Mayfield is a very special enclave of 13 contemporary townhouses in the sought after northwest section of West Chester borough.   Noted for its distinctive architecture, the  outstanding mature trees and  the original mansion house which is the centerpiece of the community,  Mayfield is a  little known  borough treasure.      522 William Ebbs Lane is the largest floor plan in the community, featuring a first floor master bedroom and ensuite, a full finished basement with bath, and a 2 car garage.   The kitchen was completely renovated several years ago, with a wall taken down creating the  open floor plan today's buyers want.   There's a cozy den right off the kitchen, and patio door to enclosed outdoor sitting area.      Upstairs are 2 bedrooms and a hall bath,    The house is a downsizer's dream - casual sophistication but without worry about lawn care and snow removal!  Two blocks to West Chester Country Club or the West Chester library, and a short stroll to the center of town for shopping and dining.  Mayfield offers the best of both worlds...borough living but in a tucked away serene setting.