The 421-a tax abatement, a program allowing developers a multiyear grace period before paying taxes in exchange for developing on specific parcels of land, isn’t just a pipe dream for Manhattanites anymore. In fact, the 421-a savings are being passed on to the residents of the Lower East Side apartments for sale at Extell-developed One Manhattan Square. The 800-foot-tall glass-clad tower, situated just steps from the East River, not only offers a 20-year tax abatement, but it boasts a long list of amenities, including an attended lobby, concierge, state-of-the-art fitness center, bowling alley, full basketball court, performance space, wine and cigar rooms, children’s playroom, and teen arcade, as well as a spa that contains a 75-foot indoor swimming pool and a hammam.