Issaquah allows two accessory dwelling units with a single-family home or middle housing, attached or detached, each up to 1,000 square feet. Three local requirements stand out: a size ratio between the house and its ADUs, a separate street address for every unit, and a walkway leading to each ADU entrance.
Those details, together with hillsides, creeks and planned communities, make Issaquah one of the more site-specific ADU markets on the Eastside. This guide explains the current standards in the city’s Land Use Code, what they mean for the floor plan, and how permitting works, including a newer self-certification path for detached units. For the regional picture, see our overview of ADU code changes across Puget Sound.
Issaquah’s May 2025 Code Package
Issaquah adopted a set of middle housing and ADU ordinances in May 2025 to meet state law. Ordinance 3092 rewrote the ADU standards in Land Use Code 18.500.090, replacing a 2023 version of the same section, and companion ordinances updated zoning tables and parking standards. The package removed owner occupancy, confirmed two ADUs per lot and kept the city’s critical areas protections in place.
Two ADUs, Within the Lot’s Density Limits
The code allows two ADUs, attached or detached in any combination, with one single-family detached house or with middle housing. The condition is that the lot complies with the allowed density and minimum lot size for the principal unit. On a lot that is already smaller than the zone’s minimum, ask the city how that condition applies before you assume two units.
When ADUs share a lot with a middle housing building, such as a duplex or townhouses, they count toward the lot’s density. A house with two ADUs and a fourplex are therefore different paths, not stackable ones.
Three Size Tests for an Issaquah ADU
Most cities check one number. Issaquah checks three things, and the floor plan has to pass all of them.
What the 1,000 Square Feet Leaves Out
Each ADU may be up to 1,000 square feet, excluding garage, storage and deck areas. A cottage with an attached single-car garage or a storage room can therefore keep its full living area. Our guide to the maximum ADU size in Washington explains how this compares with the state floor and other cities.
The 25 Percent Rule
The principal unit must have a gross floor area at least 25 percent greater than that of the largest ADU on the lot. Put simply, the largest ADU can be no more than 80 percent of the main home’s size. A 1,000-square-foot ADU needs a main home of at least 1,250 square feet; next to a 1,100-square-foot rambler, the ADU tops out at 880 square feet. Owners of smaller older homes should run this number first.
Carving the ADU From the Existing Home
An ADU created within the main house may exceed the 1,000-square-foot cap if it does not require an exterior expansion, apart from minimal changes needed to make it a working separate residence. The 25 percent rule still applies, which in practice limits how much of a house can become the ADU.
For a compact detached unit that clears all three tests on most lots, a plan like The Country Cottage at 785 square feet is a useful reference point.
Height Borrowed From the Principal Unit
Issaquah does not set a separate ADU height. An ADU must meet the height required for the principal unit in its zone. In the SF-E, SF-S and SF-SL single-family zones, the base building height is 30 feet. In the SF-D zone it is 25 feet or two stories, whichever is lower.
Thirty feet allows a two-story detached unit on most flat lots. On the steep lots common around the city’s hillsides, how height is measured from grade becomes the deciding factor.
An Address and a Four-Foot Walkway
Every ADU in Issaquah must have a separate address from the principal unit. Settling the address with the city during permitting avoids delays at final inspection, and a clearly posted number helps emergency responders find a rear unit.
There is also a walkway rule: a four-foot-wide path must run from the address side of the street right-of-way or driveway to the ADU’s primary entrance. On a sloped lot that path may need steps or a ramp, so it belongs in the site plan, not the landscaping budget at the end.
Where the rear yard abuts an alley, a new ADU is not subject to a rear setback, unless the city routinely plows that alley. Other setbacks follow the zone. Existing nonconforming structures may be converted to ADUs, and the city cannot refuse a condominium sale just because the unit was first permitted as an ADU.
Parking Capped Rather Than Required
ADUs follow the parking standards in Chapter 18.604, which list no minimum for an accessory dwelling unit and a maximum of one space per unit. In other words, Issaquah does not require a new parking space for an ADU, and it limits how much parking you can add for one.
The main house keeps its own requirement, which depends on lot size, so if an ADU project removes an existing garage or driveway space, check that the house still meets its minimum.
Two Fees Issaquah Waives for Accessory Units
Issaquah’s municipal code says a development permit for an accessory unit shall not be assessed a school impact fee, and the traffic impact fee chapter contains the same exemption. Other permit, review and utility connection fees still apply, and new sewer connections in King County carry a regional capacity charge. Confirm the treatment of park and fire fees with the city when you apply.
Self-Certified DADU Plans and the Permit Path
ADU applications go to Issaquah’s Community Planning and Development department through MyBuildingPermit; the city does not accept paper submittals for detached ADU permits. An optional pre-application meeting for ADU projects is available and is worth booking for sloped or creekside lots.
Under a 2025 state law, Washington cities may let licensed architects certify that a detached ADU design meets building codes. Issaquah’s City Council approved such a program in July 2026, and the city’s DADU permit page lists a self-certified option for qualifying projects outside geologically hazardous areas, filed with an affidavit of self-certification. Site plan review and utility requirements still apply, and the city audits a share of self-certified projects. To see how review fits into the whole schedule, from design to move-in, read how long an ADU takes to build.
Hillsides, Creeks, Olde Town and the Highlands
Issaquah’s geography shapes ADU projects as much as its code. Lots on the slopes of Squak, Cougar and Tiger mountains often contain steep slope or landslide hazard areas, which add geotechnical review and rule out the self-certification path. Properties near Issaquah Creek and its tributaries may include flood or stream buffers.
Olde Town, the historic core, has its own district standards in addition to the general rules. Master-planned communities such as Issaquah Highlands and Talus carry community association rules and, in places, development agreement standards, so review those documents along with the city code.
Flatter neighborhoods in the valley, with single-story homes on mid-size lots, are often where the 25 percent rule matters most. To see how these rules land on your parcel, talk with the United Signature team serving Issaquah, which handles ADU design, permitting and construction in the city.
FAQ
How large can an ADU be next to a 1,200-square-foot house in Issaquah?
About 960 square feet. The main home must have at least 25 percent more gross floor area than the largest ADU, and 960 times 1.25 equals 1,200. Confirm how the city measures both buildings before you finalize the plan.
Does Issaquah charge school impact fees on a new ADU?
No. Accessory units are exempt from both school and traffic impact fees under Issaquah’s municipal code, though permit, review and utility charges remain.
Do I need a separate address for a basement ADU in Issaquah?
Yes. The requirement applies to every ADU, attached or detached, so a basement unit also receives its own address.
Can I add a parking space for my Issaquah ADU?
You can add one, but no more. The parking table sets no minimum for an ADU and a maximum of one space per unit.
Is the self-certified DADU option available on every Issaquah lot?
No. It is limited to qualifying projects outside geologically hazardous areas, which excludes many hillside lots. Those projects go through standard review.