The Eastside Seattle ADU & DADU Guide (2026)

If you own or are buying a property in Bellevue, Kirkland, Redmond, Sammamish, Mercer Island, or Seattle and you've started wondering about an Accessory Dwelling Unit, you're catching the Eastside at an unusual moment. Three things are happening at once: Washington State's 2023 ADU law (HB 1337) and 2026 statewide implementation have stripped away most of the local restrictions that used to make ADUs hard; Kirkland has launched a Pre-Approved DADU Program that cuts months off permitting; and Eastside buyer demand for properties with ADU potential — multigenerational living, rental income, work-from-home flexibility — keeps climbing.

This guide is the practical version. What an ADU actually is. What it costs. How long it takes. When it adds value at resale and when it doesn't. Which lots qualify and which don't. And the red flags experienced architects and brokers learn to spot before a homeowner spends $20,000 on a design that can't be permitted.

Where the numbers come from matters: this guide draws heavily on the Earth Advantage All About ADUs (Seattle, February 2026) course taught by Seattle architect Sheri Newbold, AIA — the source of the most-citable cost and timeline figures you'll see below. Eastside city specifics come from the cities' own published code and zoning pages. Where information applies only to Seattle (or only to Portland in the case of the appraisal data), the guide says so explicitly.

ADU vs. DADU vs. JADU: What These Actually Are

The first thing to know is that the "ADU" label covers three quite different things, and the difference matters for cost, zoning, and resale.

An ADU (Accessory Dwelling Unit) is exactly what the name suggests: a secondary, fully self-contained living unit that's accessory to the primary house on a single-family lot. To count as a "dwelling" in Seattle and most Eastside cities, the unit must have both a kitchen and a bathroom — that's the test that separates a real ADU from a rec room, a home office, or a detached studio (Earth Advantage, All About ADUs, p. 7).

An AADU (Attached ADU) is created from part of the existing house — most commonly a basement conversion, an upstairs apartment, or an addition that's connected to the main house by heated space. It can also be designed into a new house or townhouse from the start.

A DADU (Detached ADU) is a completely separate structure — a "backyard cottage." It can be standalone, built above a new or existing garage, or in some cities can include two separate dwelling units in one building. DADUs are what most people mean when they say "ADU" in casual conversation.

A JADU (Junior ADU) is the smallest category and the most regulated. Under Washington State's HB 1337 (2023, codified in RCW 36.70A.681), JADUs are typically capped at 500 square feet, must be created within the existing footprint of the primary residence, and may share a bathroom with the main house. Not every city allows them, and Eastside city policies vary — verify before assuming a JADU strategy works on a given lot.

A practical reframe: a basement conversion is usually an AADU; a backyard cottage is a DADU; a small in-law suite carved out of the existing main floor is often a JADU. Each has different rules.

Why ADUs Matter on the Eastside in 2026

The market context isn't subtle. Washington State's Office of Financial Management projects the state will add nearly two million more residents by 2050, reaching about 9.76 million people. The state Department of Commerce estimates Washington needs roughly 1.1 million new homes in the next 20 years to keep pace. Most of those can't be built on greenfield Eastside land — there isn't much left. They have to fit into existing neighborhoods, on existing lots.

That's the structural backdrop for ADUs. But the more specific Eastside backdrop is this: a quiet convergence of three trends that together make ADU-viable lots more valuable than they were five years ago.

State-level pre-emption. Washington's HB 1337 (2023) effectively forced cities to allow ADUs on most single-family lots, capped how restrictive local rules can be on size and parking, and set a 2026 statewide baseline for ADU rights. Cities that had been slow to liberalize their codes are now obligated to.

City-level programs. Kirkland is the Eastside leader, with a Pre-Approved DADU Program that publishes city-reviewed plans homeowners can use. Choosing a pre-approved plan compresses the longest part of the process — design and code review — into days instead of months. Seattle has its own pre-approved plan program. Bellevue, Redmond, Sammamish, and Mercer Island have not adopted one at Kirkland's level as of May 2026.

