Demolition is underway at 1681-1689 Third Avenue, the five consecutive sites between 94th and 95th streets that developing giant Extell acquired over the course of nine years, from 2004 to 2013. While the developer has yet to announce its specific plan for the site, it is Extell, and there are quite a lot of air rights involved, so smart money says that the plan involves an enormous, glassy tower. The fact that the area allows for the highest density residential development in the city, however, suggests that the tower could be contain other than Extell's typical huge condos for billionaires. (Rentals? Affordable housing, maybe?) Extell has filed demolition plans with the Department of Buildings for all five existing buildings, which are all four-stories and contain between 10 and 15 units apiece. Permits for nos. 1687 and 1689 were issued in July, and the permit for 1681 was issued the day before yesterday. Nos. 1683 and 1685 don't have permits yet.