This is 910 Union Street in Park Slope, right off Grand Army Plaza. A few weeks ago we noted on Curbed that the People’s Cathedral of St. Michael & All Angels was on the market and that it was being sold as a development site which could handle a “vertical enlargement.” Here’s one idea for it, courtesy of the Massey Knakal website. The big real estate firm is handling the sale of the property, whose asking price is $5.995 million. It estimates that condos on the property would go for $950 a square foot.