Bellevue ADU and detached ADU rules

Can I build an ADU or DADU in Bellevue?

Many qualifying Bellevue properties can support one or two ADUs, either inside or attached to the main home or in a separate detached building. The practical question is which option fits your lot and your plans. The answer depends on setbacks, FAR, lot coverage, trees, critical areas, utilities, and Bellevue's August 2026 ordinance package, which is newer than the online codification.

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

Bellevue allows up to two ADUs on a qualifying residential lot. Most may contain up to 1,200 square feet, with up to 300 square feet of parking or unheated storage excluded from that size limit. An attached ADU can be an addition or part of the existing house; a DADU is a separate building. That choice matters because attached ADU floor area is exempt from FAR and does not increase the FAR dwelling count, while a detached ADU counts toward both the lot's dwelling count and its FAR calculation.

Up to 2ADUs on an eligible lotAttached or detached, subject to unit-lot and site rules.

1,200 sq ftTypical maximum per ADUNarrow one-floor and existing-structure exceptions can be larger.

24 / 28 ftDetached heightThe 28-foot path is for an ADU above an existing accessory structure.

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Common homeowner questions

Answers to common Bellevue ADU questions

Can I add an ADU to my Bellevue home?

Often, yes. Bellevue allows attached and internal ADUs on qualifying residential lots. The unit can be created within the existing house or as an addition, but it must be a complete legal dwelling and meet building, utility, access, and permit requirements. Attached ADU floor area is exempt from FAR and does not increase the FAR dwelling count, which can make an addition worth comparing with a detached unit when the main house is already close to the lot's floor-area limit.

Can I build a detached ADU (DADU) in Bellevue?

Bellevue permits detached ADUs citywide on qualifying residential sites, and a lot may support up to two ADUs when all dwelling-count and site rules are met. Most DADUs are limited to 1,200 square feet and 24 feet high, or 28 feet when an ADU is added above an existing accessory structure. A DADU counts toward the lot's dwelling count and FAR, and a new ADU is not allowed on a unit lot created or developed through a unit-lot approval.

What setbacks apply to a Bellevue DADU?

The side setback is generally five feet, while front and rear setbacks depend on the zoning district. Current front setbacks range from 10 to 25 feet and rear setbacks from 10 to 15 feet. An alley can allow a zero setback, but easements, building separation, fire access, critical-area buffers, and tree-protection areas may require more room than the zoning setback alone.

How do FAR and lot coverage affect an ADU addition or DADU?

They are separate fit tests. An attached ADU's floor area is exempt from FAR and does not raise the FAR dwelling count. A DADU counts toward FAR but also moves the property into the applicable multi-dwelling FAR tier. The entire project must still meet structure coverage plus the district's hard-surface and impervious-area limits, so additional FAR does not guarantee that a detached footprint will fit.

Can trees or critical areas prevent a Bellevue DADU?

Yes. Significant or landmark trees, including protection areas for trees near a property line, can occupy the apparent backyard building area. Wetlands, streams, flood areas, habitat, steep slopes, and geologic hazards can add buffers, studies, mitigation, or a Critical Areas Land Use Permit. Check these constraints together with setbacks rather than treating the open lawn as the buildable area.

How can I tell whether an ADU will fit on my Bellevue lot?

Compare more than the ADU size limit. A useful first screen needs the parcel's zoning, lot geometry, existing house and garage, legal units, remaining FAR and coverage, setbacks, trees, critical areas, easements, access, and utility routes. Build Eastside's free feasibility study starts with the address and the home you hope to create, then identifies which attached, conversion, or detached options deserve more design work.

