Real estate agent Jim St. André with CORE, who wasn’t involved in the sale of Mr. Hirsh’s home, says reductions that sacrifice floor space for light and volume are reviving the market for brownstones, which sometimes lack natural light. “It was a groundbreaking moment for people who hadn’t considered townhouses before,” he said. One of the most expensive sales in the neighborhood, an $18.96 million townhouse sale in 2012, sold after a similar reconfiguration, he said.