Buyer demand. Multi-generational households, remote work setups, rental income from tech-driven rental markets, and the cost of assisted living all push Eastside buyers toward properties with ADU potential. That demand is uneven across cities and zones — but it's real, and prepared sellers are increasingly factoring "ADU viability" into list prices on lots that qualify.

The takeaway: properties with viable ADU potential are becoming a measurably distinct market segment from comparable properties without it. Not every lot qualifies, not every owner wants to build, but the option increasingly factors into what a prepared buyer will pay.

City-by-City: ADU Rules on the Eastside (and Seattle)

A clear summary of what each city allows and where the friction points are. Verify with the city before relying on this — zoning rules and fees change frequently, and the exact rules vary by zone within each city.

Seattle

The most documented and most permissive of the cities in this guide. Per the Earth Advantage course (pp. 56–69):

• Both AADUs and DADUs allowed on most single-family parcels. Up to two ADUs per parcel: two attached, two detached, or one of each.
• Maximum floor area: 1,000 sf for a unit with two bedrooms or fewer; up to 1,200 sf for a 3-bedroom unit; up to 1,500 sf in Lowrise zones with frequent transit if the property hasn't been purchased in the past 20 years.
• No minimum lot size. Density limit is one unit per 1,250 sf of parcel area, including ADUs. A parcel under 5,000 sf can still include four units.
• No parking requirement for ADUs (the principal dwelling still needs one space).
• No owner-occupancy requirement — the covenant was removed in 2019.
• Setbacks: typically 15 ft front and rear; 3 ft minimum / 5 ft average on the sides. DADUs without alley-facing parking can sometimes have no rear setback.
• Lot coverage: typically 50% in Neighborhood Residential zones.
• Volume: about 2,100 attached and 2,300 detached ADUs have been constructed in Seattle since 2017 (Seattle Quarterly Permitting Trends Dashboard).
• Trees: Seattle's updated tree ordinance went into effect January 21, 2026. Trees 12 inches in diameter and up are now protected; Tier 1 trees cannot be removed; Tier 2 trees only with formal proof the project cannot otherwise happen. A large protected tree on or near a project site can stop the project entirely.

Seattle is regulated by the Seattle Department of Construction & Inspections (SDCI). Pre-approved DADU plans are available at the ADUniverse gallery.

Bellevue

Both AADUs and DADUs are allowed under Bellevue Land Use Code 20.20.120. Bellevue does not currently offer a pre-approved plan program at Kirkland's level. Specifics on size, setbacks, parking, and any owner-occupancy requirements are in the LUC and have been moving in line with the state's 2026 implementation. Bellevue's ADU viability varies sharply by neighborhood — Crossroads, Lake Hills, and Newport Hills tend to have more ADU-supportive lots than the steeper, tighter Bridle Trails or West Bellevue parcels.

Kirkland

The Eastside's clear leader on ADU/DADU friendliness. Kirkland allows AADUs and DADUs and — uniquely on the Eastside — runs a Pre-Approved DADU Program as part of the Kirkland ADU Toolkit. The city has reviewed a curated set of DADU plans for code compliance; homeowners who choose one of those plans skip the longest and most uncertain part of the process — the design review.

The result is faster permitting, reduced permit fees, lower design costs (a modest royalty to the plan architect rather than a full custom design), and far fewer surprises on timeline. Combined with HB 1337 and Kirkland's broader middle-housing reform, Kirkland is currently the Eastside city where ADU optionality is most visibly priced into lot values. (See the full DADU program post here.)

Redmond

Redmond allows AADUs and DADUs, with rules covering size, setbacks, and parking under the Redmond Zoning Code. The city has been moving in the same general direction as Kirkland and Seattle and aligning with HB 1337 implementation, but does not currently offer a pre-approved plan program. ADU-viable lot stock on the Eastside often sits in Education Hill, Grasslawn, Union Hill, and the Sammamish-Plateau-adjacent neighborhoods where lots are larger.

Sammamish

Sammamish allows AADUs and DADUs (city DADU handout). Sammamish's hillside terrain, tree-protection rules, and lot-by-lot variation make ADU viability more case-specific than in flatter cities — slope, sewer/septic status, and tree cover often matter more than the zoning rules themselves. No pre-approved plan program at Kirkland's level.

