As property values skyrocket in the low-slung eastern edges of Williamsburg, small developers are searching for ways to boost a site’s density without spending exorbitant amounts of money on construction. Increasingly, they file their developments as alterations, rather than new buildings, which allows them to avoid building expensive underground parking. And the J-51 abatement gives landlords a break on their property taxes, in exchange for preserving at least 50% of the old building and keeping the new units rent-stabilized for up to 20 years. One of these enterprising developers has filed alteration applications to expand a little three-story brick apartment building at 196 Montrose Avenue, on the corner of Humboldt Street.