The Cambridge City Council unanimously passed an amendment to the  Zoning Ordinance for Inclusionary Housing Monday night, increasing the amount of affordable housing that will be included in new Cambridge developments to 20 percent of residential floor area in new developments.

“This is a crucial step in preserving affordability and socioeconomic diversity in Cambridge,” Mayor E. Denise Simmons said in a statement. “We have effectively doubled the rate of required affordable units that must be built going forward.”

Under the new inclusionary housing provisions, developments of 10 or more units will be required to allocate 20 percent of residential floor area for low- and moderate-income tenants or moderate- and middle-income homebuyers.

To preserve options for families, the ordinance encourages developers to provide three‐bedroom units in the 20 percent of floor area, even if the number of affordable units within the required floor area is reduced; in developments of 30,000 square feet or larger, it requires the creation of three‐bedroom units in the affordable  component. Monthly costs for the affordable units will be set at 30 percent of household income.