How big can a detached ADU (DADU) be in Seattle?

Under current Seattle SDCI rules, a DADU may be up to 1,000 sq ft of gross floor area on most single-family lots, with a maximum height of roughly 18–24 ft depending on roof form and lot size. We confirm the exact envelope against your zone (NR1/NR2/NR3) and lot coverage before design starts.

Do I still need to live on-site to build a DADU after WA HB 1337?

No. Washington's HB 1337 (effective 2024) removed owner-occupancy requirements for ADUs in cities that have implemented the law, including Seattle, Bellevue, Kirkland and Redmond. You can rent both your primary home and the DADU as long-term rentals.

How long does a DADU project take from contract to keys in Seattle?

Plan on 6–8 months of construction once permits are issued. SDCI permit review typically adds 3–5 months on top of design. We compress that timeline by submitting through the Pre-Approved DADU program when your lot qualifies.

Will a DADU trigger a sewer capacity charge from King County?

Yes — a new DADU on King County sewer service almost always triggers a Capacity Charge (paid monthly for 15 years) plus a side-sewer permit. We include those costs in our feasibility report so the number isn't a surprise at permit submittal.

Can I get a separate address and utility meters for the DADU?

Yes. Seattle issues a distinct address for the DADU, and Seattle City Light + SPU will set independent power and water meters when requested. Separate metering is what makes a DADU financeable as a true rental income asset.

What does a DADU actually cost to build in Seattle in 2025?

Our turnkey DADU range is $325K–$525K depending on size, site conditions, and finish level. Sloped lots, deep utility runs, or rock excavation push the upper end. We deliver a fixed-price contract after the feasibility study, not a moving estimate.

What is included in DETACHED ADU?

Our DETACHED 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 DETACHED ADU different from a generic remodel?

DETACHED 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 DETACHED ADU?

DETACHED 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 DETACHED ADU require?

Standard DETACHED 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 DETACHED ADU take from contract to completion?

9–14 months end-to-end is typical for DETACHED 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 DETACHED 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 DETACHED 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 DETACHED 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 DETACHED 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 DETACHED 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 DETACHED ADU projects for income-qualified owners. Go deeper: Seattle ADU financing options (/financing/seattle).

Do you provide references for DETACHED ADU work?

Yes — we provide 5+ DETACHED 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_01 / DADU

DETACHED ADU

A detached accessory dwelling unit (DADU) is a fully independent residence on your existing single-family lot. We design and build DADUs that meet rental code, hold long-term value, and look like they belong — not like a kit dropped in the yard. Every project starts with a feasibility study and ends with a turn-key handover.

START THIS PROJECT COST GUIDE

Deliverables

SPEC_01.A

01Stamped architectural set
02Structural + MEP engineering
03Permit submittal & city liaison
04Foundation, framing, weather envelope
05Finish carpentry, cabinetry, fixtures
06Final inspection + warranty package

Spec sheet

SPEC_01.B

+Standing-seam metal roofing
+Vertical western red cedar siding
+Andersen 100-Series black aluminum windows
+Polished or stained concrete slab
+Heat-pump mini-split HVAC

ADJACENT_SPECS

02 //

Recent DADU projects

Full archive →

DADU questions

SPEC_01.FAQ

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

Q.01
How big can a detached ADU (DADU) be in Seattle?
Under current Seattle SDCI rules, a DADU may be up to 1,000 sq ft of gross floor area on most single-family lots, with a maximum height of roughly 18–24 ft depending on roof form and lot size. We confirm the exact envelope against your zone (NR1/NR2/NR3) and lot coverage before design starts.
Q.02
Do I still need to live on-site to build a DADU after WA HB 1337?
No. Washington's HB 1337 (effective 2024) removed owner-occupancy requirements for ADUs in cities that have implemented the law, including Seattle, Bellevue, Kirkland and Redmond. You can rent both your primary home and the DADU as long-term rentals.
Q.03
How long does a DADU project take from contract to keys in Seattle?
Plan on 6–8 months of construction once permits are issued. SDCI permit review typically adds 3–5 months on top of design. We compress that timeline by submitting through the Pre-Approved DADU program when your lot qualifies.
Q.04
Will a DADU trigger a sewer capacity charge from King County?
Yes — a new DADU on King County sewer service almost always triggers a Capacity Charge (paid monthly for 15 years) plus a side-sewer permit. We include those costs in our feasibility report so the number isn't a surprise at permit submittal.
Q.05
Can I get a separate address and utility meters for the DADU?
Yes. Seattle issues a distinct address for the DADU, and Seattle City Light + SPU will set independent power and water meters when requested. Separate metering is what makes a DADU financeable as a true rental income asset.
Q.06
What does a DADU actually cost to build in Seattle in 2025?
Our turnkey DADU range is $325K–$525K depending on size, site conditions, and finish level. Sloped lots, deep utility runs, or rock excavation push the upper end. We deliver a fixed-price contract after the feasibility study, not a moving estimate.

AUTHORITY / Verify the spec

Official resources & sources

KNOWLEDGE_CENTER / DETACHED ADU

In-depth guides on detached adu

Browse the full Knowledge Center →

FAQ

Frequently asked

What is included in DETACHED ADU?
Our DETACHED 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 DETACHED ADU different from a generic remodel?
DETACHED 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 DETACHED ADU?
DETACHED 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 DETACHED ADU require?
Standard DETACHED 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 DETACHED ADU take from contract to completion?
9–14 months end-to-end is typical for DETACHED 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 DETACHED 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 DETACHED 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 DETACHED 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 DETACHED 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 DETACHED 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 DETACHED ADU projects for income-qualified owners.
Go deeper: Seattle ADU financing options
Do you provide references for DETACHED ADU work?
Yes — we provide 5+ DETACHED 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