EXCLUSIVE NEW TO MARKET RIVERDALE HISTORIC DISTRICT  5-Bd. Federal-Style Gem with Patios & Grassy Yard on Private Cul-de-Sac    Nestled on nearly a quarter-acre along a quiet, private cul-de-sac, this 5-bedroom, 4-bath neo-Federal-style house was built in 1937 and designed by master architect Julius Gregory. It enjoys grassy front and back lawns, two inviting patios (including an adorable hidden one) and a fenced, gravel side yard with flagstones and a barbeque. There is a splendid view of the Hudson River from the second floor    Standout features include: a classic entry portico with elegant leaded-glass sidelights and a fanlight framing the doorway; center stair hall; living room with wood-burning fireplace and custom built-ins; formal dining room; windowed kitchen with breakfast nook and door out to the yard; main-floor guest bedroom/office with its own full bathroom; master bedroom with picture-window view of the Hudson River and the Palisades; finished 2-room basement (with family room/guest room, full bathroom, and laundry/storage room); split-system air-conditioning; private driveway; attached 1-car garage; 0.22-acre lot in landmark district.    The house is within walking distance of Wave Hill public garden and culture center, SAR Academy, the Metro-North trains and express buses to Manhattan.