What Is a DADU and Why Are Kirkland, Bellevue, and Redmond Homeowners Building Them?

If you own a home in the Greater Seattle Area, you may have heard the term DADU popping up more and more. Neighbors are building them in their backyards. Real estate listings are featuring them as selling points. And local cities like Kirkland, Bellevue, and Redmond are actively updating their codes to make them easier to build. So what exactly is a DADU, and should you consider adding one to your property?

This guide breaks down everything King County homeowners need to know about Detached Accessory Dwelling Units, from the basics of what they are to the rules, costs, and process involved in building one.

What Is a DADU?

A DADU stands for Detached Accessory Dwelling Unit. It is a completely separate, self-contained living structure built on the same residential lot as your primary home. Think of it as a backyard cottage, a carriage house, or a stand-alone guest suite. Unlike an attached ADU, which shares a wall or roof with your main house, a DADU has its own foundation, its own roofline, its own entrance, its own kitchen, bathroom, and sleeping area.

It is a fully independent dwelling unit, just on a smaller scale, and it sits on the same lot as your existing home without being physically connected to it.

Washington State Law Has Made DADUs Much Easier to Build

In 2023, Washington State passed House Bill 1337 (HB 1337), one of the most significant changes to housing law in the state in decades. The law took full effect on July 1, 2025, and it removed many of the barriers that previously made DADU construction complicated and expensive for homeowners in King County and across the Greater Seattle Area.

Here is what changed under HB 1337:

• Homeowners can now build up to two ADUs per residential lot, whether attached, detached, or a combination of both

• The owner-occupancy requirement has been eliminated statewide. You no longer need to live on the property to build or rent a DADU

• Cities cannot set a maximum DADU size below 1,000 square feet of heated floor area

• No additional parking is required if the property is within half a mile of a major transit stop, saving homeowners $5,000 to $15,000 in construction costs

• Impact fees for DADUs are capped at no more than 50 percent of what is charged for a primary residence

For homeowners in Kirkland, Bellevue, Redmond, and throughout the Eastside, these changes mean building a DADU is now more accessible, more affordable, and more financially rewarding than it has ever been.

King County DADU Rules: What You Need to Know

While state law sets the baseline, individual cities and King County itself maintain their own specific development standards. Here is what homeowners in the Greater Seattle Area should understand before starting a DADU project.

• Size limits: DADUs in King County are generally capped at 1,000 square feet of heated floor area, with some exceptions for basement conversions and urban area flexibility

• Setbacks: Your DADU must meet the setback requirements for your specific zone, which vary by city. A contractor familiar with Bellevue or Kirkland zoning will know these rules in detail

• Height restrictions: DADUs must comply with the height limits of your property zone

• Utilities and septic: If your property is not on a public sewer, you may need septic system upgrades, which can add $15,000 to $30,000 to your project cost

• Permitting: In Seattle, pre-approved DADU design plans are available through the city's ADUniverse program, which can shorten the permit review timeline to as little as two to six weeks

One important note: if your property is governed by a Homeowners Association (HOA) that had rules against ADUs before June 30, 2023, those rules may still apply. Always verify HOA restrictions before moving forward.

How Much Does a DADU Cost in King County?

Cost is one of the first questions homeowners in Kirkland, Bellevue, and Redmond ask, and it is an important one. DADU costs in King County vary significantly based on size, lot conditions, materials, and finish level.

• A new detached DADU in King County typically ranges from $175,000 to $550,000

• A garage conversion to a DADU tends to be less expensive, often in the $80,000 to $150,000 range depending on the scope of work

• Rental income from a one-bedroom DADU in the Greater Seattle Area typically ranges from $1,800 to $2,500 per month, making the investment financially compelling over time

From start to first rental income, a realistic project timeline is 10 to 14 months: one to two months for site planning and design, six to sixteen weeks for permit review depending on the city and plan type, followed by three to six months of construction.

Why Are Homeowners Building DADUs?

The reasons homeowners in the Greater Seattle Area choose to build a DADU vary, but the most common motivations are:

• Rental income: A DADU can generate $1,800 to $2,500 per month in a market like Kirkland or Bellevue, helping offset mortgage costs or building long-term wealth

• Multi-generational living: A separate structure gives aging parents or adult children their own private space on the same property

• Home office or studio: Some homeowners use a DADU as a dedicated workspace, creative studio, or gym

• Increased property value: DADUs add meaningful value to your home, particularly in high-demand Eastside markets where housing inventory remains tight

  
  

How to Get Started with a DADU in the Greater Seattle Area

The first step is confirming that your property can support a DADU. That means verifying your zoning, lot size, setback requirements, utility connections, and whether any HOA restrictions apply. This is not something to guess at, and it is one of the most common reasons DADU projects stall early.

Working with a licensed general contractor who has direct experience building DADUs in Kirkland, Bellevue, Redmond, and across King County is the most reliable way to avoid surprises. Local knowledge of permitting timelines, zoning nuances, and utility requirements can save you significant time and money.

At Rez Construction, we work with homeowners across the Greater Seattle Area and the Bay Area on DADU projects from initial feasibility through final construction. We understand the local codes, the permit processes, and the design considerations that make a DADU work on your specific property.

A DADU is one of the most versatile and financially rewarding investments a homeowner in the Greater Seattle Area can make right now. Washington State law has opened the door wider than ever before, and cities like Kirkland, Bellevue, and Redmond are actively streamlining their permitting processes to match. Whether you want rental income, space for family, or simply more flexibility from your property, a DADU is worth a serious conversation.

Ready to find out if your property qualifies? Contact Rez Construction today for a free consultation.

Rez Construction | 844-409-1500 | hello@rez-construction.com | rez-construction.com