Will adding an ADU raise my property taxes in Mercer Island?

Yes. Under RCW 84.40.0301, the county assessor adds the new ADU's value to your parcel's assessed value at the next assessment cycle after certificate of occupancy. In Mercer Island, expect roughly +$380k of assessed value on an $484k build. At the current combined levy rate of $6.94/$1,000, that's about $2,637/yr extra — roughly $220/mo. Go deeper: Read: What an ADU does to your property tax in King County (/insights/property-tax-impact-king-county).

What's the combined property tax levy rate in Mercer Island?

Mercer Island's 2025 combined effective rate is about $6.94 per $1,000 of assessed value. That bundles state schools (RCW 84.52.065), county general, city, school district M&O, fire/EMS, library, port, and any voter-approved special levies. Washington's statutory 1% cap (RCW 84.55) applies to regular levies only — voted bonds and EMS levies sit on top. Go deeper: Read: What an ADU does to your property tax in King County (/insights/property-tax-impact-king-county).

Does Washington tax ADU rental income at the state level?

No. Washington has no personal or business income tax on rental receipts. You still file federal Schedule E (Form 1040) and pay federal income tax on net rental income, but there's no WA state income tax, which is a structural advantage over Oregon, California, or Idaho ADU owners. The city of Mercer Island may impose a B&O tax on rental gross — most cities exempt small landlords; verify with your CPA. Go deeper: Read: Seattle ADU rental comps in 2025 (/insights/seattle-rental-comps-2025).

How does depreciation actually reduce my Mercer Island ADU tax bill?

IRS Pub 527 lets you depreciate the building portion (land excluded) of a residential rental over 27.5 years, straight-line. On an $484k Mercer Island build with ~85% allocated to improvements, that's about $14,960/yr of non-cash deduction. Combined with operating expenses, most Mercer Island ADUs show a small tax loss in years 1–3 even when cash-flow positive. Go deeper: Read: What an ADU does to your property tax in King County (/insights/property-tax-impact-king-county).

What's my approximate Year-1 net taxable rental income on a Mercer Island ADU?

Rough math: market rent ~$34,200/yr − depreciation $14,960 − property tax delta $2,637 − ~35% opex (insurance, maintenance, vacancy, mgmt) = about $4,633 of taxable rental income in Year 1. Cost-segregation studies can pull more depreciation forward and often push that figure negative for high-bracket filers. Talk to your CPA — these are directional, not advice. Go deeper: Read: Seattle ADU rental comps in 2025 (/insights/seattle-rental-comps-2025).

Can I deduct ADU construction loan interest on my federal return?

Construction-period interest on the ADU loan is generally capitalized into basis under IRC §263A — not deducted in the year paid. Once the ADU is rented, ongoing mortgage interest, HELOC interest used for the build, property tax, repairs, insurance, depreciation, and a portion of utilities all flow to Schedule E as rental deductions in Mercer Island. Go deeper: Federal Way ADU financing options (/financing/federal-way).

Does Mercer Island or King/Pierce/Snohomish/Thurston County offer a property tax exemption for ADUs?

No general property-tax exemption exists for ADUs in Mercer Island. WA's senior/disabled exemption (RCW 84.36.381) caps qualifying income at roughly $58k (2025) and applies to your primary residence assessment only. MFTE is restricted to designated multifamily zones, not typical single-family ADUs. Go deeper: Read: What an ADU does to your property tax in King County (/insights/property-tax-impact-king-county).

What about REET (real estate excise tax) when I eventually sell?

Washington's graduated REET (RCW 82.45) applies to the full sale price at sale, not to the ADU specifically. Adding an ADU raises sale value, which raises REET due — but on a Mercer Island sale the ADU also typically lifts buyer pool and price/sqft. Section 1031 like-kind exchanges (federal) can defer federal capital gains on the rental portion if you trade into another investment property. Go deeper: Read: What an ADU does to your property tax in King County (/insights/property-tax-impact-king-county).

Will adding an ADU raise my property taxes in Mercer Island?

