Included are design/build firms, which can do things such as build walls and patios, and landscape-architecture firms, which provide only design services but often have relationships with contractors who can do the actual work.Landscape Professionals Arentz Landscape Architects, 4543 44th St., NW, 202-537-8020; 8014 Charnick Rd., Marshall, Va., 540-341-4330.