Can I convert my Seattle basement into an attached ADU (AADU)?

In most cases, yes. The two hard requirements are code-compliant egress (a window or door per IRC R310) and minimum 7 ft ceiling height in habitable areas. We frequently underpin or saw-cut for an egress window-well and add a code-compliant exit during the conversion.

Do I need a separate entrance for an AADU in Seattle?

Yes. SDCI requires the AADU to have its own exterior entrance independent of the primary residence. We typically add a side-yard or basement walkout entry with weather protection and a code-compliant landing.

How do I sound-proof a basement AADU from the home above?

We assemble the floor/ceiling between units with Rockwool batts, resilient channel, and double layers of 5/8 in. Type X drywall — typically reaching STC 55+. That meets WA energy code separation and lets a tenant run a vacuum without bothering the upstairs.

Will building an AADU raise my property taxes in King County?

Yes — the King County Assessor reassesses the home after permit closeout, and the added square footage and second kitchen will raise the assessed value. Most clients see the rental income comfortably exceed the tax delta within the first year.

What's the difference between an AADU and a junior ADU in Washington?

An AADU is a fully independent unit with its own kitchen, bath, and entrance, governed by the local ADU code. A junior ADU (JADU) under HB 1337 is smaller, carved from the existing footprint, and may share some facilities — but Seattle has not adopted a separate JADU classification, so we build to the AADU standard.

Can I build an AADU and a DADU on the same Seattle lot?

Yes. Under Seattle's current code (and HB 1337's two-ADU minimum), most single-family lots can host one AADU plus one DADU in addition to the primary home — three units total. We confirm tree, lot-coverage, and FAR limits before committing to the dual-ADU path.

What is included in ATTACHED ADU?

Our ATTACHED ADU scope includes design through completion: feasibility, schematic design, construction documents, permit submittal and review management, construction with one self-performed framing crew plus L&I-licensed MEP subcontractors, all required WA inspections, certificate of occupancy, and warranty. One contract, one license number, one warranty — not a coordination headache across five vendors. Go deeper: Read the Puget Sound ADU timeline guide: kickoff to keys guide (/guides/puget-sound-adu-timeline).

How is ATTACHED ADU different from a generic remodel?

ATTACHED ADU carries code, permit, and construction details that a one-off remodeler hasn't repeated enough times to optimize. Examples: IECC R406 envelope path selection, fire separation when units are within 5 ft, side-sewer slope requirements, IRC Appendix AQ when permitted as a tiny house, separate utility metering rules. We do this scope every month — repetition is what makes our first-cycle approval rate and our schedule reliability stand out. Go deeper: Read the Puget Sound ADU timeline guide: kickoff to keys guide (/guides/puget-sound-adu-timeline).

What is the typical price range for ATTACHED ADU?

ATTACHED ADU pricing in 2026 lands $330–$380/sqft all-in depending on size, lot conditions, finish tier, and AHJ. The wide range is real — small lots with difficult access, soils requiring pier-and-beam, or service upgrades push the upper end; flat lots with good access and a catalog plan land at the lower end. Our feasibility report gives you a ±15% number tailored to your specific lot in two weeks. Go deeper: Seattle ADU cost breakdown (/cost/seattle).

What permits does ATTACHED ADU require?

Standard ATTACHED ADU permit stack: building permit (covers structural, mechanical, plumbing under combined review), side-sewer permit, water meter capacity check, electrical permit, drainage review where applicable, addressing application. Critical-areas review triggers on lots mapped for steep slope, wetland, riparian, or landslide hazard. We pull all permits as a pass-through line in our contract — not marked up. Go deeper: Read the King County side sewer permits for ADUs: cost & timeline (2026) guide (/guides/king-county-side-sewer-permits).

How long does ATTACHED ADU take from contract to completion?

9–14 months end-to-end is typical for ATTACHED ADU: 6–8 weeks design, 8–16 weeks permit, 18–26 weeks construction. Faster on catalog plans with pre-app meetings. Schedule variance is driven by AHJ workload, owner selection pace, and any field-discovered conditions. We publish a Gantt at contract and update it weekly. Go deeper: Read the Puget Sound ADU timeline guide: kickoff to keys guide (/guides/puget-sound-adu-timeline).

What's the warranty on ATTACHED ADU?

One year workmanship, two years mechanical (HVAC, plumbing rough, electrical rough), 10 years structural per RCW 4.16.310. Manufacturer warranties pass through directly: LP SmartSide siding 50-year, GAF Timberline HDZ shingles, Marvin window 20-year, Bosch/GE appliances 1-year. Documented in a written warranty packet at closeout. Go deeper: Workmanship Warranty (/warranty).

Do you self-perform ATTACHED ADU work or subcontract everything?