Yes. Under RCW 84.40.0301, the county assessor adds the new ADU's value to your parcel's assessed value at the next assessment cycle after certificate of occupancy. In Mercer Island, expect roughly +$380k of assessed value on an $484k build. At the current combined levy rate of $6.94/$1,000, that's about $2,637/yr extra — roughly $220/mo.

What's the combined property tax levy rate in Mercer Island?

Mercer Island's 2025 combined effective rate is about $6.94 per $1,000 of assessed value. That bundles state schools (RCW 84.52.065), county general, city, school district M&O, fire/EMS, library, port, and any voter-approved special levies. Washington's statutory 1% cap (RCW 84.55) applies to regular levies only — voted bonds and EMS levies sit on top.

Does Washington tax ADU rental income at the state level?

No. Washington has no personal or business income tax on rental receipts. You still file federal Schedule E (Form 1040) and pay federal income tax on net rental income, but there's no WA state income tax, which is a structural advantage over Oregon, California, or Idaho ADU owners. The city of Mercer Island may impose a B&O tax on rental gross — most cities exempt small landlords; verify with your CPA.

How does depreciation actually reduce my Mercer Island ADU tax bill?

IRS Pub 527 lets you depreciate the building portion (land excluded) of a residential rental over 27.5 years, straight-line. On an $484k Mercer Island build with ~85% allocated to improvements, that's about $14,960/yr of non-cash deduction. Combined with operating expenses, most Mercer Island ADUs show a small tax loss in years 1–3 even when cash-flow positive.

What's my approximate Year-1 net taxable rental income on a Mercer Island ADU?

Rough math: market rent ~$34,200/yr − depreciation $14,960 − property tax delta $2,637 − ~35% opex (insurance, maintenance, vacancy, mgmt) = about $4,633 of taxable rental income in Year 1. Cost-segregation studies can pull more depreciation forward and often push that figure negative for high-bracket filers. Talk to your CPA — these are directional, not advice.

Can I deduct ADU construction loan interest on my federal return?

Construction-period interest on the ADU loan is generally capitalized into basis under IRC §263A — not deducted in the year paid. Once the ADU is rented, ongoing mortgage interest, HELOC interest used for the build, property tax, repairs, insurance, depreciation, and a portion of utilities all flow to Schedule E as rental deductions in Mercer Island.

Does Mercer Island or King/Pierce/Snohomish/Thurston County offer a property tax exemption for ADUs?

No general property-tax exemption exists for ADUs in Mercer Island. WA's senior/disabled exemption (RCW 84.36.381) caps qualifying income at roughly $58k (2025) and applies to your primary residence assessment only. MFTE is restricted to designated multifamily zones, not typical single-family ADUs.

What about REET (real estate excise tax) when I eventually sell?

Washington's graduated REET (RCW 82.45) applies to the full sale price at sale, not to the ADU specifically. Adding an ADU raises sale value, which raises REET due — but on a Mercer Island sale the ADU also typically lifts buyer pool and price/sqft. Section 1031 like-kind exchanges (federal) can defer federal capital gains on the rental portion if you trade into another investment property.

CAP_16 / TAX / MERCER ISLAND

ADU property tax & rental income tax in Mercer Island, WA

The exact reassessment, levy, and depreciation math we walk every Mercer Island client through before they break ground — pulled from current county assessor data, IRS Pub 527, and Washington's no-income-tax framework. Not tax advice; ground truth for your CPA conversation.

The Mercer Island tax numbers

Build cost (800sqft DADU): $484k; All-in, this market
Assessment lift: +$380k; Post-CofO reassessment
Levy rate: $6.94/$1k; 2025 combined effective
Annual tax delta: +$2,637; ≈ $220/mo
Year-1 depreciation: $14,960; 27.5-yr SL on basis
Yr-1 net taxable rent: $4,633; Directional only

Levy rate is the 2025 combined effective rate for a representative TCA inside Mercer Island (state + county + city + schools + fire/EMS + library + special districts). Your parcel's exact rate varies by TCA — confirm on the county assessor parcel viewer before underwriting.