Official sources: Bellevue LUC 20.20.120: accessory dwelling units · Bellevue Ordinance 6931 enactment · Bellevue LUC 20.20.390: floor area ratio · Bellevue LUC 20.20.538: middle-housing dimensions · Bellevue LUC Part 20.25H: critical areas · Bellevue LUC 20.20.900: tree retention · Bellevue LUC 20.50.020: FAR definitions · Bellevue 2026 tree-code update

What is allowed on one lot

LUC 20.20.120, as amended by Ordinance 6931, allows up to two ADUs on a lot in a district that permits a single-family dwelling or middle housing. They may be attached or detached. A detached ADU counts toward the maximum dwellings on the lot; an attached ADU does not. A new ADU is prohibited on a unit lot created or developed under a unit-lot approval, so parent-lot and unit-lot status must be checked first.

Two attached or internal ADUs.
One attached and one detached ADU.
Two detached ADUs where dwelling count, FAR, coverage, trees, access, and building separation allow them.
Conversion of a legal existing structure despite qualifying setback or coverage nonconformity; expansions need separate review.

Official sources: Bellevue LUC 20.20.120: accessory dwelling units · Bellevue Ordinance 6931 enactment · RCW 36.70A.681: limits on local ADU regulation

How much room the property may have for an ADU

Maximum size: The typical maximum is 1,200 square feet. The Director may allow more where an attached ADU occupies an entire floor of the principal dwelling or where an addition or conversion to an existing detached accessory structure meets the codified exception. A guest-cottage conversion without expansion is exempt. Up to 300 square feet per ADU of parking or unheated storage is excluded from the ADU size limit.
Height: A DADU is limited to 24 feet, or 28 feet when added above an existing accessory structure. An attached ADU uses the principal building's height standards.
Setbacks: A DADU follows the current middle-housing dimensions: front yards are 25 feet in LL-1, 20 feet in LL-2, and 10 feet from SR-1 through MDR-2; rear yards are 15 feet in LL-1, LL-2, SR-1 and SR-2, and 10 feet from SR-3 through MDR-2; side yards are five feet. A zero setback is allowed along an alley. Other zero common-side conditions are narrow and should not be assumed for an ordinary DADU.
FAR, lot coverage, or impervious area: Structure coverage is generally 40 percent in LL-1 through SR-3 and 45 percent in SR-4 through MDR. Hard-surface and impervious limits vary by district and must both be checked. Bellevue's FAR table depends on total dwelling count. An attached ADU's floor area is exempt and it does not increase the FAR unit count; a detached ADU counts. The limited FAR exemption for up to 300 square feet of parking or unheated storage per dwelling applies to qualifying middle housing, not automatically to an ordinary single-house garage. Current maximum FAR ranges from 0.5 for one dwelling on a lot of 10,000 square feet or less through 1.5 for six dwellings, with separate large-lot formulas.

Official sources: Bellevue LUC 20.20.120: accessory dwelling units · Bellevue LUC 20.20.010: base residential dimensions · Bellevue LUC 20.20.538: middle-housing dimensions · Bellevue LUC 20.20.390: floor area ratio

When a house is near or over FAR

One target size can produce three very different Bellevue answers

Start with the total living area you want, then decide whether some of that total will be a real second home. Bellevue treats one house, an attached ADU, and a detached ADU differently. This can rescue a program that is over FAR, but it does not create free area or prove that the buildings fit on the ground.

Set one total program

Living area plus every normally counted area, including an enclosed garage.

Choose the legal configuration

One home, part of the total as an attached ADU, or part of the total in a DADU.

Run two separate fit tests

FAR controls counted floor area. Coverage and setbacks control what can occupy the site.

Do not apply one FAR rule to every Bellevue housing configuration.
Configuration	FAR dwelling count	What counts toward FAR	Why a homeowner might consider it
One house	One	The house and enclosed garage generally count. The 300-square-foot middle-housing parking allowance is not applied to an ordinary single-house garage.	This is the conservative default used by the feasibility study when no second dwelling is part of the brief.
House + attached ADU	Still one	The attached ADU's floor area is exempt. The rest of the house and its normally counted areas remain in the one-dwelling calculation.	It can create a legal independent home without consuming FAR for the ADU itself, which can help when the base house is close to its ceiling.
House + DADU	Two	The detached ADU counts toward FAR. The lot moves to the two-counted-dwelling tier, but the DADU consumes part of that larger ceiling.	It provides a physically separate home and may raise the lot's FAR ceiling. It is not free extra floor area.
Qualifying middle housing	Actual middle-housing unit count	The dwelling-count table applies. Up to 300 square feet per middle-housing dwelling used for parking or unheated storage is exempt.	A true multi-unit project can use a higher FAR tier, but it changes the project, plan set, permitting path, and building requirements.