Mercer Island

Mercer Island allows ADUs under its city code. Lot character (waterfront, hillside, narrow streets) drives most ADU viability questions on the Island — fewer lots are obviously DADU-ready than in Bellevue or Redmond. Verify with the city before assuming a plan works.

Other Eastside / North-end cities

Kenmore, Bothell, Issaquah, Renton, Newcastle, Snoqualmie, and Woodinville each have their own ADU rules and most are allowable. The state's HB 1337 floor applies. If you're evaluating a property in any of these jurisdictions, the same investigation applies: zoning + lot + utilities + trees + slope before committing to a strategy.

What an ADU Actually Costs (Seattle / Eastside, 2026)

This is the section most articles get wrong. The honest answer is: more than people expect.

The most credible recent build-cost figure for the Seattle metro comes from Earth Advantage (Sheri Newbold, AIA, All About ADUs, Feb 2026, p. 85): a new 1,000-square-foot custom DADU, code-built and turnkey at medium to medium-high quality, starts at $600,000–$700,000 and goes up. That's for an arms-length professional contractor in 2026 Seattle pricing.

The cost categories the course breaks out:

Soft costs (typical 1,000 sf custom DADU, Seattle, 2026):

• Utility scope and locate: ~$500
• Tree consultant: ~$1,000
• Survey: ~$3,200 (often required for full-property development)
• Construction permit (Seattle, new 1,000 sf): $5,248 under the 2026 Fee Subtitle
• Architectural design fees: 7–15% of construction cost
• Structural design fees: 0.5–2% of construction cost
• Other consultants (geotech, etc.): variable, mostly for unusual sites

Hard costs (construction):

• New DADU (1,000 sf custom turnkey, Seattle, medium to medium-high quality): $600K–$700K starting (Earth Advantage, p. 85)
• Basement conversion to AADU: starting around $250K for a custom turnkey project (Earth Advantage, p. 85)
• Garage conversion or build-above-garage: highly variable depending on existing structure condition

Why so much? Earth Advantage attributes the cost level to a structural skilled-trade labor shortage across North America, citing the HBI Construction Labor Market Report (Fall 2025). And as the course bluntly puts it: an ADU "packs a lot of higher-cost items — kitchen, bath, heating system, stair, utility work — into a relatively small space."

Eastside numbers will track Seattle's closely on a per-square-foot basis (same trades, same supply chain, similar permit complexity), with two adjustments: Kirkland's pre-approved plan program meaningfully reduces design and permitting costs for projects that fit a published plan; and Mercer Island, Sammamish hillside, and tightly-treed Bellevue lots can have meaningfully higher sitework costs than a flat Seattle lot.

The right way to think about it, per the course: there are three buckets of cost — intangible (your time, energy, decision fatigue), soft (design, permitting, surveying), and hard (construction itself). Most homeowners underestimate the first two.

Financing an ADU

Most ADUs aren't paid for in cash. The most common Seattle/Eastside financing paths:

• HELOC (Home Equity Line of Credit) during construction, refinanced into a single mortgage afterward. Most flexible if you already have substantial equity.
• Construction loan that converts to a mortgage. Requires the bank to appraise the property + the future ADU at a value high enough to support the loan.
• Cash, family loans, or other unsecured credit.
• Craft3 ADU Loan — a Pacific Northwest CDFI that makes ADU loans up to $250,000. Worth checking for owners who don't have enough equity for a full HELOC.
• Renovation-style mortgage products: Homestyle (conventional, for updating homes upon purchase or refinance) and 203K (FHA-backed) are sometimes used. The Earth Advantage course notes that 203K loans now allow ADU rental income to be used for qualification.

Per the course (p. 92), construction loans for ADUs have been issued through Washington Federal; Homestyle loans through Prospect Mortgage, Umpqua, Mechanics Bank, and Eagle Home Mortgage; and 203K loans through HomeStreet Bank and Prospect Mortgage. Lender lists change — verify currently available products before relying on any specific name.

