ADU Architect in Bellevue, WA

TL;DR: Bellevue adopted middle housing code changes on June 24, 2025, allowing up to two ADUs per lot in urban residential zones. If you’re ready to build a detached ADU or DADU on your Bellevue property, hiring an experienced architect is the fastest path from idea to permit. This post explains what the design and permitting process looks like, what it costs, and why professional design pays off.

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I’m David Meade, AIA, NCARB, and for the past several years I’ve worked with Eastside homeowners on accessory dwelling units across Bellevue, Kirkland, Redmond, and the broader King County area. Bellevue’s regulatory landscape changed significantly on June 24, 2025 — exactly one year ago today — when the City Council adopted sweeping middle housing code amendments. Those changes opened the door for detached ADUs and cottage housing citywide, and the phone calls from Bellevue homeowners have not stopped since.

This post is not a deep-dive into the rules themselves (we have a dedicated Bellevue ADU rules 2026 page for that). What I want to do here is walk you through what it actually looks like to hire an architect and design an ADU in Bellevue — the process, the costs, the timeline, and why doing it right from day one saves you money.

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What Changed in Bellevue on June 24, 2025

Before last year’s middle housing code update, homeowners in most Bellevue neighborhoods could build one attached ADU. Detached ADUs were not allowed in standard residential zones. That changed overnight.

Under the new code:

• Two ADUs per lot are now permitted on any lot in an urban residential zone that allows a single-family dwelling
• Detached ADUs (DADUs) are fully legal and can be built in the backyard, detached from the main house
• The pre-approved plans pilot program launched in 2026, offering city-reviewed DADU and cottage housing plans that can shorten permit review times

These changes align Bellevue with Washington State’s HB 1337, which requires cities of Bellevue’s size to allow at least two ADUs per lot. For homeowners in Bridle Trails, Crossroads, Newport Hills, Somerset, Woodridge, and West Bellevue, this means new options that simply did not exist 13 months ago.

That said, the new code comes with design standards — setbacks, height limits, FAR calculations, tree retention requirements, and utility connection rules — that vary by zone and lot configuration. Getting those details right is what an architect is for.

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Why You Need an ADU Architect in Bellevue (and Not Just a Builder)

Washington State does not require a licensed architect for every ADU project. A small DADU under a certain threshold may not trigger a mandatory architectural seal. But “not required” and “not worth it” are two different things — and in Bellevue, they’re very different things.

Here’s what I see happen when homeowners skip professional design:

Permit rejections add months. Bellevue’s Development Services reviews ADU permits carefully. A plan set that doesn’t address all the current middle housing code standards, or that mis-applies a setback rule, gets kicked back. Every revision cycle adds weeks to your timeline.

Contractors can’t price what isn’t designed. Without a complete set of construction documents — architectural drawings, structural notes, and specifications — contractors give you ballpark numbers that shift once they understand the actual scope. The bids you get on a full architect-designed plan set are more accurate and more competitive.

Design adds value that a spec plan doesn’t. A pre-approved plan from Bellevue’s pilot program is a practical tool for speed. But it was designed for a generic lot, not yours. Your property in Newport Hills or Somerset has a specific orientation, existing tree canopy, grade changes, and neighbor adjacency that all affect how the ADU should be sited, fenestrated, and detailed. A custom-designed ADU will typically rent for more, appraise higher, and integrate better with the main house than a cookie-cutter plan dropped onto your lot.

I always say: the architect’s fee is the cheapest insurance you’ll buy on a $200,000–$400,000 project. Learn more about how much does an architect cost in Seattle and what drives that number.

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The ADU Design Process at Piper Cole Architects

When a Bellevue homeowner calls us about an ADU project, here’s what the process looks like from first conversation to breaking ground.

Step 1: Free Consultation and Site Evaluation

We start with a free consultation — by phone or in person — to understand your goals. Are you building a DADU for rental income? A multigenerational suite for aging parents? A home office with a bedroom for a live-in caretaker? The intended use shapes everything from the floor plan to the permitting category.

We’ll also do a preliminary site evaluation using your assessor’s parcel data and any existing survey or as-built drawings. This lets us identify lot coverage constraints, setback requirements for your specific zone, and any obvious site challenges before we commit to a design direction.

Step 2: Schematic Design

Once we understand the site and the program, we develop schematic options — typically two or three approaches that differ in footprint, orientation, or massing. For a detached ADU, this is where we work through whether a one-story or two-story scheme makes sense, how we handle the entry and parking access, and how the ADU relates visually to the main house.

For clients in Bellevue, we’re designing under the 2025 middle housing code from the start. No surprises at permit.

Step 3: Design Development and Construction Documents

After you select a direction, we develop the design into a full set of construction documents: floor plans, elevations, sections, site plan, and the details a contractor needs to build accurately. We coordinate with a structural engineer for the foundation and framing, and we’ll flag any mechanical or utility considerations early.

Step 4: Permit Submittal and Review

We prepare and submit the permit application to the City of Bellevue on your behalf. Standard ADU permit review in Bellevue typically runs 8–12 weeks. If your project qualifies for the pre-approved plans pilot, that timeline can compress to 4–6 weeks — and we’ll tell you upfront whether a pre-approved plan or a custom design is the better path for your situation.

We handle all comment responses and revision rounds. You don’t have to decode reviewer notes or figure out what “correct the setback dimension on Sheet A1.1” means.

Step 5: Construction Administration

Once permits are in hand and you’ve selected a contractor, we provide construction administration: reviewing submittals, answering contractor RFIs, and conducting site visits at key milestones. This keeps the build on track and ensures the finished ADU matches what was designed and permitted.

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Detached ADU or Attached ADU: Which Is Right for You?

One of the first questions we work through with Bellevue clients is whether to build a detached ADU (DADU), an attached ADU (AADU), or — now that two ADUs are allowed — both. For a full comparison, see our ADU vs DADU in King County 2026 guide.

Here’s a quick summary of how I think about the choice:

Detached ADUs (DADUs) offer more privacy for both the primary household and the ADU occupant. They’re typically preferred for rental use, where tenant separation is a selling point. They’re also easier to design for multigenerational use without creating awkward shared-entry situations. The tradeoff is cost: building a fully detached structure with its own foundation, utilities, and exterior envelope costs more than converting existing interior space.

Attached ADUs (AADUs) can be more cost-effective because they share the main house’s foundation and envelope. They’re a strong choice if you have an underused basement, a large attached garage, or a wing of the house that could be separated. The privacy separation is less clean, but for family use, that often doesn’t matter.

Two-ADU strategies — where a homeowner builds both a DADU and an AADU on the same lot — are now possible under Bellevue’s 2025 code. I’m seeing this approach on larger lots in Somerset and Woodridge, where homeowners want to maximize rental income while also providing a unit for a family member. This type of project requires careful coordination of utilities, lot coverage, and parking to stay within code, and it’s definitely a situation where professional design and permitting guidance is not optional.

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ADU Construction Costs in Bellevue, 2026

Clients always ask about cost. Here’s my honest answer for Bellevue in 2026.

A detached ADU in Bellevue typically costs between $200,000 and $400,000 to construct, depending on size, complexity, finishes, and site conditions. That range reflects hard construction costs — it does not include architectural design fees, engineering, permit fees, or utility connection costs, which together typically add 10–20% to the total project budget.

The low end of that range applies to a straightforward one-story DADU on a flat lot with simple utility hookups and mid-range finishes — think 600–700 square feet, basic but livable. The high end reflects two-story DADUs with higher ceilings, quality finishes, significant site work (grading, retaining walls), or challenging utility connections.

Attached ADUs are often less expensive, particularly if you’re converting existing conditioned space, because you’re not building a new envelope from scratch.

What drives your number up: hillside lots (common in Newport Hills and Woodridge), significant tree removal or protection, distance from the main utility connections, and premium finishes. What brings it down: flat lots, proximity to existing utilities, simple rectangular floor plans, and using Bellevue’s pre-approved plan options where eligible.

