ADU Architect in Bellevue, WA | DADUs, Garage Conversions & AADUs | Piper Cole

> TL;DR: Bellevue banned detached ADUs (backyard cottages) until July 1, 2025. Ordinances 6851 and 6852 changed everything — up to two ADUs per lot, no owner-occupancy requirement, and the right to sell an ADU as a separate condo. If you’ve been waiting, or if you had no idea this was even possible, you’re not alone. This post covers what the new rules actually allow, what a Bellevue ADU realistically costs in 2026, and how my firm — Piper Cole Architects — helps Eastside homeowners navigate the full process from site feasibility through permit approval.

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The Biggest ADU News in Bellevue’s History — and Most Homeowners Missed It

I’ve been practicing architecture on the Eastside for years, and I can count on one hand the zoning changes that genuinely move the needle for residential clients. July 1, 2025 is one of them.

Before that date, Bellevue prohibited detached ADUs outright in standard residential zones. A backyard cottage — the kind that can house an aging parent, a college-age kid, or generate $2,000–$2,800 per month in rental income — simply was not legal in most of Bellevue.

Ordinances 6851 and 6852, adopted June 24, 2025, changed that in response to two state mandates: HB 1337 (the ADU reform bill) and HB 1110 (middle housing). The key provisions now in effect:

• Up to two ADUs permitted per lot — one attached, one detached, or two of certain configurations
• No owner-occupancy requirement — you do not need to live on the property to build or rent an ADU
• ADUs can be sold as separate condominium units — a new exit strategy for homeowners building for equity
• Impact fees capped at 50% of the primary unit’s fees (per RCW 36.70A.696)
• No additional parking required if the property is within ½ mile of a major transit stop

If you’ve been curious about building an ADU in Bellevue and assumed it was prohibited or too complicated — the rules have changed. The question now is whether your specific lot works.

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What Bellevue’s New ADU Rules Actually Allow in 2026

Here is what the current code permits as of July 1, 2025. Note that Bellevue’s code is still being refined, and I recommend calling Bellevue Development Services at (425) 452-6875 or scheduling a pre-application meeting before finalizing any design:

Parameter	Bellevue (Post-July 2025)
Detached ADU (DADU) permitted?	Yes — newly allowed as of July 1, 2025
ADUs per lot	Up to 2
Max DADU floor area	Minimum 1,000 sq ft must be allowed (state floor)
Max DADU height (sloped roof)	24 ft
Max DADU height (flat roof)	18 ft
Side and rear setbacks	5 ft in most residential zones
Owner occupancy required?	No
ADU saleable as condo?	Yes
Parking required near transit (½ mi)?	No
Permit submission portal	MyBuildingPermit.com
Typical review timeline	4–8 weeks

Bellevue’s permit review runs meaningfully faster than Seattle’s 8–14 week average, which matters when construction costs compound with carrying time. For more on the permit process specifically, see our Bellevue building permit guide for 2026.

For a full comparison of state ADU law and how it applies across King County cities, see Washington ADU Laws 2026 — Eastside Edition and ADU vs. DADU in King County — 2026.

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The Site Feasibility Question Most Contractors Skip

Here is where I have to be direct with Bellevue homeowners: not every lot that is *legally* permitted to have an ADU is *physically* suited for one.

Bellevue’s Critical Areas Ordinance (LUC 20.25H) applies to properties with steep slopes (≥15% grade), wetlands, streams, or shoreline proximity. Buffer setbacks from these features can range from 25 to more than 200 feet — enough to eliminate the buildable area on a significant portion of a lot, or make a detached ADU infeasible entirely.

The western hillside neighborhoods — parts of West Bellevue, Enatai, Vuecrest, and portions of the Factoria plateau — have considerable steep-slope and drainage sensitivity. Eastgate and the Cougar Mountain area have stream and wetland designations that affect siting. Even properties that look flat on street view can carry geotech constraints that add $20,000–$80,000 to a project budget before the first permit is filed.

What I do before any Bellevue client commits to design is a site feasibility review: we look at topographic survey data, King County GIS critical areas layers, the property’s existing structures and easements, setback envelopes, tree canopy, and utility service locations. That 90-minute exercise has saved more than one client from spending $15,000 in design fees on a project that couldn’t get permitted.

If your lot is a good candidate, I’ll tell you exactly why and what the design options are. If it has constraints, I’ll tell you that too — and we’ll figure out whether there’s a viable path or whether a different project type makes more sense.

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Bellevue ADU Costs in 2026 — Real Numbers for the Eastside Market

Generic ADU cost guides are not useful for Bellevue homeowners. The Eastside market commands different labor and materials costs than Portland, Sacramento, or even Seattle’s urban core. Here is what our Eastside clients are actually spending in 2026:

ADU Type	Typical Range (All-In)
Garage conversion to ADU	$80,000–$250,000
Basement or lower-level ADU	$100,000–$350,000
Attached addition (new construction)	$150,000–$400,000
Detached DADU — 600–800 sq ft	$280,000–$450,000
Detached DADU — 800–1,000 sq ft (premium finishes)	$350,000–$650,000

Soft costs to budget separately:

• Architecture and design: $15,000–$60,000 (5–15% of construction budget)
• Structural engineering: $2,000–$4,000
• Site survey: $2,000–$3,500
• Geotechnical or arborist report (common on hillside lots): $2,000–$5,000
• Permit fees: variable; use Bellevue’s online fee estimator at permitfeeestimator.bellevuewa.gov
• Utility connections (separate meter, water, sewer): $25,000–$60,000
• Electrical panel upgrade: $5,000–$15,000

The wide range on detached DADUs reflects real variability: a flat lot in Eastgate with good utility access and mid-range finishes lands at the lower end. A sloped lot in West Bellevue with geotech requirements, arborist coordination, and premium finishes lands at the upper end. I will not quote a number until I have seen the site.

For garage conversion specifics, including when conversion makes more sense than new construction, see our post on garage conversions to ADU on the Seattle Eastside.

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One Option Almost No One Is Talking About — Selling the ADU

Under the post-July 2025 Bellevue code, an ADU can be sold as a separate condominium unit, independent of the primary residence. This is new. Before Ordinances 6851 and 6852, ADUs could not be legally separated from the main home for purposes of sale.

For homeowners who want to build a DADU for a family member and later sell it independently, or investors who want to build two units and sell one to offset construction costs, this option is worth discussing with both an architect and a real estate attorney early in the planning process. The design implications — separate entrance, separate utilities, sound separation between units, appropriate unit demarcation — need to be built into the project from the start, not retrofitted at the end.

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Comparing ADU Rules Across the Eastside

Many of our Bellevue clients also own or are considering properties in neighboring cities. Here is how the rules compare:

	Bellevue	Kirkland	Redmond	Sammamish
DADU allowed?	Yes (July 2025)	Yes	Yes	Yes
Two ADUs per lot?	Yes	Yes	Yes	Yes
Owner occupancy req’d?	No	No	No	Not required
Permit timeline	4–8 weeks	4–12 weeks	~60 days	Varies
Near-transit parking exemption	Yes (½ mi)	Yes	Yes	Yes

We have published detailed guides for ADU design in Sammamish and custom home design in Bellevue if those are more relevant to your situation.

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Why Hire an Independent AIA Architect — Not a Design-Build Package

Design-build contractors who handle both architecture and construction are common in the ADU space. There are good ones. But the incentive structure is worth understanding: when the same firm designs and builds your project, they benefit financially from the construction budget, not just the design fee. That creates pressure — subtle or not — to propose solutions that keep construction in-house rather than solutions that are best for your project.

As an independent AIA architect and NCARB-certified professional, my fee covers design only. I have no financial stake in which contractor you hire or what your construction contract looks like. My job is to produce permit-ready drawings that accurately represent what you want to build, coordinate with Bellevue Development Services on your behalf, and help you select and manage a contractor who is right for your project — not mine.

AIA contracts also provide meaningful protections for homeowners that standard design-build agreements often do not: defined scope, clear change order procedures, professional liability, and a licensed professional who signs and seals the drawings submitted to the city.

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How the Piper Cole Process Works for Bellevue ADU Clients

1. Initial consultation — We discuss your goals, budget range, and timeline. I give you an honest read on whether your project is likely feasible before you spend money on design.
2. Site feasibility review — Topographic and critical areas analysis, setback envelope, utility assessment. This is where we confirm the project makes sense on your specific lot.
3. Schematic design — Floor plan options, exterior massing, and material direction. We refine until the design matches what you want to build.
4. Permit documents — Construction drawings and specifications prepared for submission through MyBuildingPermit.com.
5. Permit coordination — We respond to Bellevue Development Services review comments and track your application through approval.
6. Construction administration — Optional but recommended: site visits, RFI responses, and change order review to make sure what gets built matches what we designed.

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Frequently Asked Questions

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Ready to Build an ADU in Bellevue?

The window opened on July 1, 2025. The regulatory framework is now in place. What remains is the project-specific work: confirming your lot works, designing something worth building, and navigating Bellevue’s permit process efficiently.

I am David Meade, AIA, NCARB, principal of Piper Cole Architects. My practice is based in Kirkland and focused on the Seattle Eastside — Bellevue, Redmond, Sammamish, Kirkland, Mercer Island, and surrounding communities. I know the Bellevue Development Services process, the Critical Areas constraints that trip up projects, and the Eastside construction market.

If you are a Bellevue homeowner exploring an ADU, start with a conversation.

Contact Piper Cole Architects

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Sources

• Bellevue City Code Amendments — ADU Reform (Ordinances 6851 & 6852): bellevuewa.gov/ADU-reform
• Bellevue Middle Housing Code Amendments: bellevuewa.gov/code-amendments/recent-code-amendments/middle-housing-code-amendments
• Bellevue Critical Areas Ordinance Update: bellevuewa.gov/code-amendments/recent-code-amendments/critical-areas-ordinance-update
• Washington State ADU Statute: RCW 36.70A.696
• Permit Submission Portal: MyBuildingPermit.com