Clyde Hill ADU and detached ADU rules
Can I build an ADU or DADU in Clyde Hill?
Clyde Hill allows two attached or detached ADUs, but it categorically prohibits an ADU on a lot containing a designated critical area or buffer. On an otherwise eligible lot, the normal R-1 setbacks, not the smaller ordinary accessory-structure rear yard, control a DADU.
Quick definitions. An accessory dwelling unit (ADU) is a complete additional home on the same property. A detached ADU (DADU) is in its own building. FAR or GFA limits how much enclosed floor area the property may contain.
Bottom line
What the current rules mean
An eligible lot may support up to two ADUs, each no larger than 1,000 square feet and 25 feet high. A detached unit generally needs the full 30-foot front and 35-foot rear yards plus the lot-area-based side yards. Before drawing anything, obtain the City's critical-area determination and Bellevue Utilities' water/sewer capacity certification.
On this page9 sections
What is allowed on one lot
Up to two attached or detached ADUs may accompany a single-family dwelling. Clyde Hill does not allow ADUs in addition to middle housing on the same lot.
- Two attached or internal ADUs.
- One attached and one detached ADU.
- Two detached ADUs.
- Conversion of an existing structure, including a qualifying nonconforming footprint, subject to size, parking replacement, building safety, and the parcel-wide critical-area rule.
Official sources: CHMC Chapter 17.39: accessory dwelling units · Clyde Hill official ADU guide · RCW 36.70A.681: limits on local ADU regulation
How much room the property may have for an ADU
- Maximum size
- Each ADU is limited to 1,000 square feet.
- Height
- Each attached or detached ADU is limited to 25 feet under the ADU chapter, with Clyde Hill's grade and height-measurement rules still applying.
- Setbacks
- The normal R-1 yards apply; a DADU does not receive the smaller rear setback for ordinary accessory structures. Front is 30 feet and rear is 35 feet. Side yards are generally 15 feet each on lots of at least 15,000 square feet, 10 feet each above 10,000 but below 15,000, and 15 feet combined with a five-foot minimum on lots of 10,000 square feet or less. Upper-story setback planes can also apply. A qualifying alley lot line may use the state-authorized zero setback.
- FAR, lot coverage, or impervious area
- Structural coverage is 30 percent and impervious coverage is 60 percent. Clyde Hill does not use a separate base residential FAR, so the house, garage and DADU footprints plus parking, driveways, gravel, walkways, utilities, and drainage surfaces must fit the applicable coverage ledger.
Official sources: CHMC Chapter 17.39: accessory dwelling units · CHMC Chapter 17.16: R-1 district
Make the most of the lot
ADU allowances worth checking before you design
An ADU can be a home for family, a rental, or a way to use space you already have. Before ruling it out, check the allowances below. They are specific code provisions, not a way around permits.
Allowance review: September 19, 2026. Confirm your parcel, application date, and proposed layout with the city before paying for detailed plans.
No blanket ADU floor-area bonus found
Clyde Hill's R-1 rules use structural and impervious coverage rather than a general residential FAR limit. Adding an ADU does not create a separate coverage allowance. The normal limits are 30% structural coverage and 60% impervious coverage.
Clyde Hill residential coverage rules ↗
An existing garage may offer a better starting point
CHMC 17.39.030(H) allows conversions of existing structures. It expressly allows an existing footprint that does not meet today's setbacks or lot coverage. That can make a garage conversion worth testing when a new DADU cannot fit.
The 1,000 sq ft ADU cap, parking requirements, and other applicable rules still apply. This is not permission to expand a nonconforming footprint. Ask which walls, roof, and footprint must remain if major rebuilding is needed.
The alley exception is narrow
A DADU may reach a lot line next to a public alley that is not routinely snowplowed. A private lane does not qualify. New DADUs do not receive the reduced rear yard for ordinary accessory buildings.
The chapter's section called “Exemptions” protects legally nonconforming existing ADUs. It is not extra floor area for a new one. A critical area or buffer on the lot remains a local eligibility blocker.
Clyde Hill ADU conversion, exemption, and setback provisions ↗
Ask about the state conversion protection
RCW 36.70A.681(1)(j) requires cities to allow ADU conversions of existing structures even when they conflict with current setbacks or lot coverage. It does not give a blanket right to enlarge or replace the building. The statute also has critical-area and watershed exceptions.
Ask the planner to confirm that your proposed work is a protected conversion rather than new construction. Keep the written answer with your project records.
Washington's ADU conversion protections and exceptions ↗
Compare three layouts before choosing one
- Inside the house. Could an existing floor become a separate home without adding a footprint?
- Inside an existing outbuilding. Can it be converted safely, and what parking or utility work would that trigger?
- A new DADU. Does its footprint fit after coverage, yards, trees, access, and drainage are checked?
Keep the unit-size limit, total counted floor area, building coverage, and impervious area as separate calculations. Passing one does not mean the others pass. A nominal second unit must be a real, permitted home, not just a label on a bedroom.
Parking, occupancy, renting, and separate sale
- Parking
- The current code requires two spaces per ADU on lots above 6,000 square feet, one per ADU below 6,000, and none within a half-mile walking distance of a major transit stop.
