Zoning describes an envelope. The site describes what is left of that envelope once trees, slopes, water and drainage have taken their share. Projects often stall in the gap between the two. It usually happens several thousand dollars into the design fees, and often after the owner has settled on a particular corner of the yard.
1. Trees, in Seattle
Seattle's tree protection code, at 25.11 as amended by Ordinance 127300 with effect from 21 January 2026, does more than restrict tree removal. It also takes ground out of your buildable area whether or not you touch the tree.
How a tree protection area is measured
A protected tree's is a circle around the trunk with a radius of one foot for every inch of trunk diameter. A 30 inch tree therefore generates a 30 foot radius, a circle roughly 60 feet across.
In zones, required tree protection areas are not counted as buildable area. Neither is any strip less than 10 feet wide left between a protection area and a setback. And where there are several trees, the protection areas are analyzed in combination rather than one tree at a time.
One 30 inch tree on a 40 by 100 foot lot. Drawn to scale. The circle is wider than the lot, so it crosses both side lot lines.
Worked example: one Douglas fir
A typical Seattle back yard behind a house on a 40 by 100 foot lot leaves maybe 40 feet by 35 feet of usable rear yard once the house and the setbacks are accounted for.
Put a single 30 inch Douglas fir anywhere in that space. Its protection area is 60 feet across, which is wider than the lot. Whatever ribbon of ground survives at the edges is then likely to be less than 10 feet wide between the circle and the setback, which the code also removes. There is no buildable area left, and moving the cottage elsewhere does not help, because the excluded ground stays excluded.
Two 14 inch trees at opposite corners produce two 28 foot circles that, analyzed in combination, can do the same job between them.
Whether the tree can come out at all depends on its tier, which is set by , the trunk diameter at a standard height:
| Tier | What it covers | Removal on a development site |
|---|---|---|
| Tier 1 | Heritage trees, any size | Essentially never, except hazard or emergency |
| Tier 2 | 24 inches DSH and over, tree groves, plus certain listed species below that size | Only for hazard, emergency, or development capacity |
| Tier 3 | 12 inches up to 24 inches | Generally removable as part of a development permit |
| Tier 4 | 6 inches up to 12 inches | Generally removable as part of a development permit |
Removed Tier 1, 2 and 3 trees require replacement achieving canopy roughly proportional to what was there before, on site, off site, or by payment in lieu. Tree service providers must be registered with the City, carry a Seattle business license and Washington contractor registration, hold at least $1,000,000 in commercial general liability insurance naming the City as an additional insured, and have a currently credentialed ISA certified arborist on staff. Public notice of reportable work is posted on SDCI's website at least three full business days ahead, and six for removal of a protected tree, so the work can't be done on short notice.
One point of legal tension is unresolved. State law bars tree retention mandates for ADUs that are more restrictive than those for principal units. Seattle's tree code is written to apply equally to both, so it likely survives that test, but its practical effect falls hardest on structures in back yards. That is our reading rather than a settled conclusion.
An arborist survey is inexpensive. Have an arborist measure and map the protection areas on your lot before you commission plans. A morning's work shows whether the site can take a cottage.
2. Critical areas and state ADU protections
Seattle calls them . King County calls them . Both mean mapped hazard and habitat: landslide-prone ground, steep slope erosion hazards, liquefaction, peat settlement, abandoned landfills, flood-prone areas, wetlands and riparian corridors.
A critical area designation removes state ADU protections
RCW 36.70A.681(4) says the state ADU provisions do not apply to lots designated with critical areas or their buffers.
Taken literally, your right to two units, the rule that a city cannot cap your unit below 1,000 square feet, and the rule that it cannot cap your height below 24 feet no longer apply as a matter of state law. The local jurisdiction's own rules then govern.
A counts too. A lot can lose its protections without containing a wetland at all, just by falling inside someone else's wetland buffer.
Seattle adds two more consequences. Lots containing riparian corridors, wetlands and their buffers, submerged lands or shoreline setbacks, or a designated steep slope non-disturbance area lose those areas from the lot area used to calculate density and lot coverage, and the applicant may instead choose a reduced density calculation, with a floor of at least one dwelling unit on any lot in existence when the ordinance took effect. And such lots are disqualified from the pre-approved plan fast track altogether. Relief pathways exist. One covers a steep slope area that is less than 20 feet in vertical rise and 30 feet or more from other steep slope hazard areas, where no adverse impact results. Projects in wetlands or riparian corridors, though, require formal director review rather than counter approval.
King County's buffers run wider in rural areas than in urban ones. Outside shoreline jurisdiction, stream buffers on Type S or F waters are 200 feet outside the urban growth area and 180 feet inside it. Where the riparian area overlaps a steep slope or landslide hazard, the width extends up to the hazard, capped at twice the standard buffer. That cap is a ceiling, not an automatic doubling. Structures must sit 10 feet back from the outside edge of any critical area or buffer, so the effective exclusion is the buffer plus ten. And where no geological report is submitted for a steep slope, the buffer defaults to 50 feet, rising to 75 feet where any part of the hazard area reaches into a coastal high hazard or sea level rise risk area. That default is often wider than the buffer a report would have supported.