The honest takeaway on financing: the right product depends on equity position, income, project size, and how the ADU will be used. A Kirkland homeowner using a pre-approved DADU plan to add rental income looks very different to a lender than a Bellevue homeowner doing a custom basement conversion to house an aging parent. Both are doable; the financing path is different.

Rental Income Expectations

There's no honest single number for "what does an ADU rent for on the Eastside." Rents track location, size, finish quality, and the broader rental market. The Earth Advantage course uses these examples for Seattle (p. 89), and they're a reasonable starting frame:

• Owner lives in main house, rents the ADU: ~$2,400/month for a 1,000 sf DADU in Seattle. About $26,400/year.
• Owner lives in the ADU, rents the main house: ~$3,800/month for the main house. About $45,600/year.

Eastside numbers for tech-corridor cities (Bellevue, Kirkland, Redmond) tend to run higher than Seattle for comparable units in good locations near Microsoft, Amazon (Bellevue), or Google (Kirkland) — Eastside rental demand from tech professionals is unusually strong and unusually rate-sensitive. Premium-zone neighborhoods like Medina or West Bellevue rarely produce conventional rental cash flow proportional to build cost; the play there is more often multigenerational living or eventual condoization, not yield.

The payback math, per the course, is roughly:

• Build for ~$600K, rent for $2,400/month → about 19 years to gross-payback the build cost (gross rent, not net cash flow after taxes/insurance/maintenance).
• Build for ~$600K, owner moves into ADU and rents the main house at $3,800/month → about 11–12 years.

But the Earth Advantage course's most important framing on this — and the line worth pulling out — is on p. 90: "Payback is often about something besides $$$." The course explicitly broadens the math to childcare, eldercare, vehicle and meal sharing, and a grandparent aging in place. The most-citable comparison: $600K to build a DADU, vs. $10,000–$15,000 per month — $120,000–$180,000 per year — for assisted living. When the alternative is institutional care, an ADU that lets a parent live nearby pays back fast even if no rent is collected.

Resale Value Impact

This is where the data is thinnest and the marketing claims are loudest. Be careful here.

The clearest research the Earth Advantage course cites comes from Portland appraiser Abdur Abdul-Malik, SRA, AGA, presented at "ADU Hour" (buildinganadu.com/adu-hour). The Portland case-study figures (paraphrased):

• Garage-conversion ADU: roughly +10–17% net (the ADU adds value; losing the garage subtracts some).
• ADU as addition to the house: ~+24.6% (single case study).
• New house with basement ADU included (speculative property): initial sale +6.1%, second sale a year later +16%.
• Detached ADU on a lot where the DADU is similar in size to the main house: ~+71.2%.

Two important caveats: these are Portland numbers, not Seattle/Eastside. And these are case studies, not market-wide regressions. They're directionally useful — DADUs in particular can add substantial appraised value when the lot supports a DADU sized similarly to the main house — but they shouldn't be cited as "expect this on your Bellevue lot."

Two pieces of guidance that hold across markets:

Permitted matters. Per the course (p. 49): "Most of what look and function like ADUs are, in fact, not permitted ADUs." A buyer evaluating a property with an existing "rental unit" should verify the permit. Unpermitted ADUs come with insurance risk, financing risk, and a property-tax-reassessment risk if the next owner wants to legalize it.

Property tax reassessment is real. Two anecdotal King County (Seattle) examples from the course (p. 95): one owner saw assessed value go up about $71,000 after a studio ADU was permit-finaled (~5 years later); another on the same block saw +$194,000 after a 2-bedroom ADU. These aren't market predictions; they're real reassessments. Assume some reassessment; don't assume none.

Typical Timeline (First Investigation to Certificate of Occupancy)

The Earth Advantage course's clearest timeline framing for a Seattle DADU (p. 85): 6–8 months for design and permitting, then 8–9 months for construction. Total: about 14–17 months from project kickoff to occupancy. Eastside cities track close to this, with one important variation:

Kirkland's pre-approved DADU plans can compress the design + permit window meaningfully — closer to 2–4 months from plan selection to permit issuance for a project that fits a published plan, since the design review is already done.

