Secluded serenity on Williamsburg's North Side.     Located on Skillman Avenue between Graham and Manhattan Avenues, this modern 1-Bedroom is endowed with a rare feature; a 450-square foot private garden, descended into from the apartment's smaller covered terrace. Gazing southward into the garden, apartment 2B rests within a leafy tree canopy; a quiet retreat from the city, where the windows stay open and one can entertain comfortably inside, or out.     At 663 interior square feet, this apartment makes ergonomic use of its footprint, allowing for a separate but conversational kitchen with substantial prep and storage space, a coat closet, pantry, and a master bedroom with a walk-in closet. Handy features include two zones of air conditioning, an integrated sound system with three independent zones, and an irrigation system for the planted garden. There is also a full size dishwasher, a washer/dryer in the apartment, and a private storage room in the basement.     142 Skillman Avenue, an eight-unit condominium built in 2008, is the beneficiary of both a 15-year tax abatement and low monthly common charges. The building has video intercom, and is close to the Graham Avenue and Metropolitan Avenue thoroughfares, which are rich with shopping and dining. 142 Skillman is two blocks from the L train at Graham Avenue, and four blocks from the G and L trains at Metropolitan Ave/Lorimer Street.