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In Seattle, “backyard cottage” is the everyday name for a detached accessory dwelling unit, or DADU: a separate, permitted home on the same lot as a house, with its own living, sleeping, kitchen and bathroom space. The phrase has no legal meaning of its own. When you apply for a permit, the code’s DADU rules are what count.
City websites, listings and builders mix the friendly name with the technical one. This short guide explains where the term came from, which official words sit behind it, and what a backyard building needs to legally count as a cottage. For the attached-versus-detached distinction in more depth, see our explainer on the difference between an ADU and a DADU.
Where the Name “Backyard Cottage” Came From
Seattle allowed attached accessory units in all single-family zones in 1994. Detached units came later: the city first tried them in Southeast Seattle in 2006 and then opened them up citywide by 2010.
As detached units spread, “backyard cottage” became the city’s plain-language label for them. Seattle’s planning office ran a long-term project called Encouraging Backyard Cottages, which led to the 2019 code changes and to the ADUniverse website launched in September 2020.
DADU and AADU: The Words the Code Actually Uses
Seattle’s code speaks of accessory dwelling units and divides them into two kinds. The planning office describes a detached ADU as one in a separate structure from the main house, also called a backyard cottage or carriage house. An attached ADU sits within or is attached to the main house, and is often called an in-law apartment or basement suite.
Washington law draws the same line. RCW 36.70A.696 defines an attached ADU as one located within or attached to the principal home, and a detached ADU as one that consists partly or entirely of a separate building on the same property. So “backyard cottage” maps to the detached category under both city and state rules.
Where You Will See Each Term
Permit applications, plan sets and inspection records use DADU and AADU. Guidance from SDCI, Seattle’s permitting department, on establishing a DADU notes that the unit is also known as a backyard cottage or mother-in-law unit, which is a good example of the two vocabularies meeting.
ADUniverse, the city’s ADU website, uses both and hosts pre-approved detached plans. Real estate listings are the least precise: “cottage,” “guest house,” “carriage house” and “studio” can describe anything from a fully permitted DADU to an uninsulated shed with a bed in it. A listing’s wording proves nothing about the permit history.
What a Backyard Building Needs to Count as a Cottage
SDCI describes a DADU as a room or set of rooms designed and established by permit to be a separate dwelling unit. To qualify, it needs all of the following.
Living and sleeping space for a household.
A kitchen, meaning permanent cooking facilities, not just a sink and a microwave.
A bathroom inside the unit.
A lockable entrance door of its own.
A permit that establishes it as a dwelling unit, which brings building, energy, fire and land use review.
The state definition of a dwelling unit points the same way: complete independent living facilities with permanent provisions for living, sleeping, eating, cooking and sanitation.
Studios, Sheds and Guest Rooms That Fall Short
A backyard office, art studio or gym without a kitchen is an accessory structure, not a dwelling. It can be useful and fully legal, but it cannot be rented out as a separate home. A guest room inside the house with its own bathroom but no kitchen remains part of the main house. A detached garage is not a cottage either until it is converted under a permit; our article on turning a garage into an ADU covers what that conversion involves.
Adding a kitchen to a permitted office later turns it into a dwelling, which needs its own permit, utility work and code review.
Why the Label Matters for Buyers, Builders and Landlords
If you are buying a home advertised with a cottage, check the permit record, since an unpermitted unit can affect rental plans, insurance and resale. If you are planning one, the word you use does not matter, but the type of permit does, and the current limits on size, height and placement are summarized in our guide to Seattle’s ADU and DADU rules. Builders and permit reviewers will talk in DADU terms, so it helps to know that is simply the formal name for the cottage you have in mind.
Seeing Whether a Cottage Fits Behind Your House
Whether a detached unit works on a given parcel comes down to its dimensions, slope, trees, existing structures and sewer route. A no-cost feasibility report on your lot maps those factors from public records, and United Signature’s Seattle ADU page shows how the company designs and builds backyard units in the city.
FAQ
Is a carriage house the same thing as a backyard cottage?
In Seattle usage, yes. A carriage house is usually a detached unit built above or beside a garage, often facing an alley, and it is permitted as a DADU like any other backyard cottage.
Does Seattle’s code use the phrase “mother-in-law unit”?
Not as a defined term. It is an informal name that people apply to both attached and detached units, and city guidance mentions it only to explain what readers may already call the unit.
How can I tell whether an existing backyard cottage was permitted?
Search SDCI’s online permit records by address and look for a permit that established a detached accessory dwelling unit, along with a final inspection. If you only find permits for a shed, garage or studio, the space was not approved as a dwelling.
Can I advertise a backyard office as a cottage when I sell?
You can describe it accurately, but presenting a space without a permitted kitchen and bathroom as a dwelling can mislead buyers and create problems during inspection and appraisal. Describe it as a studio or office unless it was permitted as a DADU.
Do other Washington cities use the term “backyard cottage”?
Some do informally, but most codes, and state law, say “detached accessory dwelling unit.” You will also hear “backyard home” or simply “detached ADU.” Whatever the label, the city’s own code sets the rules, and our overview of how cities adopted HB 1337 shows how those codes differ.