Instead of classic beach houses with cedar shingles, developers are building more sleek, contemporary homes with flat roofs and lots of glass. When it comes to new construction, traditional houses with cedar shingles and gambrel roofs have given way, over the last few years, to sleek, contemporary homes or modern farmhouses with gables, standing-seam metal roofs and board-and-batten siding. White shingles and black windows are all the rage. Interiors are modern; moldings are passé. Beyond the dunes, from Westhampton Beach to Montauk, new forward-looking modern beach houses designed for indoor-outdoor living — with flat roofs and floor-to-ceiling walls of glass — pepper the landscape. “The styles are pretty much cyclical,” said James McDonough, a principal of McDonough and Conroy Architects, in Bridgehampton. Though traditional architecture is tried and true, “it’s in and out,” he said. “Now it’s back to modern.” Recently Mr. McDonough collaborated on the design of a $53.9 million contemporary spec house on Meadow Lane in Southampton, known as Billionaire’s Row. “You begin to see the modern at the water,” he said. “It’s really where people would experiment. It’s where it would b...