Backyard cottage in Seattle or on the Eastside: can your lot take an ADU?

Whether your lot can take an ADU or a backyard cottage comes down to four checks, in this order: how many units your city allows, how large the unit may be, whether the lot has room once coverage, floor area, yards and height are counted, and whether a critical area, shoreline or tree rule takes part of the lot out of play. Washington sets a floor for every city; each city sets its own numbers above it. In Seattle and Bellevue the surprise is floor area: adding a detached unit changes the floor area your lot is allowed. My first basic feasibility check is free and usually takes about a day, and an in depth analysis is $1,500 over 2 to 3 weeks. I am David Meade, an architect in Kirkland. I design the unit and administer construction. I do not build.

The sources below were read on 5 November 2026. Where a city’s code site sat behind a bot check, it was not bypassed. I am not an attorney, and the city’s answer controls.

What to check first on your lot

Work through these in order. Each can end an ADU plan or change its shape.

• The city and the zone. Seattle, Kirkland, Bellevue, Redmond and Sammamish each write their own ADU numbers. Check whether any part of the lot is a critical area or in the shoreline district.
• Units and size. How many ADUs the city allows and the largest it permits.
• Room on the lot. Coverage, floor area, yards, height and the space required between buildings.
• What the survey shows. Trees, slopes, utilities and where the house sits.

What Washington sets for every city

RCW 36.70A.681 is the state floor. Cities may allow more, not less (RCW 36.70A.681). It says a city:

• must allow at least two ADUs on lots in an urban growth area that allow single-family homes, attached or detached;
• may not require the owner to live on the lot;
• may not set a maximum ADU floor area below 1,000 square feet or a roof height limit below 24 feet, with a narrow exception tied to the main house’s own height limit;
• may not impose setbacks, yard coverage limits or tree retention mandates stricter than for a principal unit;
• must allow a detached ADU at an alley lot line (unless the city routinely plows that alley), and conversions of existing structures such as detached garages even if they break current setback or coverage rules;
• may not require off-street parking within one-half mile walking distance of a major transit stop.

Subsection (4) says these rules do not apply to lots designated with critical areas or their buffers. See my Washington ADU laws guide and ADU versus DADU.

Backyard cottage Seattle: what the 2026 rules decide

Seattle’s neighborhood residential update took effect on 21 January 2026 (OPCD). The ADU section is SMC 23.42.022 and the lot standards are in chapter 23.44, both as Ordinance 127376 amends them (ordinance PDF). Here is what the text says for neighborhood residential lots.

Question	What the source says	Section
Units	No lot may have more than two ADUs, attached, detached or stacked. ADUs count in density. They follow the same standards as a house unless the zone says otherwise.	23.42.022.C to E, J
Size	Up to two bedrooms: 1,000 square feet gross. Three or more: 1,200. Not counted: up to 250 square feet of attached garage, underground stories and 35 square feet of bike parking. Some Lowrise lots have a different cap, not covered here.	23.42.022.G
Yards	Rear yard for an ADU is 5 feet, or none at an alley. A house needs 15 feet at the rear. Side: 5 feet average, 3 minimum. Front: 15 feet on lots with one or two dwelling units. Buildings with floor area stay 5 feet apart.	23.44.090 Table A, fn 3; 23.44.100
Coverage	50 percent for structures. OPCD says ADUs count.	23.44.080.A
Height	32 feet, plus a pitched roof allowance of up to 5 feet (3 for a shed roof). An accessory structure in a required setback is limited to 12 feet. No lower cottage limit was found; ask SDCI.	23.44.070
Parking, shoreline	No off-street parking is required. In the Shoreline District, ADUs must comply with chapter 23.60A, which was not read.	23.42.022.I, A

Floor area is the rule most people miss. Seattle’s floor area ratio (FAR) depends on units on the lot, and OPCD’s summary says that count includes accessory units (OPCD summary). Table A in 23.44.050 gives 0.6 for less dense than 1 unit per 4,000 square feet, 0.8 from there down to 1 per 2,201, and 1.0 from 1 per 2,200 down to 1 per 1,601, with higher figures for some stacked unit projects. The ADU’s own floor area counts; the old ADU exemption is gone from the text that was read. The ordinance does exempt Type A accessible units under the Seattle Building Code, and floor area underground or no more than 4 feet above grade (23.44.050.C), so ask SDCI how that applies. Lots under 5,000 square feet may use 2,500 square feet if that is more. The city’s October 2026 clean up draft would also rename the table’s column, to show it is “based on actual density of the development, not allowed density”, and would narrow the accessible unit exemption. It is a draft, not law.

SDCI’s Tip 116B is dated 18 August 2023 and still says DADUs are not permitted on shoreline lots (Tip 116B). It predates the 2026 ordinance, so it is not used for lot rules here.

Worked lot: can a cottage fit on 6,500 square feet?

The lot is hypothetical: 50 feet wide and 130 deep, no alley, in a Seattle neighborhood residential zone outside the shoreline district. The house is two stories, 30 by 40 feet, set 25 feet back, with 2,400 square feet of floor area. I assume all of it counts, since the definition of chargeable floor area was not read. The cottage is two stories, 20 by 25 feet, which reaches the 1,000 square foot cap.

