The Boston Design Center’s roof looks like the top of any old building. “We have air conditioning . . . some boilers . . . but mostly an empty rooftop,’’ says Design Center manager Julie Rogowski. But that will change next year. Higher Ground Farm, also known as farming partners John Stoddard and Courtney Hennessey, have the Design Center’s approval to turn the 55,000-square-foot space on top of the building into a working farm in February. The duo will grow produce up there that will be sold to restaurants and to the community. Rogowski said locals won’t have access to the farm space (imagine the liability), but that she hopes to use Higher Ground’s produce for the Design Center’s in-house events.