The apartment market in King and Snohomish counties stabilized in the third quarter, which is surprising considering the weak economy and newly constructed units coming onto the market, according to Tom Cain of Apartment Insights, an apartment market research firm.

The firm tracks properties with 50 or more units.

It said vacancy fell slightly in the third quarter, to 6.8 percent, but is up from 4.7 percent a year ago.

The submarket that includes Arlington, Marysville and Monroe continues to be the strongest, at 3.1 percent vacancy, Cain said. Kirkland improved the most; its vacancy declined 2 percent, to 6 percent.

Des Moines and SeaTac were the weakest submarkets, at 10.2 percent and 10 percent vacancy, respectively.

Age-restricted properties, at 5 percent vacancy, continue to outperform the market, Cain said. Rent-restricted properties ended the third quarter at 7.3 percent vacancy.

The overall King/Snohomish vacancy rate for properties, including those in lease-up is 8.8 percent, a half percentage point improvement from the second quarter, Apartment Insights reported.

Three condo projects in Seattle's Lake Union area — Domaine, Equinox and Rollin Street — added 503 units to that market, which will drive up vacancy, Cain said.

Rent incentives in King/Snohomish increased to $94 per month, or 9.1 percent.

Incentives were highest in Class B properties (9.2 percent) and lowest in Class C properties (6.6 percent). Class A properties averaged 8.6 percent. Incentives were higher in Snohomish County (10.6 percent) than in King (8.6 percent).

Maple Valley, North Bend, Newcastle and Sammamish offered incentives of 12 to 18 percent.

In King/Snohomish, rents increased a dollar on average in the third quarter, to $1,038 per month.

Areas with the highest rents are Kirkland, $1,516; Bellevue West, $1,391; and Seattle downtown, $1,384.

The lowest rents were recorded in SeaTac, $810; and the Marysville/Monroe area and Edmonds, at $814.

Cain said he expects the third quarter's relatively positive performance to be a pause in what will be a continuing decline for the King/Snohomish apartment market.

He said projections by the Employment Security Department show a significant reduction in jobs through the second quarter of next year, resulting in weaker demand for apartments. Also, 3,530 units are scheduled for completion by the end of the year in King/Snohomish, and that is a huge supply for a three-month period, he said.

However, it is likely not all will be finished this year due to delays that traditionally occur in development, he said.