Construction has exploded along the High Line ever since it opened: condos hover over the rehabilitated track and look out onto the Hudson, while the new location of the Whitney Museum is making headway on the southern end of the park as Google moves into its NYC headquarters to a building just a few short blows away.  Now, the historic Chelsea Market may be looking at a facelift following approval from the New York City Council for increasing density in the building by developers, Jamestown Properties. The proposed vertical extension, which has made a brief appearance on a few architecture blogs, will provide the additional in demand office and retail space in the Chelsea neighborhood.