LEDGER_A / PROPERTY TAX

What the assessor does after CofO

New construction reassessment. RCW 84.40.0301 requires the assessor to add the value of the ADU at the next assessment date after CofO. In Mercer Island, that lift is typically ~$380k on an $$484k build.
Levy math. Assessed value × levy rate ÷ 1,000. $$380k × $6.94 = $2,637/yr (~$220/mo).
1% cap on regular levies (RCW 84.55). Regular district levies can only grow 1%/yr on the existing base, but new construction value is added on top — that's why your bill jumps the first year.
Voted levies + bonds. School bonds, EMS levies, and capital bonds sit outside the 1% cap. Mercer Island bond cycles can move your effective rate ±$0.30/$1k year-over-year.

LEDGER_B / RENTAL INCOME TAX

What flows to Schedule E

No WA state income tax. Unlike Oregon or California landlords, Mercer Island ADU owners owe zero state income tax on rental receipts. Federal only.
Depreciation (IRS Pub 527). 27.5-yr straight-line on the building basis (~85% of build cost). On this Mercer Island project: ~$14,960/yr of non-cash deduction.
Cost-segregation upside. A cost-seg study breaks out 5/7/15-yr property (appliances, flooring, landscaping) for accelerated depreciation — often $$5k of additional Year-1 deduction.
Operating deductions. Insurance, maintenance, management, utilities (tenant-paid portion excluded), and HOA dues all deduct against Schedule E rental income.
Passive loss rules (IRC §469). If your AGI is under $100k, up to $25k of passive rental losses can offset W-2 income; the allowance phases out by $150k AGI.

Where these numbers come from

RCWRevised Code of Washington 84.40.0301 (new construction assessment), 84.55 (1% levy cap), 84.36.381 (senior/disabled exemption), 82.45 (REET).
IRSPublication 527 (Residential Rental Property), Publication 946 (How to Depreciate Property), IRC §263A (UNICAP), §469 (passive activity).
ASSESSORKing, Pierce, Snohomish, and Thurston County 2025 levy rate publications; parcel-level TCA midpoints used where ranges apply.
MARKETBuild cost: Golden State ADU 2023–2025 bid archive scaled by Mercer Island cost multiplier. Rent: Zillow/Apartments.com 1-2BR midpoint, Q4 2025.

We're builders, not CPAs. This page is an underwriting starting point. Confirm every figure with a licensed tax professional before filing or signing a loan.

