Whether it be a guest house, home office or for aging parents an ADU (Accessory Dwelling Unit) is a great answer to use that extra space.

ADUs are an affordable type of home to construct in Washington and Idaho because they do not require paying for land, major new infrastructure, structured parking, or elevators.

ADUs can provide as much living space as many newly-built apartments and condominiums, and they’re suited well for couples, small families, friends, young people, and seniors.

ADUs can provide a source of income for homeowners.

Work from home in a separate space.

ADUs give homeowners the flexibility to share independent living areas with family members and others, allowing seniors to age in place as they require more care.

Guests and family are welcome and comfortable.
WA House Bill 1337 (codified as RCW 36.70A.680, 681 and 696) requires fully planning cities and counties to allow two ADUs on all residential lots that allow single family homes within an urban growth area, with some limitations. ID does not have a single unified set of ADU requirements as regulations vary by locale.
ADU in Downtown Coeur d' Alene