When it became clear to Spokane contractor Ron Barnes that his parents Orville and Marjorie Barnes, both in their mid-80s, needed assistance in day-to-day tasks, he looked into building a home to accommodate both generations.

Sketching it out on a napkin—and clearing it with his wife, Vicki—Barnes began to pursue development of a home with what the city of Spokane refers to as an accessory-dwelling unit.

Having lived a few paces from his parents in the South Hill home for nearly a year now, Barnes says, "It actually has worked out pretty darn well. If something happens, we're right here."

While Barnes built the house for personal reasons, he says he sees a niche market for this type of house.

"There may be a need for a half dozen of them a year," says Barnes, who is vice president of On-Top Realty Inc. and involved in Construction Services, the company's contracting division.

Known informally as mother-in-law apartments or granny flats, accessory-dwelling units are defined by the city of Spokane as units that provide separate living quarters while maintaining the look and scale of a single-family home.

To qualify as an accessory-dwelling unit, the secondary living quarters must have no more than 800 square feet of living space and account for less than half of a home's total living space. Garage space isn't factored into the square footage proportions. Also, a property owner is to occupy one of the units the majority of the time and may not charge the other party living there rent.

Tami Palmquist, associate planner at the city of Spokane, says the city developed formal guidelines for accessory-dwelling units in 2006. Since then, the city has permitted 36 such projects.

"There are more out there," she says, "but that's the number that have actually gone through the city's process."

Palmquist says the city has issued 10 permits so far this year for accessory-dwelling units. While that's less than 1 percent of the 224 total single-family residences for which the city issued permits through October, it's up substantially from the three accessory-dwelling units that the city permitted in 2012. In both 2010 and 2011, the city issued permits for six such units.

Joel White, executive officer of the Spokane Home Builders Association, says he doesn't know of any of that organization's members who are actively marketing granny flats, but that a number of builders are prepared to build them if a buyer requests one to be built.

Jim Greenup, a real estate agent with Windermere North Spokane LLC, tried to develop a small multigenerational housing neighborhood on the Five Mile Prairie in the late 2000s. He says people liked the concept, but that the development never gained traction.

He says he's considering revisiting the idea.

"Now, I'm working with some people. They are looking for that type of thing, and they aren't finding it. I have a lot or two left that I want to try to do something on," Greenup says.

Palmquist says accessory-dwelling unit projects permitted by the city have been split evenly between new construction and remodels of existing homes. In most instances, she says, new construction that incorporates such a unit involves constructing an independent living space over a garage.

The Barnes' project is different in that it's a one-level house with no basement and no stairs. From the exterior, there isn't any discernable difference between it and the other homes in the neighborhood, yet the home and the accessory unit share no walls. The primary living space has about 1,800 square feet of floor space, and the accessory unit has 800 square feet of space.

The home and accessory unit share the front door, but both parties can access their living spaces separately from an attached three-car garage. A large dining room is situated between the two living spaces, which either group can use to entertain privately. There is a shared backyard patio, but the primary residence has a small, separate patio that isn't visible from the accessory unit.

Barnes says the home is appraised at just over $300,000, and he expects homes of this sort would appeal to higher-end buyers looking to build for between $325,000 and $500,000.

Nick Barnes, secretary/treasurer of On-Top Realty and Ron Barnes' son, says he expects such homes to appeal to seniors and their families who want to preserve the family's wealth by trying to avoid extended periods of time in a retirement home.

"I would think this would be attractive to people with net worth of $300,000 to $900,000," Nick Barnes says. "You don't lose any of your net worth."