The Javits Center is planning a $1.5 billion expansion that will double the exhibit space and add a grassy rooftop park, a new report says. The expansion would also add a fifth-floor ballroom overlooking the Hudson River and a 40-story hotel, Crain’s New York Business says. Political, business and community leaders agree that expanding the center is crucial to the Big Apple’s economy, the weekly says. The Javits Center has dropped to 14th in exhibit space nationwide and is too small to accommodate 43 of the largest 200 conventions. Meanwhile, new centers are going up in Washington and Boston. But financing the expansion remains a problem, Crain’s says. While the center is controlled by the state, the Pataki administration has always insisted that the city pay a big portion of the cost of the project, and the state and city aren’t close to reaching an agreement. The center now runs from West 34th to 39th streets between 11th and 12th avenues. Under the expansion, there would be a new building that would run to 42nd Street. The new building would be five stories high, one more than the existing structure. Exhibit space would nearly double to 1.3 million square feet. Meeting space would increase tenfold, to 300,000 feet. The hotel would be built at the corner of 42nd Street and 11th Avenue on land owned by developer Larry Silverstein, who holds the World Trade Center lease. Javits officials say Silverstein is willing to sell. Center facts * 745,860 feet of exhibit space * Ranked 14th in terms of exhibit space * Loses dozens of conventions to other cities each year * Expansion has been proposed since 1995 * Proponents say expansion would pump $1.5 billion a year into city’s economy.