Seattle-area rents jumped 4.1 percent in the second quarter to an average $1,284 a month, as strong demand for apartments pushed the vacancy rate in King and Snohomish counties to its lowest level in at least nine years, according to a new study.

Over the year, the average rent has risen by $94 or 7.9 percent, accelerating from a 6.8 percent annual gain in the first quarter, according to Seattle-based Apartment Insights Washington, a market-research firm.

The figures are for rents for new leases, not including utilities or other fees, from a survey in May of apartment properties with at least 50 units.

Those averages mask a wide range of rents: At $1,912 a month, West Bellevue had the highest overall rent, an annual increase of 6.4 percent. Downtown Seattle ranks the highest on a per-square-foot basis, at $2.43, up 5.7 percent from the first quarter. The lowest rents are in SeaTac ($901) and Des Moines ($923).

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Apartments in Seattle’s Ballard neighborhood saw the biggest increase in rents. The average asking rent was 12.3 percent higher over the quarter, rising to $1,628.

But Ballard also had a vacancy rate of 8.6 percent, the highest in Seattle. And when new apartments that just opened are included, the vacancy rate shoots up to 18 percent.

The apartment boom in Ballard has led to a doubling of the inventory over the past six years, said Tom Cain, head of Apartment Insights Washington. When the units now being built are complete, Ballard’s inventory will have quadrupled.

New units rent for a premium, and they’re part of what’s driving up market rents, Cain said.

Across King and Snohomish counties, the average apartment rent in a property built since 2010 was $1,754, while the average rent in a property built in the 1970s was $1,019.

As some tenants trade up for newer apartments, landlords of older properties may be able to raise rents, too.

Properties in King and Snohomish counties built before 1950 raised rents in the second quarter 5.2 percent, more than any of the more recent vintage properties, Apartment Insights’ data shows.

Sanjay Bhatt: