Charming as they are, the antique houses that make the island of Nantucket, Massachusetts, an architectural treasure also have their quirks. Low ceilings, small rooms, and floor plans designed for another era tend to make them incompatible with the life of a bustling modern family. After renting a few such idiosyncratic houses over the years, a Midwestern couple with three teenage sons knew what they did — and didn’t — want when they set about building an island getaway of their own. “The house had to be traditional,” says Jeff Spoelker, the architect whom the owners tapped to design their retreat, “but without the creaks and cracks.” The place also had to have what he calls a “beachy functionality.” That meant making it durable, comfortable, and suited for entertaining a steady stream of overnight guests. “We wanted the house to be a place where we could create memories,” says the homeowner, who attended college in Boston. Her husband, an avid sailor with great affinity for the ocean, is a New England native.