An accessory dwelling unit (ADU) is a residential living unit providing independent living facilities and permanent provisions for sleeping, cooking, sanitation, and living on the same lot as a single-family house or other housing unit.  As of July 1, 2021, cities are prohibited from requiring the provision of off-street parking for ADUs within a quarter mile of a high capacity transportation system stop, a rail stop, or certain bus stops unless the city determines that on-street parking is infeasible for the ADU.

Cities with more than 20,000 people, and counties with more than 125,000 people or that are required to plan under the Growth Management Act are required to incorporate in their development and zoning regulations recommendations made by the then Department of Community, Trade, and Economic Development for encouraging the development of accessory apartments in 1993. 

The ADUs may be subject to various local development regulations, potentially including, among other things, maximum floor space limits, lot setback requirements, and roof height limits.

Impact fees are assessed by a local government on a new development to help pay for the increased services that will be required because of that development.  Other fees on development may include permit fees and plan review fees. 

A short-term rental is a lodging use, outside of a hotel, motel, or bed and breakfast, in which a room is offered for a fee for fewer than 30 consecutive nights. 

Real property subject to property taxes includes the land itself, as well as all structures on the land. 

Cities and counties may offer incentives for the development or construction of ADUs, including by waiving or deferring fees, including impact fees, deferring the payment of taxes, or waiving specific regulations. Such incentives may only be offered if the ADUs are subject to binding commitments that the units will not regularly be offered for short term rental and the local government has a program to audit compliance with the commitments.