Construction workers have begun grading land for a 244-unit apartment project in Rohnert Park, the first of two new market-rate developments to be built in Sonoma County in the past four years.

Heavy equipment is preparing the site of the future Fiori Estates apartments. The 10-acre property in west Rohnert Park once was part of the Sonoma County Crushers baseball stadium.

The developer, Stockton-based A.G. Spanos Companies, characterizes Fiori Estates as a luxury complex that will include a clubhouse, gym, pool and an outdoor fireplace and barbecue area. It will feature nine three-story apartment buildings.

The construction gets underway as county apartment rents have increased an average of 10 percent in the last year, according to RealFacts, a Novato-based apartment market research firm.

The average rent has risen to $1,407 a month. The county's occupancy rate stands at 97.5 percent.

"We think it's a really strong rental market," said Alexandros Economou, a regional vice president for A.G. Spanos.

The project sits northwest of Costco along the future extension of Dowdell Avenue. An empty field separates the property from Business Park Drive and the southern entrance to the Graton Resort & Casino that will open next week.

Fiori Estates will feature one-, two- and three-bedroom units.

The first units may be available to rent in about a year, Economou said. The entire development is expected to be completed by the spring of 2015.

A.G. Spanos doesn't reveal the cost of such projects.

Apartment managers have reported strong demand for apartments in the county this year, with applicants far outnumbering available units.

One reason: Very few apartments have been built in Sonoma County since the Great Recession began in late 2007.

The county hasn't had a new market-rate apartment complex built since 2009, according to RealFacts. For Rohnert Park, Fiori Estates is the first major residential project in a number of years, said Marilyn Ponton, the city's development services manager.

But Rohnert Park is preparing for more growth. The city has a total of more than 4,000 single- and multi-family housing units that have been approved in development plans and is about to install a $13 million sewer trunk line to accommodate the new construction.

Along with Fiori Estates, a 270-unit apartment complex is proposed to be built in Santa Rosa.

Cleanup work has begun for the $34 million Range Ranch apartments to be built near Coddingtown Mall at Jennings and Range avenues. Construction is expected to begin there next month, according to the developer, the Wolff Co. of Scottsdale, Ariz.

The rental market comprises a sizable portion of the county's 180,000 houses, apartments and condominiums. Four in 10 housing units here are occupied by renters, according to the U.S. Census Bureau.