Our ADU design services page has more detail on how we structure fees and what’s included.

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Bellevue Neighborhoods We Design ADUs In

We work with homeowners across Bellevue. Some of the neighborhoods we’ve been most active in since the 2025 code changes:

West Bellevue — Large lots, high land values, and strong rental markets make DADUs a compelling investment. The ADU rental income can offset carrying costs significantly.

Somerset and Woodridge — Hillside neighborhoods with scenic views but complex topography. Site-specific design is critical; a pre-approved plan rarely works without modification on these lots.

Newport Hills — Established neighborhood with strong community character. We pay close attention to design compatibility with the existing housing stock.

Bridle Trails — Larger lots mean more room for a detached ADU without eating into usable yard space. These projects often target multigenerational living.

Crossroads — More diverse housing stock, strong rental demand, and a mix of lot sizes. We’ve done both AADUs and DADUs here depending on what the lot supports.

If you’re in a Bellevue neighborhood not listed here, reach out — we work throughout the city and can quickly assess what your lot can support under the current code. See our Bellevue architect location page for more on our work in the area.

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Frequently Asked Questions

How much does it cost to build an ADU in Bellevue, WA?

In 2026, a detached ADU (DADU) in Bellevue typically costs between $200,000 and $400,000 in hard construction costs. Add 10–20% for architectural design, engineering, permit fees, and utility connections. Attached ADUs that convert existing space can be less expensive, depending on the scope of work. Site conditions, finishes, and project complexity are the biggest variables.

Do I need an architect for an ADU in Bellevue?

Washington State does not require a licensed architect for every ADU project. However, Bellevue’s permit review process requires complete, code-compliant plan sets. Hiring a licensed architect significantly reduces the risk of permit rejections and revision cycles, and produces construction documents that contractors can bid accurately. For two-story structures, complex sites, or two-ADU projects, architectural involvement is strongly advisable. We offer a free consultation to help you assess what level of design support your project needs.

How long does it take to get an ADU permit in Bellevue?

Standard residential ADU permit review in Bellevue currently takes approximately 8–12 weeks from a complete submittal. Projects using plans from Bellevue’s pre-approved DADU plans pilot program may qualify for a shorter review period of approximately 4–6 weeks. Add 4–8 weeks of design and drawing preparation before submittal, and plan for at least one revision round. Total time from first consultation to permit in hand is typically 5–7 months for a custom-designed project.

Can I build two ADUs on my Bellevue property?

Yes. Under the middle housing code adopted by Bellevue on June 24, 2025, up to two ADUs are permitted on any lot in an urban residential zone that allows a single-family dwelling. You could build both a detached ADU and an attached ADU, or two detached ADUs on a sufficiently large lot. Each must comply with its own size, setback, and design standard requirements. We can assess your specific lot’s capacity during a free consultation.

What is the maximum size for an ADU in Bellevue?

Under Bellevue’s current code, both attached and detached ADUs can be up to 1,000 square feet, or 60% of the primary dwelling’s floor area, whichever is less. For the pre-approved plans program, Bellevue has reviewed DADU designs up to 1,200 square feet. Exact size allowances depend on your specific zone and lot configuration. Contact us or review the Bellevue ADU rules 2026 page for details on how the size limits apply to your property.

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Work With an ADU Architect Who Knows Bellevue

Bellevue’s new middle housing code is one of the most significant zoning changes the city has made in decades. It creates real opportunity for homeowners — but only if you navigate the design and permitting process correctly.

At Piper Cole Architects, we’ve been tracking these code changes from the beginning. We understand the setback rules in Somerset as well as the utility connection realities in Crossroads. We know how Bellevue’s Development Services reviews ADU plans, and we know how to write a permit set that gets approved without unnecessary revision cycles.

If you’re ready to move from “I’ve been thinking about an ADU” to “my ADU is under construction,” let’s talk. Contact us for a free consultation — no pressure, no commitment. We’ll tell you what your lot can support, what it will cost, and what the timeline looks like.

*David Meade, AIA, NCARB — Principal, Piper Cole Architects*