Mercer Island, WA · COMPARISON_GUIDE
Detached ADU vs. Attached ADU in Mercer Island
Same code chapter, different economics. AADUs are cheaper but share walls. DADUs cost more but rent and resell as standalone units.
Side-by-side comparison
| Metric | Detached ADU (DADU) | Attached ADU (AADU) | Winner |
|---|---|---|---|
| Base cost | $180K–$400K | $95K–$320K | Attached ADU (AADU) |
| Shared walls | None | 1–3 shared | Detached ADU (DADU) |
| Permit complexity | Higher — new structure | Lower — envelope work | Attached ADU (AADU) |
| Rent achieved | $2,400–$3,500 | $1,950–$2,800 | Detached ADU (DADU) |
| Resale lift | Strong | Moderate | Detached ADU (DADU) |
Best for
Detached ADU (DADU)
Owners optimizing for rental income and resale
Best for
Attached ADU (AADU)
Owners optimizing for budget and shorter timeline
FAQ — Mercer Island owners
- Can I build both a DADU and an AADU on one lot?
- In Seattle and most King County cities, yes — current code allows two ADUs per single-family lot, one detached and one attached.
Other comparisons for Mercer Island owners
Same comparison, nearby cities
Which makes sense for your Mercer Island lot?
$1,500 feasibility includes a side-by-side recommendation for your exact site.
Request Mercer Island feasibilityFrequently asked
Should I choose Detached ADU vs. Attached ADU in Mercer Island?
The Detached ADU vs. Attached ADU decision in Mercer Island comes down to four variables: lot geometry (does the configuration physically fit setbacks?), cost delta (is the marginal price worth the marginal benefit?), permitting risk (does the configuration trigger additional reviews?), and resale/rental impact (what does the Mercer Island market actually pay for this configuration?). Our comparison page works through all four with current Mercer Island data so you make the call with numbers, not vibes.
Go deeper: Read the Puget Sound ADU timeline guide: kickoff to keys guide
What does Detached ADU vs. Attached ADU cost in Mercer Island, WA?
Detached ADU vs. Attached ADU pricing in Mercer Island for 2026 typically falls within the $330–$380/sqft all-in band, with specific position driven by size, finish tier, and any AHJ-specific reviews the configuration triggers. The comparison tables show side-by-side cost ranges so you can model both options against your budget and target rent or hold value.
Does Detached ADU vs. Attached ADU affect Mercer Island permit timeline?
Sometimes yes. Some Detached ADU vs. Attached ADU configurations are reviewed under the streamlined ministerial track required by RCW 36.70A.681, while others trigger additional review (design review for a historic district, drainage review for higher impervious, or critical-areas review on mapped lots). We name the specific review tracks that apply so the timeline difference is concrete, not abstract.
How does Detached ADU vs. Attached ADU compare on rental yield in Mercer Island?
Detached ADU vs. Attached ADU rental yield in Mercer Island depends on configuration's appeal to the local tenant pool. Detached + 1BR typically commands a 5–10% premium over attached or studio at the same square footage because tenants value privacy and full kitchen. Run the side-by-side rent comp on the page for current quarter Mercer Island data.
Go deeper: Read the Puget Sound ADU timeline guide: kickoff to keys guide
Which Detached ADU vs. Attached ADU option has better resale value in Mercer Island?
Mercer Island appraisers value separately-metered, permitted ADUs via income approach (cap rate × NOI) or sales comp. Detached, larger, and fully-permitted always outperforms attached or smaller in appraised value per dollar spent, by roughly 6–14% on average. The compare tables quantify the spread for your specific Detached ADU vs. Attached ADU decision.
Go deeper: Read the Puget Sound ADU timeline guide: kickoff to keys guide
Is Detached ADU vs. Attached ADU eligible for fast-track permitting in Mercer Island?
Mercer Island's fast-track tracks (catalog plans, ministerial review under HB 1337) apply to many Detached ADU vs. Attached ADU configurations but not all. Configurations triggering design review, critical-areas review, or non-standard structural details fall outside fast-track. Our compare page calls out which fast-track each option is eligible for so the timeline expectations are honest.
Go deeper: Read the Puget Sound view corridor & height restrictions on ADUs (2026) guide