Our good friend 525 Union Avenue, a Karl Fischer building that was a poster child for Sunday construction work in Williamsburg and the Department of Building’s inability to do anything about it, is going on the market as a “luxury rental” building. The apartments start at $2,395 for a 1BR and top out at more than $5,000 per month. The building is about a half-block from the BQE and within a block of four major construction projects on Union Avenue and Roebling Street.