There are places where history continues to live, where each stone tells a century, where the silence of mornings is that of grand estates. The Phélypeaux Wing is one of those places. Attached to the Château de Pontchartrain since the 17th century, rehabilitated with the rigor of master builders of yesteryear under the aegis of the DRAC and the Architects of the Buildings of France, it now offers one of the few heritage triplexes available in Île-de-France.This 3-room triplex extends over three levels in the East wing of the Château, developing 75.98 m² of living space plus a private terrace of 17.28 m², oriented south. The living room, with its generous volumes and high ceilings, opens onto the honor courtyard and the classified park. The two bedrooms upstairs enjoy absolute tranquility in a preserved estate away from urban turmoil. Engineered parquet flooring, Bosch/Grohe equipped kitchen, high-end fittings — each finish respects the spirit of the place.The location is incomparable: 25 minutes from Paris via N12, 33 minutes from Paris-Montparnasse via the Transilien Line N, 18 minutes from the Saint-Quentin-en-Yvelines employment basin — France's second economic hub with 145,000 employees (Airbus, Renault, Thales, Valeo, Bouygues) — and 30 minutes from the Paris-Saclay campus (HEC Paris, Polytechnique, CentraleSupélec). A strategic crossroads between royal countryside and tomorrow's economy.Tax-wise, this triplex is a first-rank heritage opportunity. Under the Historic Monument scheme, the €501,200 of work is fully deductible from total income, with no cap on tax niches. Monthly savings effort: €3,150. Estimated market rent at €927/month. Delivery is scheduled for November 1, 2026 — an opportunity to seize urgently, in a program of only 13 units.