“ADU Architect in Sammamish, WA

By David Meade, AIA, NCARB | Piper Cole Architects

> TL;DR: Sammamish allows up to two ADUs per lot with no owner-occupancy requirement and no ADU impact fees — one of the more flexible ADU environments on the Eastside. A custom detached ADU on a Sammamish lot runs $350,000–$650,000 all-in, with terrain and tree codes being the dominant site-cost drivers. Permit review typically takes 6–14 weeks through Sammamish Development Services.

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What Sammamish Homeowners Can Build: ADU Rules Under Current Code

Sammamish allows up to two ADUs on a lot with a detached single-family residence. You can build two attached, two detached, or one of each — and a detached ADU can be attached to a detached garage, which often simplifies setback compliance.

Key size limits:

• Detached ADU (DADU): maximum 1,000–1,200 sq ft (confirm the current cap with Sammamish Development Services, as a 2025 Comprehensive Plan update may have raised it in some zones)
• Attached ADU (AADU): up to 50% of the primary residence square footage, or 1,000 sq ft, whichever is larger

Setbacks for a stand-alone DADU require a 5-foot side yard when abutting R-8, R-12, or R-18 zoned property and a 30-foot front setback along arterial streets. ADUs do not count against your Floor Area Ratio, and Sammamish charges no ADU impact fees — a real cost advantage over some neighboring cities.

For a broader look at how ADU rules vary across King County, see our guide to ADU vs. DADU options in King County.

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How HB 1337 Changed ADU Rules in Sammamish — and What It Means for You

Washington’s HB 1337, effective 2023 with required city compliance, rewrote the baseline rules that cities like Sammamish must follow. The changes that matter most to Sammamish homeowners:

• Owner-occupancy is eliminated. You no longer need to live on the property to rent an ADU. Sammamish cannot reinstate this requirement. You can rent both the primary home and the ADU simultaneously.
• Impact fee cap. Cities may not charge more than 50% of standard impact fees for ADUs. Sammamish already charges nothing, so this is a floor, not a ceiling.
• Minimum size guaranteed. The state requires cities to allow at least one ADU of 1,000 sq ft — Sammamish already exceeds this.
• Condo conversion. HB 1337 opens a pathway to sell an ADU as an independent condominium in some cases — a significant long-term financial planning option.

For a full breakdown of how state law interacts with local Eastside codes, read our post on Washington ADU laws for Eastside homeowners.

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Sammamish vs. Kirkland vs. Bellevue: ADU Rules Side by Side

Rule	Sammamish	Kirkland	Bellevue
Max ADUs on lot	2	2	1 (AADU only)
DADU allowed	Yes	Yes	No
DADU max size	1,000–1,200 sq ft	1,200 sq ft	N/A
AADU max size	50% of home or 1,000 sq ft	1,200 sq ft	40% of home / 300–800 sq ft
Owner occupancy	Not required	Not required	Required in one unit
Impact fees	None	Reduced	Standard

The practical takeaway: Bellevue does not allow detached ADUs at all under current code, and still requires the owner to live in one unit. Sammamish homeowners have meaningfully more flexibility.

If you are comparing jurisdictions, see our detailed write-ups on Bellevue ADU rules in 2026 and Kirkland ADU rules.

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What Does an ADU Actually Cost in Sammamish? A Realistic 2026 Budget

For a detached ADU in Sammamish, plan for:

• All-in range: $350,000–$650,000
• Per square foot: $450–$600 for modest finishes; $600–$800+ for higher-end work

Sammamish-specific cost drivers:

• Rolling terrain and hillside lots. Many Sammamish parcels have grade changes that require engineered foundations, retaining walls, or stepped floor plates. We regularly see $30,000–$80,000 in additional site work on sloped lots compared to a flat Kirkland parcel.
• Tree retention ordinance. Sammamish has one of the stricter tree codes on the Eastside. If protected trees fall within the footprint or root zone, the design has to work around them — which affects placement, foundation type, and sometimes size.
• Utility connections. In lower-density Sammamish neighborhoods, the nearest water and sewer connections can be further away. Separate utility hookups typically run $25,000–$60,000.
• No impact fees. Sammamish charges nothing for ADU impact fees, which partially offsets the terrain and utility costs above.

Soft costs — architecture, structural engineering, permit fees, and geotechnical reports on sloped lots — typically add 12–18% on top of construction costs.

Rental income context: A well-designed 600–1,000 sq ft ADU in Sammamish currently rents for $2,000–$3,200 per month, making the investment math work for most homeowners within 8–12 years at current rates.

