ADU & construction glossary

An ADU that shares one or more walls with the primary home — typically a basement or upper-floor conversion.

A self-contained second dwelling on a single-family lot, with its own kitchen, bath, sleeping area and entry.

A free-standing ADU built as a separate structure from the main house — Seattle's backyard cottage.

A cap on total built square footage as a ratio of lot area — Seattle's NR3 is 0.5 FAR for the primary home.

Washington's 2023 statewide ADU law — two ADUs per lot, no owner-occupancy, no extra parking.

A small interior ADU under 500 sq ft created within the primary home's existing footprint.

The percentage of a lot occupied by buildings — typically capped at 35–45% for residential zones.

The minimum distance a structure must be from a property line — usually 5 ft side, 5–10 ft rear for DADUs.

A separate aesthetic review board process triggered by certain lot types or larger projects.

A separate right-of-way permit required when modifying or adding a driveway approach to a public street.

A pre-design report confirming what's actually buildable on your lot before architecture spend.

A licensed engineer's report on soil-bearing capacity and slope stability — required on ECA lots.

A library of vetted DADU designs that bypass full architectural plan review for faster permitting.

Seattle's permitting agency — issues all building, land use, and electrical permits inside city limits.

A mandatory air-leakage test for new construction — WSEC requires ≤4.0 ACH50 for ADUs.

The code-required escape opening from every sleeping room — minimum 5.7 sq ft clear opening.

A relaxed code section for dwellings under 400 sq ft — lower ceiling and stair requirements.

A signed document where a contractor or supplier waives their right to file a lien against the property.

A factory-built home certified to federal HUD code — different legal class from a modular or stick-built ADU.

A factory-built ADU shipped to site in finished modules — same code as stick-built, faster shell only.

The final list of small defects and incomplete items that must be resolved before project close-out.

The contractual milestone when an ADU is usable for its intended purpose — triggers final payment and warranty start.

A structured pass through a design to cut cost without changing functional intent.

Washington's building energy code — currently WSEC 2021, requires heat pumps and high-performance envelopes.

The increase in your home's appraised value attributable to adding an ADU — typically $220K–$380K for a DADU.

A loan that underwrites against the after-completion value of the property instead of current value.

A rental of fewer than 30 nights — Seattle requires an STR license and caps hosts at two units.

An electrified HVAC unit that's now effectively required for new ADUs under Washington's energy code.

A 15-year monthly fee King County levies on every new dwelling connected to its sewer system.

The private sewer line connecting a house to the city main — usually the owner's responsibility to upgrade.

Seattle/King County overlay covering steep slopes, wetlands, liquefaction zones and riparian buffers.

Hard surfaces that prevent rainwater from soaking in — triggers stormwater review above 2,000 sq ft new.

An old rule requiring the owner to live on-site — eliminated statewide by HB 1337 in 2024.

The public-owned strip including the street, sidewalk, and planter area in front of every lot.

Seattle's mandatory rental registration program — every rented ADU must be registered and periodically inspected.