FAQ

ADU tax questions, answered for Mercer Island

Will adding an ADU raise my property taxes in Mercer Island?
Yes. Under RCW 84.40.0301, the county assessor adds the new ADU's value to your parcel's assessed value at the next assessment cycle after certificate of occupancy. In Mercer Island, expect roughly +$380k of assessed value on an $484k build. At the current combined levy rate of $6.94/$1,000, that's about $2,637/yr extra — roughly $220/mo.
Go deeper: Read: What an ADU does to your property tax in King County
What's the combined property tax levy rate in Mercer Island?
Mercer Island's 2025 combined effective rate is about $6.94 per $1,000 of assessed value. That bundles state schools (RCW 84.52.065), county general, city, school district M&O, fire/EMS, library, port, and any voter-approved special levies. Washington's statutory 1% cap (RCW 84.55) applies to regular levies only — voted bonds and EMS levies sit on top.
Go deeper: Read: What an ADU does to your property tax in King County
Does Washington tax ADU rental income at the state level?
No. Washington has no personal or business income tax on rental receipts. You still file federal Schedule E (Form 1040) and pay federal income tax on net rental income, but there's no WA state income tax, which is a structural advantage over Oregon, California, or Idaho ADU owners. The city of Mercer Island may impose a B&O tax on rental gross — most cities exempt small landlords; verify with your CPA.
Go deeper: Read: Seattle ADU rental comps in 2025
How does depreciation actually reduce my Mercer Island ADU tax bill?
IRS Pub 527 lets you depreciate the building portion (land excluded) of a residential rental over 27.5 years, straight-line. On an $484k Mercer Island build with ~85% allocated to improvements, that's about $14,960/yr of non-cash deduction. Combined with operating expenses, most Mercer Island ADUs show a small tax loss in years 1–3 even when cash-flow positive.
Go deeper: Read: What an ADU does to your property tax in King County
What's my approximate Year-1 net taxable rental income on a Mercer Island ADU?
Rough math: market rent ~$34,200/yr − depreciation $14,960 − property tax delta $2,637 − ~35% opex (insurance, maintenance, vacancy, mgmt) = about $4,633 of taxable rental income in Year 1. Cost-segregation studies can pull more depreciation forward and often push that figure negative for high-bracket filers. Talk to your CPA — these are directional, not advice.
Go deeper: Read: Seattle ADU rental comps in 2025
Can I deduct ADU construction loan interest on my federal return?
Construction-period interest on the ADU loan is generally capitalized into basis under IRC §263A — not deducted in the year paid. Once the ADU is rented, ongoing mortgage interest, HELOC interest used for the build, property tax, repairs, insurance, depreciation, and a portion of utilities all flow to Schedule E as rental deductions in Mercer Island.
Go deeper: Federal Way ADU financing options
Does Mercer Island or King/Pierce/Snohomish/Thurston County offer a property tax exemption for ADUs?
No general property-tax exemption exists for ADUs in Mercer Island. WA's senior/disabled exemption (RCW 84.36.381) caps qualifying income at roughly $58k (2025) and applies to your primary residence assessment only. MFTE is restricted to designated multifamily zones, not typical single-family ADUs.
Go deeper: Read: What an ADU does to your property tax in King County
What about REET (real estate excise tax) when I eventually sell?
Washington's graduated REET (RCW 82.45) applies to the full sale price at sale, not to the ADU specifically. Adding an ADU raises sale value, which raises REET due — but on a Mercer Island sale the ADU also typically lifts buyer pool and price/sqft. Section 1031 like-kind exchanges (federal) can defer federal capital gains on the rental portion if you trade into another investment property.
Go deeper: Read: What an ADU does to your property tax in King County

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INTERNAL_LINKS / Deep Map

ADU property tax & rental income tax in Mercer Island, WA

The Mercer Island tax numbers

What the assessor does after CofO

What flows to Schedule E

Where these numbers come from

ADU tax questions, answered for Mercer Island

Will adding an ADU raise my property taxes in Mercer Island?

What's the combined property tax levy rate in Mercer Island?

Does Washington tax ADU rental income at the state level?

How does depreciation actually reduce my Mercer Island ADU tax bill?

What's my approximate Year-1 net taxable rental income on a Mercer Island ADU?

Can I deduct ADU construction loan interest on my federal return?

Does Mercer Island or King/Pierce/Snohomish/Thurston County offer a property tax exemption for ADUs?

What about REET (real estate excise tax) when I eventually sell?

Continue exploring Mercer Island ADU economics

Built in Mercer Island — what we know that the spec sheet won't tell you.

Environmental & locational intelligence

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0.5-mile amenity density

Active NWS weather alerts

Mercer Island ADU tax angles your CPA probably hasn't run — get the brief.

ADU property tax & rental income tax in Mercer Island, WA

The Mercer Island tax numbers

What the assessor does after CofO

What flows to Schedule E

Where these numbers come from

ADU tax questions, answered for Mercer Island

Will adding an ADU raise my property taxes in Mercer Island?

What's the combined property tax levy rate in Mercer Island?

Does Washington tax ADU rental income at the state level?

How does depreciation actually reduce my Mercer Island ADU tax bill?

What's my approximate Year-1 net taxable rental income on a Mercer Island ADU?

Can I deduct ADU construction loan interest on my federal return?

Does Mercer Island or King/Pierce/Snohomish/Thurston County offer a property tax exemption for ADUs?

What about REET (real estate excise tax) when I eventually sell?

Keep going on Mercer Island

Continue exploring Mercer Island ADU economics

Rooftop solar at this address

Tree · Grass · Weed (3-day)

0.5-mile amenity density

Active NWS weather alerts

Mercer Island ADU tax angles your CPA probably hasn't run — get the brief.