The two formulas most homeowners need

FAR ceiling by counted dwelling

One counted dwellingLot ≤ 10,000 sq ftLot area × 0.50Lot > 10,000 sq ft(First 10,000 × 0.50) + (remainder × 0.30)

Two counted dwellingsLot ≤ 10,000 sq ftLot area × 0.60Lot > 10,000 sq ftLot area × 0.50

See the 3–6 dwelling middle-housing FAR tiers

Counted dwellings	Lot ≤ 10,000 sq ft	Lot > 10,000 sq ft
3	0.80 FAR	0.60 FAR
4	1.00 FAR	0.70 FAR
5	1.20 FAR	0.75 FAR
6	1.50 FAR	0.90 FAR

What usually counts toward Bellevue FAR, and what does not

The ADU size calculation and the property's FAR calculation are different ledgers. An area can be excluded from the ADU's 1,200 sq ft size cap without being excluded from FAR.

Use a labeled code-area schedule; do not assume every garage or storage allowance is an FAR exemption.
Area	Usual FAR treatment	Important nuance
Ordinary living area	Counts	Ground and upper floors of all buildings generally count.
Enclosed garage	Generally counts	Do not apply the middle-housing parking exemption to an ordinary one-house garage.
Attached ADU floor area	Excluded	The unit must be a complete legal dwelling. It does not add a counted FAR dwelling.
DADU floor area	Counts	The DADU adds a counted dwelling, which can move the property to a higher FAR tier.
Carport	Excluded	An enclosed garage is not a carport.
Qualifying lower level	May be excluded	The level must meet Bellevue's grade-based test, generally with less than five feet exposed above finished grade.
Unfinished, nonhabitable attic	Excluded	A finished or habitable attic can be treated differently.
Qualifying porches, decks, exterior balconies, and interior floor openings	Generally excluded	Each feature must meet the definition; enclosing it can change the result.
Space 18 ft or taller	Generally counts twice	Double-height rooms can consume FAR faster than their physical floor plate suggests.
Middle-housing parking or unheated storage	Up to 300 sq ft per dwelling may be excluded	This is a narrow qualifying middle-housing rule, not a blanket ADU-garage exemption.

FAR is not footprint

Coverage can still stop a program that passes FAR

Structure coverage measures the ground occupied by structures. FAR measures counted floor area across levels. Stacking living area can reduce footprint without reducing FAR. An attached ADU uses the principal-home coverage framework; a DADU adds another structure and uses Bellevue's DADU/middle-housing coverage framework.

LL-1 through SR-3: 35% base one-home coverage → 40% with DADU/middle housing
SR-4 through MDR: 40% base one-home coverage → 45% with DADU/middle housing

Coverage is still not a placement plan. Setbacks, building separation, trees, critical areas, easements, driveway and fire access, utilities, hard surface, and impervious area can make the usable envelope smaller.

Do nearby Eastside cities offer the same FAR strategy?

No. Washington sets a broad ADU baseline, but each city's capacity rules work differently.

City	Can an ADU change the floor-area result?	The practical limiter
Bellevue	Yes. Attached ADU area is FAR-exempt. A DADU counts, but moves the lot to the applicable two-dwelling tier.	Separate coverage, placement, legal-unit, tree, critical-area, access, and utility tests.
Kirkland	Different. An attached ADU gives no FAR relief. A qualifying DADU behind the main house can exclude up to 500 sq ft on lots under 8,500 sq ft or 800 sq ft on larger lots.	No higher ADU FAR tier; hardscape counts toward lot coverage, and the DADU location controls the exclusion.
Clyde Hill	No comparable FAR strategy because Clyde Hill does not use a general residential FAR.	30% structural coverage, 60% impervious coverage, full R-1 yards, and no local ADU path on a critical-area or buffer lot.
Medina	No comparable FAR strategy because Medina does not use a general residential FAR.	Zone- and lot-based structural/impervious coverage after three gates: no critical area or buffer, no shoreline jurisdiction, and public sewer required.

