BAKER / WOHL ARCHITECTS is currently designing a comprehensive modernization of Jackson Gardens Apartments, a 45-unit state-assisted public housing site near Central Square in Cambridge.   The design features contemporary additions to the existing 60-year old Georgian style brick buildings to accommodate new stair towers and added living space.  The old apartments are being enlarged and reconfigured to meet modern standards.  New accessible apartments will accommodate families with mobility challenges. New building systems will enhance comfort and sustainability.  The work will be phased to allow approximately half the residents to remain in their homes throughout construction.