Apartment occupancy in Riverside and San Bernardino counties is expected to hover around 97% this year despite an influx of new apartment space into the market, according to a study. Rents in the two-county region are expected to rise between 8% and 8.5% during the next two years, with average rents reaching $1,000 a month for the first time, according to the Casden Real Estate Economics Forecast, which was released March 30 by the Lusk Center for Real Estate at USC. Three-bedroom units in the Inland Empire could reach $ 1,400 a month by the end of 2005. Rents in two submarkets -southwest Riverside County along with Riverside/Corona/Moreno Valley - could jump 10% during the next two years, according to the report. Rents in Los Angeles and Orange counties are expected to rise between six percent and eight percent during the next two years,according to the study.The region is plagued by a shortage of apartment units, and cities need to build more high-density apartments in urban areas, Casden Forecast director Raphael Bostic said. The average monthly rental rate in Los Angeles County last year was $ 1,300, while Orange County averaged $ 1,260 and the Inland Empire averaged $ 900, according to the study. The Casden Forecast uses apartment transactions, new building permits, job data and leasing activity to predict rent and vacancy levels in Los Angeles, Orange, Riverside and San Bernardino counties.