Step	Rule	Arithmetic	Result
Coverage cap	50 percent of the lot	50 x 130 = 6,500. 6,500 x 0.5 = 3,250	3,250 square feet
Coverage used	Structures count, ADU included	House 30 x 40 = 1,200. Cottage 20 x 25 = 500. 1,200 + 500 = 1,700	1,550 left
FAR, house alone	1 unit on 6,500 is less dense than 1 per 4,000	6,500 x 0.6 = 3,900	Cap 3,900. The 2,400 house leaves 1,500.
FAR, house plus one cottage	2 units: 6,500 / 2 = 3,250 per unit, the 0.8 band	6,500 x 0.8 = 5,200. House 2,400 + cottage 1,000 = 3,400. 5,200 to 3,400 = 1,800	Cap rose 5,200 to 3,900 = 1,300. 1,800 left.
FAR, house plus two ADUs	3 units: 6,500 / 3 = 2,167 per unit, the 1.0 band	6,500 x 1.0 = 6,500	Cap 6,500
Room behind the house	Cottage rear yard 5 feet. Separation 5 feet.	House back wall 25 + 40 = 65. 65 + 5 = 70. Rear limit 130 to 5 = 125. 125 to 70 = 55	55 feet of depth for a 25 foot cottage

Hypothetical lot, not a quote. A corner, slope, critical area or existing garage changes the arithmetic. See my Seattle zoning guide and critical areas guide.

The cottage uses 1,000 square feet of floor area but raises the cap by 1,300, so it fits with room left. A 3,500 square foot house plus the cottage is 4,500, under 5,200 but over the 3,900 cap with no ADU. Count every unit before you size anything.

Kirkland, Bellevue, Sammamish and Redmond

Each Eastside city writes its own numbers above the state floor, so I show them side by side.

Item	Kirkland	Bellevue	Sammamish
Units	Two: one attached and one detached, or two of either type. One in the Residential-L Shoreline Environment, per the city’s ADU page.	Up to two per lot where a single-family home is allowed.	Up to two on lots with a detached single-family home.
Size	1,200 square feet gross. Not counted: space under 5 feet of ceiling, uncovered rooftop decks and patios, up to 200 square feet of porches or decks.	1,200 square feet. The Director may approve more in three listed cases. Up to 300 square feet of parking or unheated storage is exempt.	Detached: 1,000 square feet. Attached: 50 percent of the house or 1,000, whichever is larger.
Height	The zone’s single-family limit.	24 feet, or 28 feet when proposed as an addition over an existing accessory structure.	Stand-alone detached ADU: 24 feet. Attached to another structure: 35 feet.
Yards	Same as a house in the zone, but a detached ADU may be 5 feet from the rear line if the part in the rear yard is no taller than 15 feet above average building elevation and the rear yard does not abut an access easement.	Same as middle housing under LUC 20.20.538, a table that was not read. May sit on an alley lot line.	Stand-alone: 5 feet side and rear. Front: 20 feet in R-1, 15 in R-4 and R-6, 10 in denser zones.
Floor area	In low density zones, the first 500 square feet (lot under 8,500) or 800 (lot 8,500 or more) of a detached ADU or garage behind the main structure is excluded.	A detached ADU counts as a unit in the table. Attached ADUs are exempt.	ADUs do not count toward the house’s FAR.

Kirkland. The city says two ADUs are allowed per property (Middle Housing page), and its ADU Regulations page gives the shoreline limit. The numbers come from Ordinance O-4905, adopted 17 June 2025, KZC 115.07, 115.42 and 115.115 (ordinance PDF); later amendments and the zone tables were not read. On a 7,500 square foot lot, an 800 square foot detached ADU behind the house counts 800 to 500 = 300 toward floor area. On a 9,000 square foot lot the first 800 are excluded, so it counts 0. See my Kirkland ADU page.

Bellevue. Council adopted Ordinance 6851 on 24 June 2025 (council minutes). The text read was the city’s unsigned ordinance copy and the 24 June 2025 strike draft, not the codified code. Its floor area table gives lots of 10,000 square feet or less 0.5 for one unit and 0.6 for two: on a 9,000 square foot lot, 9,000 x 0.5 = 4,500 for the house, and 9,000 x 0.6 = 5,400 with a detached ADU. The city’s 2023 ADU page (ADU reform) still says detached ADUs are not allowed; it predates the ordinance. The city’s 2026 omnibus package (Ordinance 6931, unsigned draft read) also amends these sections, with council action listed for 28 July 2026. The figures above do not change in that draft, but its adoption was not confirmed. See my Bellevue zoning guide.

Sammamish and Redmond. The Sammamish numbers come from the city’s ADU page (Sammamish ADUs); its code was not read. Redmond’s zoning code site returned an access error and was not bypassed, so I give no Redmond ADU numbers. Redmond’s permit page says detached ADUs need sprinklers (Redmond permits). Ask the city for its ADU section.

What can take part of the lot out of play

• Critical areas. OPCD says Seattle reduces allowed density and lot coverage in proportion to the share of the lot in a critical area such as a shoreline, creek, wetland or steep slope. Seattle’s ordinance also leaves riparian corridors, wetlands and their buffers out of the lot area used for coverage.
• Trees. Seattle requires development with new dwelling units to plant or keep trees to reach a point total set by units per lot area (23.44.120).
• Sewer. Tip 116B, from 2023, says King County bills a capacity charge about three months or more after the city reports your sewer connection. Ask King County for the amount.
• Old covenants. The state bars owner occupancy requirements. SDCI’s ADU page lists a release form for an older covenant (SDCI ADU page).

What to ask before you hire or build

• The city. Ask how it applies its floor area rule to your lot with a detached ADU, and whether any ADU rule changed after the version used here. Get the answer in writing.
• A surveyor. Ask for boundaries, easements, trees, slopes, the side sewer and the house and garage positions on one drawing.
• Your title report. Look for recorded covenants and easements.
• Yourself. Decide between a new cottage, a garage conversion and a unit inside the house. See my garage conversion page and ADU cost guide; I do not price here.

How the feasibility check works with me

The first basic feasibility check is free and usually takes about a day. The city decides the final numbers.

Step	Time or cost	What it is
Free first basic feasibility check	Free, usually about a day	You send the address, any survey, and what you picture.
In depth analysis, optional	$1,500, 2 to 3 weeks	Goes further than the first check.
Design and permit	Fixed fee, agreed before design starts. Design to permit submission typically 2 to 4 months	I draw the unit and prepare the permit set.
Construction	Built by a contractor	I administer construction. I do not build.

Planning figures from Piper Cole, not quotes. For Seattle review times see my ADU permit timeline, and for design ADU architect Seattle.

I have been an architect since 1996, an AIA member since 1992 and an NCARB member since 1996.

Questions

Can I build an ADU on my lot?

Often, but the lot decides. Washington requires cities to allow at least two ADUs on lots in an urban growth area that allow single-family homes, except that the statute does not apply to lots with critical areas or their buffers. Then your city’s size, height, yard, coverage and floor area rules decide whether a unit fits. My first basic check is free and usually takes about a day, and I design the unit and administer construction. I do not build.

How many ADUs can I have on a Seattle lot?

Two. SMC 23.42.022.C says no lot may have more than two ADUs, and they may be attached, detached or stacked. The ADUs count when density is calculated, which matters because Seattle’s floor area ratio rises with the number of units on the lot. The rules took effect on 21 January 2026 through Ordinance 127376.

How big can a backyard cottage be in Seattle?

The ordinance caps an ADU with up to two bedrooms at 1,000 square feet of gross floor area and one with three or more bedrooms at 1,200. Up to 250 square feet of attached garage, underground stories and 35 square feet of bike parking are not counted. The cottage must still fit under the lot’s coverage and floor area limits, which is why a worked lot matters more than the cap.

Does a detached ADU count toward floor area ratio?

It depends on the city. In Seattle, OPCD says ADUs count toward density, FAR and lot coverage. Bellevue’s June 2025 strike draft counts a detached ADU as a unit in its table and exempts attached ADUs. Kirkland excludes the first 500 or 800 square feet of a detached ADU or garage behind the house, depending on lot size. Sammamish’s page says ADUs do not count toward the house’s FAR. Check your city’s current text.

Do I have to live on the property or add parking?

State law bars cities from requiring the owner to live on the lot, and from requiring off-street parking within one-half mile walking distance of a major transit stop. Seattle’s code says no off-street parking is required for an ADU. Kirkland requires none within one-half mile of a major transit stop. Bellevue’s June 2025 strike draft requires none under 1,000 square feet or within one-half mile of a major transit stop. Sammamish adds none if the property already has four or more spaces. Confirm each with the city.

Can I convert my garage to an ADU?

State law says a city must allow ADUs converted from existing structures, including detached garages, even when they break current setback or coverage rules. Seattle and Kirkland have conversion provisions of their own, and Kirkland’s text exempts conversions from current setback and lot coverage requirements on the condition that existing nonconformities are not increased. The building code still applies, so see my garage conversion page and ask the city.

Can I build an ADU on a lot with a steep slope, creek or shoreline?

Possibly, with less room. State law’s ADU rules do not apply to lots with critical areas or their buffers, and OPCD says Seattle reduces density and coverage in proportion to the share of the lot in a critical area. Seattle sends shoreline lots to chapter 23.60A. A survey and the city’s critical area maps come before a design.

What does your feasibility check cost and how long does it take?

The first basic feasibility check is free and usually takes about a day. An in depth analysis is $1,500 and takes 2 to 3 weeks. If you then hire me, the fee for design and permit is fixed and agreed before design starts, and design to permit submission typically takes 2 to 4 months. For what a cottage costs to build, see my ADU cost guide.

Authoritative sources

Regulatory requirements referenced in this guide are set by the official agencies below. Rules change, always confirm current requirements with the source:

• Seattle Dept. of Construction & Inspections (SDCI)
• Seattle ADUniverse (official city ADU resource)