Adding a second home to a Seattle lot usually starts with one big choice. You can build the unit into the main house or put it in the yard as its own structure. The attached versus detached ADU decision shapes privacy, cost, construction and how the whole property works. Neither option is right for every lot.

This guide compares the two paths in plain terms. It covers privacy, building rules, utilities and site fit. Keep in mind that what you can build always depends on your property and the code in effect when you apply.

Attached Versus Detached ADU Basics

An attached unit sits within or connects to the main house. A basement apartment is a common example, and so is a unit over an attached garage. A detached unit, by contrast, stands apart in the yard as a backyard cottage.

Seattle’s current code allows up to two accessory units on a lot, in attached, detached or stacked forms. Our guide to two ADUs in Seattle explains the combinations. Many owners, for example, pair a basement unit with a cottage.

Shared Zoning Rules

Both types follow the same core rules in most respects. Under the version of SMC 23.42.022 in effect as of this writing, a unit with up to two bedrooms may reach 1,000 square feet, and a unit with three or more may reach 1,200 square feet. The code also requires no off street parking for accessory units.

Still, other standards shape each option differently. A cottage has to fit within setbacks, height limits and lot coverage on its own footprint. An attached unit, meanwhile, often works within an existing structure. Our guide to Seattle ADU size explains what counts toward the floor area cap.

Privacy and Daily Life

Privacy is often the biggest difference people notice. A cottage has its own walls, roof and entry, so sound and foot traffic stay more separate. An attached unit shares at least one wall or floor with the main home.

That shared structure is not always a drawback. For instance, it can suit a family member who wants independence but likes being close by. On the other hand, it can be a harder fit for a tenant who keeps different hours. So think about who will live in each space, both now and later.

Outdoor space matters too, since a cottage can have its own patio or garden area. An attached unit may share the yard with the main home instead.

Fire Separation and Building Code

The building code treats the two types differently. A detached cottage is its own building, with its own walls and roof. An attached unit creates a two family dwelling, which normally needs fire rated walls or floors between the units.

Section R302 of the Seattle Residential Code includes one. An accessory unit built within an existing home need not count as a separated unit if all required smoke alarms in both units are interconnected. The code also calls for separate heating controls for an accessory unit. Your designer can confirm how these rules apply to your plans.

Utilities and Systems

Utilities follow the structure. An attached unit can often draw from service lines that already reach the house, though capacity still needs review. A detached cottage, however, usually needs new lines run across the yard for water, sewer and power.

SDCI notes that an ADU project may also need electrical service changes or new service from Seattle City Light. Our guide to DADU electrical upgrades explains how designers check electrical load.

Construction Differences in an Attached Versus Detached ADU

A cottage is new construction from the foundation up. It brings site work, a new foundation and a full building envelope. As a result, it often has a more predictable scope, since crews are not working around existing walls.

An attached unit, by contrast, usually means remodeling. Crews may uncover older wiring, framing or moisture issues once walls are open. In addition, work inside a home can affect the people living there during construction, from noise to limited access.

Permits for Each Option

SDCI’s accessory dwelling unit page lists the permit paths. Adding a unit within an existing house and building a detached unit both need a construction addition or alteration permit. Legalizing an existing unit, meanwhile, needs a construction permit to establish use.

Either way, SDCI says the unit must meet current codes. That review can surface different issues for each type, such as fire separation for an attached unit or site drainage for a cottage.

Choosing an Attached Versus Detached ADU

Start with your lot and your house. A large basement with good headroom may point toward an attached unit, while a deep yard may favor a cottage. Trees, slopes and access also matter. For example, a narrow side yard can make it hard to bring materials to a rear cottage.

Budget plays a role as well, though costs vary widely by site. A detached build includes a foundation, roof and new utility runs. An attached conversion may avoid some of that work, yet it can reveal hidden repairs. So ask for a clear scope on each option before comparing numbers.

Next, think about who will live there and how much separation they need. Then ask a designer to sketch both options against the current rules. Ultimately, the attached versus detached ADU choice comes down to how each one fits your property, your budget and the way you want to live.