Accessory dwelling units have become one of the hottest topics in King County real estate. Whether you call them ADUs, DADUs, backyard cottages, or in-law suites, these secondary living spaces are transforming how Eastside homeowners use their property.

At Prolific Design-Build and Restoration, we’ve been building ADUs and DADUs across Issaquah, Bellevue, Sammamish, Redmond, and surrounding communities. Here’s what you need to know if you’re considering one.

ADU vs DADU: What’s the Difference?

An ADU (Accessory Dwelling Unit) is a self-contained living space built within or attached to your existing home. Think of a basement conversion with its own kitchen, bathroom, and separate entrance. A DADU (Detached Accessory Dwelling Unit) is a standalone structure on your property — the classic “backyard cottage.”

Both are fully permitted, code-compliant living spaces with their own kitchen, bathroom, and sleeping area. The main difference is whether the unit shares a wall with your primary residence.

Current Regulations in King County

Washington state passed legislation in recent years that significantly expanded ADU rights for homeowners. Most single-family zones in King County now allow both an attached ADU and a detached ADU on the same lot, meaning you could potentially have three dwelling units on one property.

Key requirements vary by jurisdiction. Size limits typically cap at 1,000 square feet for a DADU, though some cities allow up to 1,200. Setback requirements, height restrictions, and design standards differ between Issaquah, Bellevue, Kirkland, and unincorporated King County. Owner-occupancy requirements have been relaxed in many jurisdictions, opening the door for rental income even if you don’t live on-site.

Parking requirements have also been reduced or eliminated in many areas, especially near transit. This is a significant change that makes ADU projects more feasible on smaller lots.

What Does an ADU Cost to Build?

In the King County market, a basement ADU conversion typically costs $80,000 to $150,000 depending on the condition of the existing space, plumbing access, and finish level. A new DADU built from the ground up generally runs $180,000 to $350,000 for a quality build with proper foundations, utilities, and finishes.

The main cost drivers are site preparation, foundation work, utility connections (sewer, water, electrical), and finish quality. Permit fees add $5,000 to $15,000 depending on the city. Impact fees, which fund schools and transportation, can add another $5,000 to $20,000 in some jurisdictions.

Why Homeowners Are Building ADUs

The reasons we hear most often are rental income, multigenerational living, and property value. A well-built DADU in Bellevue or Sammamish can generate $2,000 to $3,500 per month in rental income. For multigenerational families, having aging parents or adult children in an independent space on the same property offers both proximity and privacy.

From a value perspective, an ADU can add significant equity to your property. In strong Eastside markets, a quality DADU can increase property value by $150,000 to $250,000 or more.

The Build Process

A typical DADU project takes 6 to 10 months from design to move-in. The process starts with a feasibility assessment — we evaluate your lot, check zoning requirements for your specific city, and confirm utility access. From there, we move into design, permitting, and construction.

At Prolific, we manage the entire process as a design-build firm. That means one team handles architecture, engineering, permitting, construction, and final inspection. No juggling multiple contractors or playing middleman between your architect and builder.

Interested in exploring an ADU or DADU on your King County property? Schedule a free feasibility consultation with Prolific Design-Build and Restoration. We’ll assess your lot, walk you through the local requirements, and give you a realistic cost estimate.