Sammamish vs. Issaquah ADU rules
Sammamish's standout rule is that ADUs are exempt from FAR and hardscape-coverage calculations on single-family lots (SMC 21.04.030.C) — a lever for anyone with a large existing house. Issaquah has no such exemption, and adds a constraint Sammamish doesn't: the principal home must have gross floor area at least 25% larger than the biggest ADU, which can bind before the 1,000 sq ft cap does on a modest house (LUC 18.500.090(F)(2)). Permitting also differs — Issaquah publishes target review times (four weeks for first review, three for second, one for third and later), while Sammamish's is unpublished, estimated at 8–14 weeks by secondary sources. Both cities dropped owner-occupancy.
✓ Verified against code · August 19, 2026
Sammamish rules run under SMC 21.05.010.C(5) / 21.04.030.C (Sammamish Municipal Code), adopted by Ordinance O2024-578, in effect since January 1, 2025.
✓ Verified against code · August 19, 2026
Issaquah rules run under LUC 18.500.090 (Issaquah Land Use Code), adopted by Ordinances 3092 / 3093, in effect since May 27, 2025.
Rule by rule
| Rule | Sammamish | Issaquah |
|---|---|---|
| ADUs allowed per lot | Two ADUs per lot with a single-family home — two attached, two detached (in one or two structures), one of each, or conversions of existing structures like a detached garage.SMC 21.05.010.C(5)(a)(i)–(ii) | Two ADUs in any combination of attached or detached, with a single-family residence or with middle housing (the lot must meet density and minimum lot size for the principal unit; nonconforming lots also qualify for up to two).LUC 18.500.090(F)(1), (C) |
| Maximum size | 1,000 sq ft; an ADU entirely within the existing residence may be the larger of 50% of the home's floor area or 1,000 sq ft.SMC 21.05.010.C(5)(a)(iii) | 1,000 sq ft each, excluding nonliving areas like garages, storage sheds, and decks (unless the ADU is located in them). Internal conversions and existing-structure conversions may exceed 1,000 sq ft if no exterior expansion is needed. One catch: the principal home must have gross floor area at least 25% larger than the largest ADU.LUC 18.500.090(F)(2)(a)–(c) |
| Detached ADU height | 24 ft for detached ADUs in every residential zone (the primary residence gets 35 ft in R-1 through R-8, 60 ft in R-12/R-18).SMC 21.04.030.C height table | No separate DADU cap — ADUs comply with the principal unit's height per Ch. 18.404 (roughly 30 ft in most single-family zones).LUC 18.500.090(F)(3) |
| Detached ADU setbacks | Side 5 ft and rear 5 ft in every residential zone — but only for stand-alone DADUs; a DADU combined with another structure (garage, pool house) takes standard structure setbacks. Accessory-structure front setbacks run 10–20 ft by zone (30 ft on arterials).SMC 21.04.030.C + conditions (4), (7) | Standard zone setbacks — with one exception: new ADUs face no rear-yard setback where the rear yard abuts an alley (unless the city routinely plows snow on it).LUC 18.500.090(F)(4)(a)(1) |
| Off-street parking | No additional space required when the parcel already has four or more; and no minimum parking at all for homes, ADUs, and middle housing within a quarter mile of an existing or planned transit stop — counting planned stops is unusually generous.SMC 21.05.010.C(5)(a)(iv); 21.06.030 | Zero minimum — and a maximum of one space per ADU. Tandem parking satisfies any space you choose to provide.Table 18.604.080(A) (Ord 3093); LUC 18.500.090(F)(5) |
| Owner-occupancy | None — no owner-occupancy condition appears anywhere in the adopted ADU regulations.SMC 21.05.010.C(5)(a) (verified absence) | None — Ordinance 3092 struck the owner-occupancy criterion and covenant. ADUs can be sold and owned separately, and the code bars prohibiting condominium conveyance.Ord 3092 Exh. C; LUC 18.500.090(F)(8) |
| Lot coverage / floor area | ADUs are exempt from FAR and hardscape-coverage calculations on single-family lots — a significant design lever. The 3,000 sq ft per-building footprint cap still applies, and the two ADUs together count as one dwelling unit for density.SMC 21.04.030.C ('Exempt' row) + condition (10); 21.05.010.C(5)(a)(v) | ADUs don't count toward lot density when associated with a detached single-family home (they do count with middle housing). Standard zone impervious-surface and coverage limits apply.LUC 18.500.090(D)(2) |
| Permit path & timeline | Building permit via MyBuildingPermit.com. No published review timeline (secondary sources suggest roughly 8–14 weeks for single-family-class review).City Permit Center page | Level 1 (administrative) review plus a building permit, e-submittal only via MyBuildingPermit.com. Published target review times: four weeks for first review, three for second, one for third and later — targets start when fees are paid.LUC 18.500.090(B)–(C); city Target Review Times handout (April 2026) |
Sammamish: worth knowing
- The FAR/hardscape exemption is the standout: on Sammamish's large-home lots, an ADU doesn't eat the main house's floor-area budget — the reverse of Seattle's 2026 change.
- The 'planned transit stop' parking waiver means lots near future transit corridors already qualify.
- The city's inclusionary fee-in-lieu ($34.45/sq ft, enacted March 2025) exempts projects of 1,500 sq ft or less, and ADUs up to 1,000 sq ft are excluded from the affordable-housing calculation.
- The city's DADU handout No. 310 (Sept 2023) is dangerously stale — 18-ft height, one DADU, two parking spaces, façade-matching — none of it current.
- No pre-approved ADU plan program.
Sammamish: on our watchlist
The SMC is current through Ordinance O2026-595 (May 2026); whether any 2025–26 ordinance touched the ADU sections is unverified because the code host blocks automated access — no ADU changes surfaced in searches. We re-check each pass.
The 'two ADUs count as one dwelling unit' density phrasing is loose — worth confirming the city's interpretation if density is load-bearing for a project.
Issaquah: worth knowing
- The 25%-larger principal-unit rule is unique in the region — on a small house, it can bind before the 1,000 sq ft cap does.
- Issaquah sets a parking MAXIMUM for ADUs (one space) — most cities only set minimums.
- A 4-ft-wide walkway to the ADU entrance is required, and a street-facing ADU door must be screened or mitigated.
- ADUs may share the principal dwelling's utility connection and meter if fixture counts and fire-flow demands pencil out — the city reviews capacity at submittal.
- An explicit legalization path exists for unpermitted pre-existing ADUs via inspection.
- No pre-approved ADU plan program — the city's FAQ says so explicitly.
Issaquah: on our watchlist
Ordinance 3114 (October 2025) — subject matter unverified; we confirm it didn't amend the ADU or parking chapters on the next pass.
The 'Lot Density Covenant' section (retitled from the old owner-occupancy covenant) is ambiguous about whether any recorded covenant is still required for new ADUs — worth a one-line confirmation with Issaquah CPD.
Full detail: Sammamish · Issaquah · back to the Washington hub. Not legal advice — every fact above cites the code section it comes from; check the cite, or ask your city’s planning department to confirm your specific lot.