A steep drop in housing and office construction has hit the city’s building industry hard, with spending falling off 20 percent — $6.6 billion less than last year, a report released yesterday found.

Construction spending for 2009 is projected at $25.8 billion, compared with last year’s record of $32.4 billion, according to the New York Building Congress analysis.

But the report also found a silver lining, projecting that construction activity will stabilize next year, and will likely even post modest gains in 2011.