Questions to answer before treating this as a design solution

Is the additional home attached or detached, and is it a complete legal dwelling rather than ordinary house space?
How much floor area belongs to the base house, enclosed garage, attached ADU, DADU, and any other structure?
Does any lower-level area qualify for Bellevue's grade-based exclusion, based on a surveyed design?
Does parking or storage actually qualify for a specific exemption, or is the team applying a middle-housing rule to a single house?
Do ADU size, setbacks, height, tree retention, critical areas, access, utilities, fire separation, and permit requirements still allow the configuration?

Use this as a concept-screening path, not a guaranteed workaround. Ask the architect to prepare a labeled code-area schedule and have Bellevue confirm the dwelling count, ADU legality, exclusions, coverage tier, and unit-lot eligibility before relying on the extra capacity.

Official sources: Bellevue LUC 20.20.120: accessory dwelling units · Bellevue Ordinance 6931 enactment · Bellevue LUC 20.20.010: base residential dimensions · Bellevue LUC 20.20.538: middle-housing dimensions · Bellevue LUC 20.20.390: floor area ratio · Bellevue LUC 20.50.020: FAR definitions

Parking, occupancy, renting, and separate sale

Parking: No ADU parking is required when the unit is below 1,000 square feet excluding garage area, or when it is within a half-mile walking distance of a qualifying major transit stop. Otherwise Bellevue requires one space per ADU.
Owner occupancy: Bellevue has no owner-occupancy requirement.
Short-term rental: Bellevue limits ADUs to stays of at least 30 days and prohibits using them for transient lodging under 30 days.
Separate sale: Separate condominium conveyance may be available, but there is no automatic fee-simple or unit-lot split. Parent-lot, unit-lot, title, lender, and condominium requirements need separate review.

Official sources: Bellevue LUC 20.20.120: accessory dwelling units · Bellevue rental-use requirements

Utilities, fire access, and the permit path

Applications use MyBuildingPermit. One or two dwellings in a structure generally use a residential BS permit, while three to six use the middle-housing path; multiple buildings may also need a separate clearing-and-grading/site permit. Sewer, storm, water, fire, septic/well, trade, critical-area, tree, and right-of-way approvals depend on the property and configuration.

Confirm the current district, overlays, parent-lot or unit-lot status, legal existing units and structures, transit eligibility, and whether an earlier application is vested.
Choose attached versus detached before calculating dwelling count, FAR, coverage, height, yards, parking, and tree credits; Bellevue treats the forms differently.
Prepare a survey-based plan showing current grade, easements, utilities, trees and protection areas, critical areas and buffers, access, drainage, hard surface, and impervious area.
Use the current BS or middle-housing building path and identify any separate site, clearing/grading, critical-area, tree, fire, utility, right-of-way, shoreline, or health approvals.
Confirm current fees and utility connection or capacity costs rather than assuming an ADU-wide exemption.

Official sources: Bellevue new-house and large-addition permits · Bellevue middle-housing building permits · Bellevue permit fees and estimator

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.

Ordinances 6931–6933 took effect in early August 2026 and are newer than the currently codified online text; use the enactments and confirm vesting.
A detached ADU changes the FAR dwelling count and tree-credit rate; an attached ADU does not.
A new ADU is not permitted on a unit lot created or developed through a unit-lot approval, even though an ADU may be possible on the parent lot.
Significant and landmark trees, including off-site or property-line protection areas, can eliminate the apparent DADU footprint.
Wetlands, streams, flood areas, habitat, steep slopes and geologic hazards can require a Critical Areas Land Use Permit, reduced density, large buffers, or professional reports.