For that reason, if you have any slope on the parcel, an early geotechnical report can buy back buildable ground that the default assumption takes away.
3. Drainage, and the 750 square foot line
is the stormwater engineering step. It adds design cost and review time. It usually also puts a physical facility somewhere on the site, and someone has to maintain it afterwards.
| Jurisdiction | Trigger |
|---|---|
| Seattle | More than 750 sq ft of new or replaced hard surface; or more than 750 sq ft of building measured by roof outline; or more than 5,000 sq ft of land disturbance; or any grading permit |
| Unincorporated King County | 2,000 sq ft of new impervious surface |
In Seattle the 750 square foot figure is the one most cottages reach. A 700 square foot cottage plus a path to the back door already clears it, and the roof outline test applies even if the ground under the eaves stays soft. Almost every detached cottage triggers drainage review, so include it in the budget from the start.
One timing point: Seattle's 2026 Stormwater Code and Manual took effect on 1 July 2026. Applications completed on or after that date comply with the 2026 code; applicants who submitted before it may elect the 2021 rules. If you are holding a set of plans priced under the older manual, reprice them.
4. Impervious surface, rurally
in rural King County is capped as a percentage of lot area: RA-2.5 at 25 percent, RA-5 at 20, RA-10 at 15 and RA-20 at 12.5. There is also a floor, and it usually decides the answer: lots of half an acre or more are allowed at least 10,000 square feet regardless of the percentage.
On a conforming rural lot that is normally generous rather than binding, and it is better to say so plainly. Where it does limit a project is on small or already heavily developed parcels, and in the places carrying the much tighter 8 percent figure. That figure is often quoted more broadly than it applies: it applies to subdivisions and short subdivisions in R-1 and RA zones inside named subbasins of Issaquah Creek and the Patterson Creek portion of Grand Ridge. It is not a general rural cap. Exceeding whichever limit applies requires a conditional use permit, a different and much longer process than a building permit.
Worked example: where the cap applies
Take a one acre parcel zoned RA-5. That is 43,560 square feet, and 20 percent of it is 8,712. But the 10,000 square foot floor is higher, so the floor governs and you are allowed 10,000.
An existing 2,400 square foot house footprint, a 900 square foot garage and 300 feet of 12 foot drive comes to about 6,900. A 900 square foot cottage, a parking pad and a fire turnaround brings you to roughly 8,600. You are under, with room to spare.
Now shrink the lot. At 15,000 square feet the percentage rules apply through the urban residential standards instead, and the same program of building runs out of allowance quickly. The cap doesn't bind on every lot. It binds on small and already-developed lots, so work it out on your own parcel rather than assuming either way.
5. Landmarks, historic districts and the things on your title
Seattle has eight historic districts (Ballard Avenue, Columbia City, Fort Lawton, Harvard-Belmont, the International Special Review District, Pike Place Market, Pioneer Square and Sand Point Naval Air Station) plus over 400 individually designated landmarks. A certificate of approval is required before altering the external appearance of a designated landmark or a property in one of those districts, and that expressly includes constructing a new building. Staff review takes up to 28 days. The board meets within 30 days of a completeness determination and decides within 45 days. It runs before and in addition to the construction permit.
Finally, some inexpensive checks that are often skipped:
- Covenants. A recorded covenant forbidding more than one dwelling, or forbidding structures other than a single family residence and an appurtenant garage, is enforceable by any neighbor in the association. The City does not enforce private covenants and will issue a permit for a building your covenants forbid. State law's protection against anti-ADU covenants reaches only associations created after 23 July 2023, which is almost none of them.
- Easements. Shared driveway, utility and access easements routinely cross exactly the rear-yard ground a cottage wants, and you generally cannot build over a utility easement.
- A survey. You will need one anyway, because SDCI relies on detailed property information to confirm critical area presence. Pulling it forward costs nothing and can save a redesign.
Next are the fees and charges outside the construction bid: Part 5.
Sources
- Seattle Ordinance 127300, tree protection, effective 21 January 2026, amending SMC 25.11; SDCI Trees and Codes guidance and Director's Rules 7-2023, 8-2023 and 12-2023.
- SDCI Stormwater Code and the 2026 Seattle Stormwater Manual, effective 1 July 2026.
- SDCI Tip 327A on environmentally critical areas, and SMC 25.09.
- King County Code 21A.24 for critical areas, including 21A.24.285 on steep slopes, 21A.24.325 on wetlands and 21A.24.358 on riparian areas; KCC 21A.09T.030 for rural impervious surface limits; KCC 9.04.030 for drainage review.
- RCW 36.70A.681(4), the critical areas carve-out.
- Seattle Department of Neighborhoods, Certificate of Approval.
- RCW 64.38.160 and RCW 64.90.350 on covenants, both limited to associations created after 23 July 2023.
Tree, critical area and drainage requirements are site-specific and are applied to your particular parcel by the reviewing agency. Nothing here substitutes for an arborist, a geotechnical engineer or a surveyor looking at your property. Read from primary documents in August 2026. General information from Hanco Electric, not design advice.