A once-in-a-generation opportunity awaits on the storied "Gold Coast" of the Hamptons: a 3.6-acre waterfront estate perched high atop a bluff with 260 feet of private beach frontage and panoramic views across Great Peconic Bay, Robins Island, and the North Fork. This extraordinary property occupies the historic site of the original 1896 bluff-side cottage known as Mostly Hall, a beloved Hamptons landmark. Today, the land has been meticulously prepared for a new legacy, offering a rare shovel-ready opportunity to build a custom residence designed by renowned Hamptons architect Brian Brady, celebrated for his refined, timeless approach to coastal luxury. With all permits secured and materials already on-site, the vision for this modern waterfront estate is ready to come to life immediately. The proposed 5,700-square-foot home blends classic Hamptons architecture with contemporary sophistication, designed to maximize natural light, waterfront living, and sweeping views from every level. The lower level is thoughtfully planned to include a private guest suite, recreation room, home gym, walk-in cedar closet, full bath, laundry, and abundant storage-creating a perfect balance of function and comfort. The main floor presents an elegant flow for both everyday living and grand entertaining, featuring a chef's kitchen with butler's pantry, breakfast room, formal dining room, a spacious living room with a dramatic fireplace, a screened porch, a garden room, a mudroom, a full bath, and the option of a first-floor primary suite. Elevator access connects all three floors, ensuring ease and accessibility throughout the home. Upstairs, the primary suite becomes a sanctuary with breathtaking bay views, joined by three additional en-suite bedrooms and a sitting room with a private balcony, creating an elevated vantage point above the coastline. Designed for the ultimate Hamptons waterfront lifestyle, the property includes a brand-new bulkhead with stairs leading directly to the private beach, and pending dock approval allows for future boat docking and direct water access. Outdoor living is at the heart of the estate's design, with expansive wraparound decks, a pool with spa, and a 450-square-foot pool house creating an exceptional setting for coastal entertaining. A two-car garage and an adjacent one-acre parcel-ideal for a guest house and tennis court-complete this unparalleled offering. Seamlessly merging history with modern luxury, and positioned along one of the most coveted stretches of waterfront in the Hamptons, this estate represents a rare chance to build your own Southampton-area waterfront legacy with architectural plans fully approved and construction ready to begin.