The full custom-project sequence (paraphrased from the course, p. 155):

1. Identify the property and the project goals.
2. Define the program (size, bedrooms, who it's for).
3. Assemble the team: architect/designer, structural engineer, arborist, general contractor.
4. Design and get preliminary budget pricing.
5. Submit for construction permit; revise budget against the permit set.
6. Develop the rest of the design (electrical, lighting, finishes).
7. Final construction budget.
8. Construction.

The Seattle-specific permit process adds steps for Pre-Application Site Visit, sewer scope, water availability certificate, tree assessment, and arborist report. Eastside cities have analogous steps. Almost every timeline overrun in this category traces back to one of three things: trees, sewer/water capacity, or unexpected site conditions (slope, soil, oil tank, shallow rock). Each of these is identifiable upfront with the right pre-purchase or pre-design investigation — which is exactly what an experienced architect and broker do before committing.

Red Flags: When to Walk Away from an ADU Project

Some lots look ADU-viable but aren't. The most useful framework from the Earth Advantage course (paraphrased from p. 11): the easiest projects are new DADUs on flat lots with an alley, basement conversions where the ceiling is already tall, garages built recently enough to meet current code, and ADUs designed into new house construction. The hard projects — and the real red flags — are the inverse:

• Sloped or hillside lots. Sammamish, parts of Mercer Island, and steep Bellevue lots add tens of thousands in sitework cost, sometimes more.
• No alley access. Without an alley, DADU placement options shrink and access during construction gets harder.
• Low-ceiling basements. A 7-foot basement is technically livable; once you add soundproofing, drainage mat, and finished floor, the ceiling drops below code minimum. Many "convert the basement" projects fail at this single check.
• Old garages needing significant retrofit. Especially garages built before current seismic and structural codes — a "garage conversion" can become a near-tear-down quickly.
• Large protected trees, especially Tier 1 in Seattle or equivalent in Eastside cities. Seattle's January 2026 tree law extended protection to 12-inch-diameter trees and up. A protected tree on or near a project site can prevent the project entirely.
• Buried oil tanks. Older houses on the Eastside and Seattle. Decommissioning costs vary; contamination changes the picture entirely.
• Low fire flow. Triggers required sprinklers in new structures, which adds cost and sometimes water-supply work.
• 6-inch sewer mains. Often require an upgrade to a larger pipe before adding a unit. 8-inch mains usually don't.
• No nearby Water Availability Certificate. Without an existing approved WAC nearby, getting one is uncertain.
• An existing "rental unit" with no permit. Inheriting an unpermitted ADU is inheriting someone else's risk: insurance, financing, taxes, and code compliance.

A practical version of all this: most ADU projects fail in the investigation phase, not the construction phase. A two-hour conversation with an architect, a brief sewer scope, and a tree walk can save $20,000 in design fees on a project that wasn't going to permit in the first place. That's where the first-pass investigation actually adds value.

Frequently Asked Questions

Do I need to live in the main house to have an ADU? Not in Seattle (the owner-occupancy covenant was removed in 2019), and not in most Eastside cities under the state's 2026 implementation of HB 1337. Verify with each city's current code; some cities still have specific situations where owner-occupancy applies.

How much does a typical Eastside DADU cost to build in 2026? Per the Earth Advantage All About ADUs course (Feb 2026), a new 1,000 sf custom turnkey DADU in Seattle starts at $600,000–$700,000 and goes up. Eastside numbers track closely. A basement-to-AADU conversion typically starts around $250,000. Add design fees of about 7–15% of construction cost, plus permits and consultants.

How long does the permit + design + build process take? About 14–17 months total in Seattle (6–8 months design and permitting, 8–9 months construction), per Earth Advantage. Kirkland's pre-approved DADU plans can compress this meaningfully. Custom Eastside projects with complications (trees, sewer, slope) can run longer.