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The Sammamish ADU Permit Process, Step by Step

All Sammamish permit applications go through MyBuildingPermit.com. Here is the sequence we walk clients through:

1. Feasibility check. Confirm lot coverage, setbacks, tree protection zones, and utility access before spending money on drawings.
2. Pre-application guidance. Sammamish Development Services offers a Project Guidance process with responses within about one week. We use this on complex lots to surface any issues before permit submittal.
3. Design development and permit drawings. Architectural plans, site plan, structural drawings, and energy compliance documentation.
4. MyBuildingPermit.com submittal. The complete package goes in online.
5. Review cycle. First review for a custom DADU in Sammamish currently takes 6–14 weeks. The city issues correction comments; we respond and resubmit.
6. Permit issuance. Once corrections are resolved, the permit is issued and construction can begin.
7. Construction. A DADU typically takes 4–9 months to build after permit issuance, depending on scope and contractor schedule.

There is no pre-approved plan program in Sammamish, so every project goes through full review. Having clean, code-compliant drawings on the first submittal is the single most effective way to compress the timeline.

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Rental Income, Aging Parents, or Home Office: Choosing the Right ADU

The right ADU for your lot depends on why you are building it. We see three primary use cases in Sammamish:

Rental income ADU: Prioritize a private entrance, separated utilities, and sound separation from the main house. A detached unit almost always commands higher rent and avoids the acoustic issues of an attached conversion.

Aging parents or multigenerational living: Proximity matters here. An attached ADU with an interior connection — or a detached unit sited close to the main house — gives family members independence while keeping them within easy reach. Single-level layouts and wider doorways are worth designing in from day one.

Home office or flex space: The smallest footprint and lowest cost. A 400–600 sq ft detached structure can serve as a studio, office, or creative space without triggering the full utility connection costs of a residential unit. Some homeowners build this first, then convert it to a rental later.

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Why Sammamish Lots Require an Architect Who Knows the Terrain

Washington state does not require an architect’s stamp for an ADU. A drafter or design-build contractor can prepare permit drawings. But on Sammamish’s rolling, tree-covered lots, the design decisions are complex enough that the licensing gap shows up in the project outcomes.

As a licensed AIA architect and NCARB certificate holder, I bring stamped drawings, liability coverage, and a design process built around code compliance from the first sketch. On hillside lots, we coordinate with a geotechnical engineer before placing the structure. On treed lots, we survey the protected tree canopy and design the footprint around it — rather than discovering a problem after permit submittal.

Learn more about our approach to architecture projects in Sammamish.

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Frequently Asked Questions

How many ADUs can I build on my Sammamish property?

Up to two ADUs are allowed on a lot with a detached single-family residence. You can build two attached, two detached, or one of each.

Do I have to live on my Sammamish property to build or rent an ADU?

No. Washington’s HB 1337, effective 2023, eliminated owner-occupancy requirements statewide. Sammamish cannot require you to live on site. You can rent both the primary home and the ADU simultaneously.

How much does a detached ADU cost in Sammamish in 2026?

All-in costs for a custom detached ADU on the Eastside range from $350,000 to $650,000. Sammamish-specific cost drivers include sloped terrain, tree retention requirements, and utility connection distances. The city charges no ADU impact fees.

How long does the Sammamish ADU permit process take?

A custom detached ADU typically takes 6–14 weeks for permit review through Sammamish Development Services. Applications are submitted via MyBuildingPermit.com. After permit issuance, construction runs 4–9 months depending on the scope.

Do I need a licensed architect to build an ADU in Sammamish?

Washington state does not legally require an architect’s stamp for an ADU. However, on Sammamish’s sloped, tree-covered lots — where setbacks, tree retention zones, and engineered foundations are common — a licensed AIA architect provides design expertise, stamped drawings, and liability coverage that a drafter or design-build contractor cannot match.

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Free ADU Consultation — Discuss Your Sammamish Lot

Piper Cole Architects serves Sammamish, Kirkland, Bellevue, Redmond, and the broader Seattle Eastside. Our ADU process starts with a feasibility conversation — we look at your lot, your goals, and the regulatory picture before you commit to anything.

Schedule a free consultation with Piper Cole Architects

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Sources Consulted

• City of Sammamish — Accessory Dwelling Units (sammamish.us)
• Sammamish Development Code 21.04.030 — District Standards
• Washington HB 1337 — ADU Reform Act (2023)
• Emerald City Build — Detached ADU Cost, Seattle and Eastside (2026)
• GatherADU — Sammamish, WA ADU Information