What's the difference between a DADU and an AADU?

A DADU (Detached Accessory Dwelling Unit) is a fully separate structure on the same lot as the main house — a backyard cottage. An AADU (Attached Accessory Dwelling Unit) is built within or attached to the primary residence — basement conversion, ground-floor wing, or upper-level apartment. Both qualify as legal ADUs under Washington HB 1337.

Which is cheaper to build, a DADU or an AADU?

AADUs are typically 25–35% cheaper than DADUs of the same size. An 800 sqft AADU in the Puget Sound runs $215k–$285k turnkey, while a DADU of the same size runs $285k–$355k. AADUs reuse the existing foundation, exterior shell, and utility services — DADUs need all of those new.

Which ADU type rents for more?

DADUs command higher rent — typically $2,100–$2,650/mo for a Seattle 1BR DADU vs. $1,700–$2,150 for a comparable AADU. Tenants pay a premium for full structural separation, private outdoor space, and no shared walls. DADUs also out-perform on short-term rental platforms when STR is allowed.

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

Yes. Washington HB 1337 (RCW 36.70A.681) requires cities to allow two ADUs on any residential lot — typically one detached and one attached, or two attached. Seattle, Bellevue, Kirkland, Redmond, Tacoma, and most Puget Sound cities now permit this configuration.

Which builds faster, DADU or AADU?

AADUs reach certificate of occupancy in roughly 6–8 months from permit submission. DADUs take 9–12 months because they involve full site work, foundation, framing, and new utility services. Seattle's median permit review is 10–14 weeks for AADUs vs. 16–22 weeks for DADUs.

COMPARE_01 / DADU vs AADU

DADU or AADU?
18 factors, one call.

Detached or attached — the most expensive decision in an ADU project, made before you talk to a designer. This is the side-by-side our project managers walk every homeowner through, with real Puget Sound cost, timeline, rent, and code consequences for each path.

MATRIX_01 / 18-Factor Comparison

Head-to-head

Factor	DADU (Detached)	AADU (Attached)	Edge
Build cost (typical 800 sqft)	$285k–$330k turnkey	$230k–$275k turnkey	AADU
$/sqft (Puget Sound)	$330–$380/sqft	$285–$345/sqft	AADU
Permit timeline (Seattle median)	16–22 weeks	10–14 weeks	AADU
Time to certificate of occupancy	9–12 months	6–8 months	AADU
Privacy from main house	Full separation	Shared wall / floor	DADU
Market rent (Seattle 1BR)	$2,100–$2,650/mo	$1,700–$2,150/mo	DADU
Appraisal lift on main property	+12–18% typical	+6–10% typical	DADU
STR (Airbnb) suitability	Excellent — private entry	Mixed — shared utilities	DADU
Required lot area	5,000 sqft+ practical	Any lot with main home	AADU
Setback complexity	5'/10' rear & side, height stepbacks	Inherits main house envelope	AADU
Utility hookup cost	$15k–$45k new services	$3k–$12k extension of existing	AADU
Tree retention exposure	High — new footprint	Low — within existing envelope	AADU
Financing options	Construction-perm, HELOC, renovation, WSHFC	HELOC, renovation, cash-out refi	DADU
Tenant separation for §121 exclusion	Clean separation supports it	IRS scrutiny on shared facilities	DADU
HB 1337 statewide right	Yes — 2 ADUs allowed	Yes — 2 ADUs allowed	Tie
Disruption during construction	Low — no impact on main house	Moderate–high — shared HVAC, walls open	DADU
Future sale-as-condo potential	Possible (where unit-lot subdivisions allowed)	Not possible without condo conversion	DADU
Energy code compliance ease	Easier — purpose-built envelope	Harder — must integrate with existing	DADU

DECIDE_01 / Pick by your goal

Ten common scenarios

Maximize long-term rental incomeDADU

Higher market rent + standalone tenant appeal + better STR convertibility

Smallest budget, fastest occupancyAADU

Lower $/sqft, ½ the permit time, no new utility services

Aging parent moving inDADU

Privacy, single-level accessibility, no shared HVAC/noise

Adult child temporary stayAADU

Cheapest, fastest, can convert back to main-house space later

Maximize property value liftDADU

Independent structure appraises stronger; better comp lift

Small lot (< 5,000 sqft)AADU

DADU setbacks usually impossible; basement/wing build works

Lot with mature trees (Seattle)AADU

Avoids tree-removal exposure under SMC 25.11

Plan to sell ADU separately somedayDADU

Required for unit-lot subdivision and easier condo conversion

Sloped or constrained siteAADU

Basement conversion sidesteps grading and foundation cost

Have a detached garage alreadyDADU

Garage-to-DADU conversion captures existing footprint

CODE_01 / By City

Size & height by city

City	DADU max	AADU max	DADU height	Note
Seattle	1,000 sqft (1,200 w/ 2-story)	1,000 sqft	18'–24' (lot-dependent)	Two ADUs allowed per lot under SMC 23.44.041
Bellevue	1,200 sqft	1,200 sqft	25'	LUC 20.30JJ — two ADUs per lot
Kirkland	1,200 sqft	1,200 sqft	24'	KZC 115 — owner occupancy removed in 2024
Redmond	1,200 sqft	1,200 sqft	25'	RZC 21.08 — two ADUs allowed
Tacoma	1,000 sqft	1,000 sqft	18'	TMC 13.06.150 — HB 1337 compliant
Renton	1,000 sqft	1,000 sqft	18'	RMC 4-2-110 — two ADUs allowed

SOURCES / Verify the code

Official code references

READ_NEXT / Related Resources

Go deeper

SERVICEDetached ADUBackyard DADU build, ground-up.SERVICEAttached ADUBasement, ground-floor, or wing AADU.GUIDEFloor Plans & SizingStudio to 1,200 sqft layouts compared.

DADU or AADU?
18 factors, one call.

Head-to-head

Ten common scenarios

Size & height by city

Official code references

Go deeper

DADU or AADU for YOUR lot? Stop guessing — get a 10-minute call with one clear answer.

Decide DADU vs AADU

DADU or AADU?18 factors, one call.

Head-to-head

Ten common scenarios

Size & height by city

Official code references

Go deeper

DADU or AADU for YOUR lot? Stop guessing — get a 10-minute call with one clear answer.

Decide DADU vs AADU

DADU or AADU?
18 factors, one call.