Official sources: Bellevue LUC 20.20.900: tree retention · Bellevue 2026 tree-code update · Bellevue LUC Part 20.25H: critical areas

Address-level ADU feasibility

What to check before assuming a detached ADU will fit

Start with the outcome you want: an ADU addition, a conversion inside the home, a converted garage, or a new backyard DADU. Bellevue treats those options differently. An address-level screen can compare them against the lot's remaining FAR and coverage, actual yards, tree-protection areas, mapped constraints, access, and utilities before you pay to adapt a plan.

Jurisdiction and zoning

Confirm the address is actually governed by Bellevue and select the correct base-zone standards.

Existing and proposed units

Count the principal dwelling, existing legal ADUs, middle-housing units, and the proposed attached or detached configuration.

Surveyed parcel geometry

Use legal lot lines, front/rear designation, alleys, access easements, and existing structures rather than a tax-map rectangle.

Capacity calculations

Test the ADU against the rules that apply locally: FAR or GFA, building or lot coverage, impervious or hard surface, and landscaping.

Mapped and field constraints

Screen critical areas and buffers, shorelines, slopes, flood hazards, significant trees, drainage, and recorded easements.

Utilities and emergency access

Verify water, sewer or septic, fire flow, hydrant and apparatus access, power, meters, and trench routes.

The engine must not use the online codification alone until Ordinances 6931–6933 are incorporated. It should route unit-lot sites, critical areas, significant or landmark tree impacts, legal nonconforming conversions, and vesting to manual review.

Do not rely on the older rule

Two different kinds of old information are risky here: pre-June 2025 material that says detached ADUs are not allowed, and the codified website that has not yet incorporated Ordinances 6931–6933 effective in August 2026. The current guide overlays the adopted enactments on the code.

Primary sources

Official Bellevue 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.

Bellevue LUC 20.20.120: accessory dwelling unitsCodified ADU size, configuration, conversion, parking, use, and development rules, read with Ordinance 6931.Bellevue Ordinance 6931 enactmentAdopted July 28 and effective August 4, 2026; current two-ADU and related amendments not yet fully codified online.Bellevue LUC 20.20.010: base residential dimensionsBase one-home structure coverage, hard-surface, impervious-area, setback, and height standards by current district.Bellevue LUC 20.20.538: middle-housing dimensionsCurrent district setbacks, height, coverage, hard-surface, impervious, and dwelling-count framework, read with the 2026 enactments.Bellevue LUC 20.20.390: floor area ratioCurrent dwelling-count FAR table and attached-versus-detached ADU treatment.Bellevue LUC 20.50.020: FAR definitionsWhat residential floor area includes and the specific exclusions for lower levels, attics, carports, porches, and decks.Bellevue rental-use requirementsCurrent 30-day minimum and transient-lodging prohibition for ADUs.Bellevue new-house and large-addition permitsMyBuildingPermit, BS permit, site, grading, drainage, trade, and submittal guidance.Bellevue middle-housing building permitsCurrent BS, MH, multiple-building and site-permit pathways.Bellevue permit fees and estimatorCurrent valuation, hourly review, utility, impact, and estimator links.Bellevue LUC 20.20.900: tree retentionCodified significant-tree, landmark-tree, credit, protection, replacement, and site-plan rules, read with Ordinance 6932.Bellevue 2026 tree-code updateOfficial summary and adopted-current materials for the August 2026 tree amendments.Bellevue LUC Part 20.25H: critical areasWetland, stream, flood, habitat, geologic-hazard, buffer, report, and permit standards, read with Ordinance 6932.RCW 36.70A.681: limits on local ADU regulationWashington's current baseline for two ADUs, size, height, parking, owner occupancy, conversions, alleys, fees, and critical-area exceptions.

What to check first

1
Confirm the parcel is inside Bellevue 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 September 11, 2026. 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.