Can I build a backyard home in Issaquah?
Since 2023, Washington law lets most city lots hold up to two more homes. Issaquah writes its own rules on top. Here’s what the code says, in plain English — and a free report on what may fit on your lot.
What Issaquah allows.
The headline standards. Tap any one for the full wording and the code section.
ADUs per lot2, attached or detached, any mixFull wording
2: any combination of attached or detached per single-family detached residence; legally nonconforming lots may qualify through IMC 18.214 (IMC 18.500.090.F.1).
Max size1,000 sq ft (garage, sheds and decks not counted)Full wording
1,000 sq ft per ADU, excluding nonliving areas such as garage, storage sheds or decks (unless the ADU is in that area). The principal unit must be at least 25% larger than the largest ADU. Conversions inside existing structures without exterior expansion may exceed the cap (IMC 18.500.090.F.2).
Max height30 ft in SF-S, SF-E and SF-SL (same as the house)Full wording
Same as principal unit: 30 ft base height in SF-S, SF-E and SF-SL; SF-D is 25 ft or 2 stories, whichever is lower (IMC 18.500.090.F.3; Table 18.404.090(A)).
Front setback20 ft in SF-S (same as the house)Full wording
Same as principal: 20 ft in SF-S (Table 18.404.090(A)). The city FAQ says ADUs must meet all applicable zoning standards.
Side setback8 ft in SF-S (same as the house)Full wording
Same as principal: 8 ft in SF-S (Table 18.404.090(A)).
Rear setback10 ft in SF-S (same as the house)Full wording
Same as principal: 10 ft in SF-S (Table 18.404.090(A)). No rear setback applies to a new ADU where the rear yard abuts an alley, unless the City plows that alley (IMC 18.500.090.F.4).
Lot coverageSF-S: 40% maximum impervious surfaceFull wording
SF-S: 40% maximum impervious surface (SF-E 30%, SF-SL and SF-D 50%). There is no separate building-coverage standard (Table 18.404.090(A)).
Parking1 extra off-street space per ADUFull wording
1 additional off-street space per ADU, per the city's ADU Submittal Requirements (Feb 2026). The code requires adequate on-site parking per IMC 18.604, and tandem parking is allowed (IMC 18.500.090.F.5).
Where a city figure reads stricter than the state floor (RCW 36.70A.681), the floor may control — the report flags it for your parcel.
Living there, selling it, plans, time and fees.
Each answer is the code’s own wording, with the section it comes from. “Not published yet” means the city hasn’t posted a figure — the report asks the city, not the internet.
Are backyard homes allowed?
Yes. ADUs are a permitted use (P) under Table 18.402.020, subject to IMC 18.500.090, and two ADUs in any combination of attached or detached are allowed per single-family detached residence (IMC 18.500.090.F.1).
Minimum lot size
None beyond the zone. An ADU may be built on any lot that meets the principal unit's minimum lot size (IMC 18.500.090.D.1). The SF-S minimum is 9,600 sq ft (Table 18.404.090(A)).
Do you have to live there?
No. IMC 18.500.090 has no owner-occupancy requirement, and the city ADU FAQ says homeowners are not required to live on-site.
Can it be sold on its own?
Yes. The City may not prohibit sale or conveyance of a condominium unit independently of the principal unit solely because it was built as an ADU (IMC 18.500.090.F.8). The city FAQ says ADUs can be sold and owned separately.
Pre-approved plans
None. The city FAQ says it offers no pre-approved ADU plans. Instead, a DADU Self-Certification Program (launched Sep 2026) lets WA-licensed architects certify building-code compliance: https://www.issaquahwa.gov/CivicAlerts.aspx?AID=6538
How long the city takes
Not published. An ADU application is a Level 1 review (IMC 18.500.090.C).
Impact fees
Not found; RCW 36.70A.681 caps ADU impact fees at 50% of the principal unit's
Plans Issaquah has already approved.
A pre-approved plan skips the design review: Issaquah has already checked the drawings, so the permit review is about your lot.
Loading the plans…
Pictures are the official pages’ own. A pre-approved plan is not a permit: the city still reviews your site.
The code is half the answer. Your parcel is the other half.
Lines and easements
Where the lot lines really are, which setbacks apply on each side, and whether a utility or access easement takes the corner you had in mind.
Critical areas and slope
Steep slopes, wetlands and streams come with buffers that no citywide number shows. The report reads the county’s maps for your parcel.
Sewer, septic, water, trees
Whether sewer reaches the lot or it’s on septic, where the water service is, and which trees a city protects — the things that decide where a unit can actually go.
What Issaquah allows, what could fit on your lot, and a planning cost range. If something won’t fit, the report says so — and shows what might.
Where this page comes from.
Read from the Issaquah municipal code and the city’s own pages on November 5, 2026. The same table feeds the HousUp report, so the report and this page never disagree. Cities amend their codes; before you design anything, confirm the current wording with the city.
2 sources, read November 5, 2026
Find out what your property could become.
Type your address. Within a few hours: what Issaquah allows, what could fit, and a planning cost range. Free, no obligation.