DADU · BACKYARD COTTAGE
Detached ADU
A small, freestanding house in the backyard with its own entry, kitchen and bath. The most private kind of ADU — and the most involved to build.
SHEETA-201
VIEWFRONT ELEVATION
EXAMPLE1BR · 26' × 32'
TYPICAL SIZE400–1,000 SF
PLANNING RANGE$170k–$585k
UTILITIESOwn runs
BEST ONDeep lots
01WHY BUILD ONE
- +Real privacyNo shared walls, separate entry and outdoor space for each household.
- +Strongest rentA standalone cottage usually rents for more than a unit inside the house.
- +Design freedomIts own layout, roof and orientation — not limited by the main house.
- +Can be sold as a condoWashington law lets ADUs be sold separately through a condominium.
02WHAT TO PLAN FOR
- −Highest cost per footA new foundation, roof and full set of systems for a small building.
- −Utility runsSewer, water and power often have to travel the length of the lot.
- −Yard spaceSetbacks, trees and lot coverage decide how big it can be.
- −Site accessNarrow side yards make excavation and deliveries slower.
03WHAT MOVES THE PRICE
Five things that set the cost.
SizeThe biggest single driver — every square foot carries structure and finish.
Utility distanceHow far the new side sewer and service lines have to run.
Site & foundationSlope, soil and access for equipment.
Finish levelCabinets, counters, flooring and fixtures.
Design pathA pre-approved plan versus a custom design and engineering.
04PRICE A DETACHED ADU
DETACHED ADU · SEATTLE
$414–639 / SF · includes new sewer, water and electric service
DETACHED ADU · {{cityUpper}}
{{perSf}} · includes new sewer, water and electric service
05QUESTIONS
Can I build a detached ADU and an attached one?
Yes. State law requires cities to allow two ADUs per lot in any combination of attached and detached.
Does it need its own address and meters?
It gets its own address. Separate meters are usually optional, but they simplify billing a tenant and are needed to sell it as a condo.
Can I add a second story?
Often. Cities must allow at least 24 feet of height, which fits two stories or a unit over a garage.
