Every backyard-home term, in plain English.

01

Accessory Dwelling Unit (ADU)

A second, legally independent home on a lot zoned for a single house, with its own kitchen, bathroom, sleeping area and entrance. ADUs can be attached (AADU) or detached (DADU).

02

Attached ADU (AADU)

An accessory dwelling unit built inside or onto the main house — for example a basement conversion, garage conversion, or addition with a separate entrance.

03

Backyard cottage

A common name in Seattle for a DADU — a small, detached home built in the backyard of an existing house.

04

Detached ADU (DADU)

A Detached Accessory Dwelling Unit — a standalone second home on the same lot as the main house, with its own foundation, utilities and roof.

05

Egress

A code-required way to exit a room in an emergency, such as a properly sized window or door. Bedrooms and basement conversions must meet egress rules to be legal living space.

06

Environmental Critical Area (ECA)

A protected or hazardous site condition — steep slope, landslide- or flood-prone ground, wetland or similar — that carries extra permitting requirements and can raise site-development costs.

07

Feasibility study

An upfront assessment of whether and how a project can be built on a specific lot, reviewing zoning, setbacks, ECAs, utilities and soils, and producing a realistic starting budget before major spending begins.

08

Footprint

The area of ground a building covers, measured at its outer walls. A DADU’s footprint is limited by lot-coverage and setback rules.

09

Growth Management Act (GMA)

A Washington state law that guides how cities and counties plan for growth. Recent state ADU reforms apply to jurisdictions planning under the GMA.

10

Hard costs

The direct cost of physically building — foundation, framing, systems and finishes. Typically 70–85% of a DADU budget.

11

Impact fees

Fees local governments charge new construction to help pay for public infrastructure. Washington’s ADU law caps the impact fees that can be charged for an accessory dwelling unit.

12

In-fill

New construction built on vacant or underused land within an already-developed neighborhood, rather than on the edge of a city. ADUs are a form of gentle in-fill housing.

13

Lot coverage

The share of a lot that structures are allowed to cover, set by zoning. It’s one of the limits that determines how large a DADU can be.

14

Owner-occupancy requirement

An old rule requiring the property owner to live on-site to have an ADU. Washington’s ADU reforms removed this requirement in many cases, making rentals more practical.

15

Setback

The minimum required distance between a structure and the property lines. Setbacks shape where — and how big — a DADU can be placed on a lot.

16

Side sewer

The private pipe that connects a building’s plumbing to the public sewer main. A new DADU usually needs its own side-sewer connection and permit.

17

Site development

All the work that prepares a lot to build on — excavation, grading, drainage, utilities, retaining walls and foundations. On difficult lots this can be 15–30%+ of the total cost.

18

Soft costs

The non-construction costs of a project — architecture, engineering, permits and geotechnical work. Typically about 10–15% of a DADU budget.

Want the bigger picture? Read the ADU & DADU FAQ, see how Washington's ADU laws changed, or estimate a build with the ADU cost calculator.