A Hamptons Retreat on a Budget, in a Hurry

For many years, Matthew and Jennifer Miller had an idyllic Hamptons escape for summer weekends away from their sweltering apartment in Manhattan: a quaint, 400-square-foot studio above a detached garage at the Bridgehampton, N.Y., home owned by Mr. Miller’s mother. “For us, it was perfect,” said Mr. Miller, 40, the founder of StudioLAB, a New…