Life at Fair Field, billionaire Ira Rennert's infamous estate, must be tricky. What to do with 63 whole acres of prime Sagaponack land? As it stands, Fair Field has a colossal 110,000-square-foot mansion, a handful of auxiliary structures, a playhouse, and even its own synagogue. Last summer, Rennert's daughter admitted that her father was trying to build an outdoor bathroom, but the Town Board got in the way. And now the Post learns that Rennert is looking to construct on the estate a 10,000-square-foot museum to house his $500 million art collection. Thing is, rather than dealing with the Town Board this time, he just didn't tell anyone! Says an unnamed building inspector, "I did find them trying to construct something so I gave them a notice of violation and a stop-work order notice." Perhaps Southampton's just concerned Rennert's museum will compete with their shiny new one?