As a Grade I listed country house owned by the National Trust, Hartwell House's architecture reflects centuries of change rather than a single style. The original house dates to the early 17th century and was later altered and extended during the 18th century, including key work by architect Henry Keene. Built in ashlar stone, the house rises over two main floors with attic rooms above. After a serious fire in the early 1960s, the roof was rebuilt in Westmoreland slate, with new dormer windows added in keeping with the original character. The north front remains strongly Jacobean, with stone mullioned and transomed windows, large projecting oriel bays, and a central porch with a carved arched doorway and bowed oriel above.
The east front, redesigned by Keene, is classical with sash and canted bay windows and a central Doric porch. The south front is Georgian, with long sash windows and projecting wings, while the west retains 17th-century stonework and a Tuscan colonnaded entrance. Inside, the Great Hall features 18th-century plaster panels, carved birds, and a classical chimneypiece. The staircase blends 17th-century architecture with later additions, and the Drawing Room, Dining Room, and Library are notable for their plaster ceilings, marble fireplaces, and original fittings. While parts of the upper floors were rebuilt after the fire, many Jacobean and Georgian details remain, and the house is designed to be accessible with features such as higher-level toilets, toilets with grab rails, wheelchair access, and an elevator.