- Owner occupancy
- No owner-occupancy requirement applies. Long-term rental is allowed.
- Short-term rental
- Short-term rental is allowed only through Clyde Hill's annual business-license and safety program, including a local 24/7 representative, insurance, inspection/occupancy compliance, parking and traffic controls, and a prohibition on events.
- Separate sale
- The code includes a unit-lot-subdivision path that may support separate ownership. A condominium or unit-lot project requires approvals beyond the ADU registration and building permit.
Official sources: CHMC Chapter 17.39: accessory dwelling units · CHMC Chapter 5.20: short-term rentals
Utilities, fire access, and the permit path
The applicant must obtain Bellevue Utilities certification of adequate water and sewer capacity. Fire/building review, stormwater, address assignment, City registration, and a King County recorded covenant or registration document also apply.
- Request or confirm the City's critical-area/buffer determination before spending on an ADU design.
- Verify legal-lot status, R-1 setbacks and planes, structural/impervious coverage, address access, parking, and existing unit configuration.
- Obtain Bellevue Utilities water and sewer capacity certification and resolve fire-apparatus access, water supply, and any sprinkler mitigation.
- Submit the land-use/ADU and building package plus drainage, tree/landscape, utility, grading, and other site documents.
- Complete inspection, City registration, and the required King County recording before occupancy.
Official sources: CHMC Chapter 17.39: accessory dwelling units · CHMC Title 15: building and fire
Rules that can change the answer for one property
A citywide ADU allowance does not prove that a detached unit fits on a particular lot. Resolve these items before buying plans or fixing the DADU location.
- Any designated critical area or buffer on the lot is a categorical local ADU blocker, even when the proposed footprint is elsewhere.
- The City's general information suggests a different route for some nonconforming lots; state law may protect an ADU where the principal home is lawful, so get a written determination.
- The smaller rear-yard allowance for an ordinary garage or accessory building does not apply to a DADU.
- Bellevue utility capacity, long private access, hose distance, water pressure, and fire access can change cost or trigger sprinklers.
- Upper-story setback planes, original grade, protected vegetation, views, drainage, and easements remain parcel-specific.
Official sources: CHMC Chapter 17.39: accessory dwelling units · CHMC Chapter 17.16: R-1 district
Address-level ADU feasibility
What to check before assuming a detached ADU will fit
Clyde Hill's feasibility screen can be numeric only after a first, categorical critical-area/buffer test. If that test is clear, the engine can apply lot-area-based side yards, the 35-foot rear yard, 25-foot height, coverage, parking, and utilities.
Confirm the address is actually governed by Clyde Hill and select the correct base-zone standards.
Count the principal dwelling, existing legal ADUs, middle-housing units, and the proposed attached or detached configuration.
Use legal lot lines, front/rear designation, alleys, access easements, and existing structures rather than a tax-map rectangle.
Test the ADU against the rules that apply locally: FAR or GFA, building or lot coverage, impervious or hard surface, and landscaping.
Screen critical areas and buffers, shorelines, slopes, flood hazards, significant trees, drainage, and recorded easements.
Verify water, sewer or septic, fire flow, hydrant and apparatus access, power, meters, and trench routes.
A public GIS layer is not enough to mark the critical-area test 'no.' The tool should require a formal or reliable City determination and should route uncertain mapping to manual review.
Primary sources
Official Clyde Hill and Washington references
These are the sources used for this guide. Check them again before filing because ordinances, fee schedules, and permit forms can change.
What to check first
- 1
Confirm the parcel is inside Clyde Hill city limits and identify its current zone.
- 2
Map the principal home, lot lines, easements, trees, utilities, driveway, critical areas, and buffers on one current survey.
- 3
Test the ADU footprint against setbacks, height, floor-area or coverage limits, hardscape, parking, and construction access.
- 4
Ask the utility providers to confirm water, sewer, power, meter, side-sewer, and capacity requirements before fixing the design.
- 5
Have the city confirm the complete land-use, building, fire, right-of-way, tree, and utility permit list in writing.
Practical worksheet
Put this guide to work
Worksheet updated . This is a planning example. Regulatory sources and data have their own dates.
Compare the configurations before picking a floor plan
A basement conversion, an attached addition and a detached backyard unit can have the same target living area but very different constraints. A conversion depends on the existing structure and usable headroom. A detached unit may create more utility trenching, access and tree conflicts. A pre-approved plan still needs a site-specific fit check.
- Option A: use existing space
- Ask the designer to evaluate structure, moisture, light, exits, fire separation and system capacity. Verify that the existing work is lawful rather than relying on a listing description.
- Option B: attach new space
- Check how the new area is counted with the primary home, how the roof and structure connect and whether occupied construction is practical.
- Option C: detach
- Test the permitted location, building separation, access, utilities, drainage and construction route. Include the whole property in coverage and tree calculations.
- Use and ownership
- Confirm the proposed rental duration, occupancy, addressing, utility arrangement and any separate-sale path under the current local rules. An ADU permit is not a completed subdivision or condominium process.
Request comparable budgets with the same inclusions: design, studies, permits, site work, utility upgrades, building work, taxes and contingency. A price for the prefabricated unit or shell is not the complete installed project cost.