What's the difference between an ADU, a DADU, and a JADU? An ADU is the umbrella term for any accessory dwelling unit. An AADU (attached) is built into or attached to the main house — usually a basement conversion or addition. A DADU (detached) is a separate structure — a backyard cottage. A JADU (junior) is a smaller unit (typically ≤500 sf) carved out of the existing main residence under WA's HB 1337 / RCW 36.70A.681.

Will adding an ADU raise my property taxes? Almost certainly, though the amount depends on the unit, the assessor, and how soon after permit-final the reassessment happens. Earth Advantage cites two anecdotal King County reassessments: +$71,000 assessed value for a studio AADU and +$194,000 for a 2-bedroom unit. Plan for some reassessment; don't plan for none.

Can an ADU be sold separately from the main house? In Washington, yes — through condominiumization (under RCW 64.34.200 and WUCIOA) or Unit Lot Subdivision. Both require a condo attorney and a surveyor and add cost, but they're how you turn one parcel with two structures into two separately-saleable units. Worth thinking about at the design stage if it's a likely future option.

I want my parent to live nearby — does an ADU make financial sense? Often yes, but the math isn't pure rental yield. Per Earth Advantage's framing, if the alternative is assisted living at $10,000–$15,000 per month ($120,000–$180,000 per year), a $600,000 DADU pays back faster than a rental-only model and provides a quality-of-life value the spreadsheet doesn't capture.

What's special about Kirkland's pre-approved DADU program? The city has reviewed a curated set of DADU plans for code compliance. Homeowners who choose one of those plans skip the longest, most uncertain part of the process. Kirkland is the only Eastside city with a program at this level as of May 2026. (See more here.)

Is an ADU the same thing as a "mother-in-law unit" or a "backyard cottage"? Roughly, yes — those are colloquial terms. "Backyard cottage" usually means a DADU. "Mother-in-law unit" usually means an AADU (basement or attached addition) used for family. Both are ADUs in code terms; both must have a kitchen and a bathroom to qualify as a dwelling.

My lot looks ADU-ready — what should I check first? The five fastest disqualifiers, in roughly the order they kill projects: large protected trees on or near the lot; sewer-main size and condition; lot slope; available water/fire flow; and existing structure condition (for conversions). A short pre-purchase or pre-design investigation can rule out a non-starter before you spend serious money on design.

The Bottom Line

ADUs and DADUs aren't a niche real-estate hobby on the Eastside in 2026 — they're an emerging structural feature of how the region's housing market is changing. Kirkland's pre-approved plans, Seattle's mature program, the state's 2026 implementation of HB 1337, and the underlying demographic math (1.1 million new homes needed in Washington over 20 years; 75% of Seattle households are 1–2 people; tripling of single-family square footage per person since 1950) all point in the same direction.

The honest takeaway from working through the actual numbers: most ADU questions are lot questions, not zoning questions. Two adjacent properties on the same Bellevue street can have very different ADU economics because of trees, slope, sewer, or what's already in the basement. The right first move on any property — purchase, sale, or current ownership — is a focused pre-feasibility investigation: zoning + lot geometry + trees + utilities + structure. Two hours with the right experienced eye saves five-figure mistakes downstream.

If you're evaluating an Eastside property for ADU viability — a purchase, a sale, or a property you already own — that focused investigation is exactly the kind of work I do at CVA Realty Group: a grounded first-pass review of zoning, lot geometry, published pre-approved plans where applicable, and the practical red flags above. The goal isn't to talk you into an ADU; it's to tell you honestly whether your lot supports one, and what it actually costs to find out.

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Information current as of May 2026. Zoning rules, fees, and timelines change frequently — verify directly with the relevant city before making decisions. This guide is informational only and does not constitute legal, tax, financial, or real estate advice. The cost, timeline, and rule figures above are sourced from the Earth Advantage All About ADUs course (Seattle, Feb 2026) by Sheri Newbold, AIA, and from the cities' own published code; Portland resale figures are from Abdur Abdul-Malik, SRA, AGA, presented at ADU Hour. Chandru Swaminathan is a real estate broker with CVA Realty Group, licensed with eXp Realty, and holds the ADU Specialist designation through Earth Advantage.