Plans for redevelopment of the back side of Federal Plaza are starting to take shape now that Montgomery County has approved a proposal for new housing on the exisiting parking lot. Federal Realty Investment Trust, which built Pike & Rose, Rockville Town Square and many other mixed-use properties in Montgomery County, is looking to meet the demand for affordable housing with easy access to Rockville’s Metro stations. Its plan calls for the construction of two high-rise residential towers, with about 500 total housing units, to be built over the next 10 years. The existing Panera will eventually be torn down — but that won’t happen for quite awhile.