We self-perform framing, exterior weather barrier, interior trim, and project supervision with W-2 carpenters. MEP trades (plumbing, electrical, HVAC) are subcontracted to L&I-licensed specialty contractors we've worked with 5+ years — WA requires separate PL, EL, HVAC/R licenses that a GC cannot legally self-perform without them. Go deeper: Read the Puget Sound ADU timeline guide: kickoff to keys guide (/guides/puget-sound-adu-timeline).

What's the change-order policy on ATTACHED ADU?

Every change order is written, priced, and signed before work proceeds. We never charge T&M without a not-to-exceed. Owner-initiated changes carry 15% OH&P; field-discovered conditions (rotted sheathing, undocumented buried oil tank, unmapped sewer) are priced at cost +10% with photo documentation. Average ATTACHED ADU project sees 4–7 change orders totaling 3–6% of contract value. Go deeper: Read the Puget Sound ADU timeline guide: kickoff to keys guide (/guides/puget-sound-adu-timeline).

What financing works for ATTACHED ADU?

HELOC (fastest, 8.0–9.5% APR), renovation loan (Fannie HomeStyle / Freddie CHOICERenovation, 7.1–7.9%), construction-to-perm (best for ground-up), or cash-out refi (only if existing rate above market). WSHFC's ADU pilot at ~3.5–4.25% applies to ADU-scoped ATTACHED ADU projects for income-qualified owners. Go deeper: Seattle ADU financing options (/financing/seattle).

Do you provide references for ATTACHED ADU work?

Yes — we provide 5+ ATTACHED ADU references on request: past clients in your target city, your target neighborhood when possible, and at your target scope. References include name, phone, project address, and a one-paragraph project summary. We refresh the reference list quarterly so the contacts are recent and reachable. Go deeper: Read the Puget Sound ADU timeline guide: kickoff to keys guide (/guides/puget-sound-adu-timeline).

SPEC_02 / AADU

ATTACHED ADU

An attached ADU shares a wall (and often a foundation) with your primary home. We specialize in carving these units out of basements, daylight floors, and underused side wings — all while preserving the existing home's character and avoiding common moisture, sound-isolation, and egress mistakes.

START THIS PROJECT COST GUIDE

Deliverables

SPEC_02.A

01Existing-conditions survey
02Egress + life-safety design
03Acoustic + thermal envelope upgrades
04Independent entry construction
05Sub-metering of power & water
06Final inspection + handover

Spec sheet

SPEC_02.B

+Rockwool sound batts in shared assemblies
+Self-adhered weather barrier
+GFCI / AFCI sub-panel
+Tankless or heat-pump water heater
+Engineered hardwood or LVT flooring

ADJACENT_SPECS

01 //

Recent AADU projects

Full archive →

REF_4376 · Aug 2023
Bellevue Wing
Bellevue, WA · 720 sqft
Side-yard expansion of a craftsman home — independent entry, full kitchen, separate utilities.

Full archive →

AADU questions

SPEC_02.FAQ

Real questions from Seattle, Bellevue, Kirkland and Eastside owners evaluating a attached adu project. Code references are current as of 2025 — confirm with us during feasibility.

Q.01
Can I convert my Seattle basement into an attached ADU (AADU)?
In most cases, yes. The two hard requirements are code-compliant egress (a window or door per IRC R310) and minimum 7 ft ceiling height in habitable areas. We frequently underpin or saw-cut for an egress window-well and add a code-compliant exit during the conversion.
Q.02
Do I need a separate entrance for an AADU in Seattle?
Yes. SDCI requires the AADU to have its own exterior entrance independent of the primary residence. We typically add a side-yard or basement walkout entry with weather protection and a code-compliant landing.
Q.03
How do I sound-proof a basement AADU from the home above?
We assemble the floor/ceiling between units with Rockwool batts, resilient channel, and double layers of 5/8 in. Type X drywall — typically reaching STC 55+. That meets WA energy code separation and lets a tenant run a vacuum without bothering the upstairs.
Q.04
Will building an AADU raise my property taxes in King County?
Yes — the King County Assessor reassesses the home after permit closeout, and the added square footage and second kitchen will raise the assessed value. Most clients see the rental income comfortably exceed the tax delta within the first year.
Q.05
What's the difference between an AADU and a junior ADU in Washington?
An AADU is a fully independent unit with its own kitchen, bath, and entrance, governed by the local ADU code. A junior ADU (JADU) under HB 1337 is smaller, carved from the existing footprint, and may share some facilities — but Seattle has not adopted a separate JADU classification, so we build to the AADU standard.
Q.06
Can I build an AADU and a DADU on the same Seattle lot?
Yes. Under Seattle's current code (and HB 1337's two-ADU minimum), most single-family lots can host one AADU plus one DADU in addition to the primary home — three units total. We confirm tree, lot-coverage, and FAR limits before committing to the dual-ADU path.

