What Is an ADU (and DADU)? Everything Seattle Homeowners Need to Know Before Building One

An ADU stands for “Accessory Dwelling Unit”—a small, self-contained home located on the same lot as a single-family house. These units typically include their own entrance, kitchen, bathroom, and living area, making them ideal for rentals, in-law suites, or guest accommodations.

What Is an ADU?

There are three main types of ADUs:

Interior ADU — Built within the existing footprint of the main home, such as a basement or attic conversion.
Attached ADU — Built as an addition to the main house, sharing at least one wall.
Detached ADU (DADU) — A completely separate structure in the backyard or elsewhere on the same property.

What Is a DADU?

A DADU, or Detached Accessory Dwelling Unit, is a standalone structure that functions independently from the main home. Often referred to as backyard cottages, laneway homes, or carriage houses, DADUs offer complete privacy, full design flexibility, and added square footage. They are popular for long-term rentals, short-term Airbnb use, and multigenerational living.

Why Build an ADU in Seattle?

Seattle is one of the most ADU-friendly cities in the country. Recent zoning changes have made it easier to build ADUs and DADUs, removing owner-occupancy requirements and even allowing two ADUs per lot in some zones. These policy updates, paired with rising property values and rental demand, make now a great time to work with an experienced ADU builder to maximize your investment.

Who Are ADUs & DADUs For?

ADUs and DADUs are incredibly versatile and can serve a wide range of homeowners depending on their needs:

Homeowners looking for rental income — long-term tenants or short-term guests provide an excellent way to generate passive income and help offset your mortgage.
Families with multigenerational living needs — keep older parents close, or provide extra space for college kids, all without sacrificing privacy.
Homeowners with frequent guests — a dedicated space for in-laws or friends visiting often.
Remote workers or entrepreneurs — a detached unit or basement conversion makes for the perfect private office, studio, or creative retreat.
Those planning to age in place — some homeowners build ADUs for themselves to downsize while renting out the main house.
Future-focused investors — ADUs and DADUs are one of the smartest ways to boost property value in Seattle’s evolving housing market.

Do ADUs Increase Property Value?

ADUs most certainly increase property value. Here are five ways how:

More livable square footage — naturally bumps up market value, especially in high-demand areas like Seattle.
Rental income potential — homes with ADUs appeal to buyers looking for built-in income opportunities.
Multigenerational appeal — flexible living arrangements make your property more desirable to a wider range of buyers.
Strong ROI — homes with ADUs sell for significantly more; in Seattle the added value can range from $100K to $300K+, depending on location, design, and finishes.
Long-term marketability — ADUs make homes more adaptable and future-proof, something buyers increasingly look for.

ADUs vs. DADUs: Same and Different

Both ADUs and DADUs are fully independent living spaces, must meet Seattle zoning and building code requirements, increase your property’s livable square footage, and can be used for rental income or personal use. The difference: interior/attached ADUs are built inside or attached to the main home with limited privacy and design options, while detached DADUs are standalone buildings that are fully private and highly customizable but typically cost more and take up backyard space.

Building an ADU in Seattle

Seattle-Specific Regulations to Know

Size & location limits — attached or interior ADUs must be no larger than 1,000 sq ft; DADUs can be up to 1,000 sq ft of living space. A lot can now have two ADUs total, depending on the zone.
Setbacks & height — DADUs typically follow a 5-foot side/rear setback; maximum height is usually 16–24 feet based on lot size and roof type.
Parking — no longer required for new ADUs in most zones.
Owner-occupancy & short-term rentals — Seattle no longer requires the owner to live on-site; short-term rentals are allowed with proper registration.
Permitting & design review — permits are required through the Seattle Department of Construction & Inspections (SDCI); historic or critical areas may need additional review.

ADU Building Costs in Seattle

Interior/Attached ADU: $100,000–$250,000+
DADU: $200,000–$400,000+

Costs vary depending on site conditions, materials, permits, and labor. While DADUs are more expensive, they tend to offer greater rental returns and resale value. Financing options include a HELOC, cash-out refinance, personal loans, and ADU construction loans.

Timeline to Build an ADU

Planning & design — 2–3 months
Permitting — 2–4 months depending on city backlog
Construction — 4–8 months depending on complexity

Build with Confidence: Partner with a Trusted Seattle ADU Builder

Whether you’re envisioning a cozy backyard cottage or converting your basement into an in-law suite, an experienced ADU builder can help. Specializing in designing and constructing beautiful, efficient spaces, professional builders can add value and functionality to your home. Ready to get started? Contact a builder today for a consultation and bring your ADU or DADU vision to life.