common questions we hear is whether an ADU will increase property taxes. The answer is yes, your assessed value will go up because you are adding livable square footage. However, the rental income or the increased resale value of your property typically far outweighs the modest tax increase.

If you are considering an ADU, the first step is a site evaluation. Not every lot is a good candidate. Setback requirements, utility access, drainage, and slope all play a role. We offer free consultations to walk your property and discuss what is feasible within your budget and timeline.