AUTHORITY / Verify the spec

Official resources & sources

KNOWLEDGE_CENTER / ATTACHED ADU

In-depth guides on attached adu

Browse the full Knowledge Center →

FAQ

Frequently asked

What is included in ATTACHED ADU?
Our ATTACHED ADU scope includes design through completion: feasibility, schematic design, construction documents, permit submittal and review management, construction with one self-performed framing crew plus L&I-licensed MEP subcontractors, all required WA inspections, certificate of occupancy, and warranty. One contract, one license number, one warranty — not a coordination headache across five vendors.
Go deeper: Read the Puget Sound ADU timeline guide: kickoff to keys guide
How is ATTACHED ADU different from a generic remodel?
ATTACHED ADU carries code, permit, and construction details that a one-off remodeler hasn't repeated enough times to optimize. Examples: IECC R406 envelope path selection, fire separation when units are within 5 ft, side-sewer slope requirements, IRC Appendix AQ when permitted as a tiny house, separate utility metering rules. We do this scope every month — repetition is what makes our first-cycle approval rate and our schedule reliability stand out.
Go deeper: Read the Puget Sound ADU timeline guide: kickoff to keys guide
What is the typical price range for ATTACHED ADU?
ATTACHED ADU pricing in 2026 lands $330–$380/sqft all-in depending on size, lot conditions, finish tier, and AHJ. The wide range is real — small lots with difficult access, soils requiring pier-and-beam, or service upgrades push the upper end; flat lots with good access and a catalog plan land at the lower end. Our feasibility report gives you a ±15% number tailored to your specific lot in two weeks.
Go deeper: Seattle ADU cost breakdown
What permits does ATTACHED ADU require?
Standard ATTACHED ADU permit stack: building permit (covers structural, mechanical, plumbing under combined review), side-sewer permit, water meter capacity check, electrical permit, drainage review where applicable, addressing application. Critical-areas review triggers on lots mapped for steep slope, wetland, riparian, or landslide hazard. We pull all permits as a pass-through line in our contract — not marked up.
Go deeper: Read the King County side sewer permits for ADUs: cost & timeline (2026) guide
How long does ATTACHED ADU take from contract to completion?
9–14 months end-to-end is typical for ATTACHED ADU: 6–8 weeks design, 8–16 weeks permit, 18–26 weeks construction. Faster on catalog plans with pre-app meetings. Schedule variance is driven by AHJ workload, owner selection pace, and any field-discovered conditions. We publish a Gantt at contract and update it weekly.
Go deeper: Read the Puget Sound ADU timeline guide: kickoff to keys guide
What's the warranty on ATTACHED ADU?
One year workmanship, two years mechanical (HVAC, plumbing rough, electrical rough), 10 years structural per RCW 4.16.310. Manufacturer warranties pass through directly: LP SmartSide siding 50-year, GAF Timberline HDZ shingles, Marvin window 20-year, Bosch/GE appliances 1-year. Documented in a written warranty packet at closeout.
Go deeper: Workmanship Warranty
Do you self-perform ATTACHED ADU work or subcontract everything?
We self-perform framing, exterior weather barrier, interior trim, and project supervision with W-2 carpenters. MEP trades (plumbing, electrical, HVAC) are subcontracted to L&I-licensed specialty contractors we've worked with 5+ years — WA requires separate PL, EL, HVAC/R licenses that a GC cannot legally self-perform without them.
Go deeper: Read the Puget Sound ADU timeline guide: kickoff to keys guide
What's the change-order policy on ATTACHED ADU?
Every change order is written, priced, and signed before work proceeds. We never charge T&M without a not-to-exceed. Owner-initiated changes carry 15% OH&P; field-discovered conditions (rotted sheathing, undocumented buried oil tank, unmapped sewer) are priced at cost +10% with photo documentation. Average ATTACHED ADU project sees 4–7 change orders totaling 3–6% of contract value.
Go deeper: Read the Puget Sound ADU timeline guide: kickoff to keys guide
What financing works for ATTACHED ADU?
HELOC (fastest, 8.0–9.5% APR), renovation loan (Fannie HomeStyle / Freddie CHOICERenovation, 7.1–7.9%), construction-to-perm (best for ground-up), or cash-out refi (only if existing rate above market). WSHFC's ADU pilot at ~3.5–4.25% applies to ADU-scoped ATTACHED ADU projects for income-qualified owners.
Go deeper: Seattle ADU financing options
Do you provide references for ATTACHED ADU work?
Yes — we provide 5+ ATTACHED ADU references on request: past clients in your target city, your target neighborhood when possible, and at your target scope. References include name, phone, project address, and a one-paragraph project summary. We refresh the reference list quarterly so the contacts are recent and reachable.
Go deeper: Read the Puget Sound ADU timeline guide: kickoff to keys guide