Former Air Raid Precautions Building

The Former Air Raid Precautions Building at Windmill Hall was constructed in 1941 by the Ruislip and Northwood Urban District Council as a gas de-contamination centre in anticipation of gas warfare against civilians. The building is designed in a style influenced by Dudok and is made of reinforced concrete, which is clad in wirecut bricks laid in English bond. It features metal-framed casements and has a flat roof. The structure is L-shaped, primarily one storey tall, with a two-storey tower on the west side.

The north facade includes four narrow single metal-framed casements and a triple casement to the right, all situated above a projecting brick stringcourse. The brickwork around the windows indicates previous fittings for external shutters. There is a flush door with a rectangular fanlight containing frosted glass, accompanied by a later low ramp for disabled access. The west tower, which houses the boiler room and water tank, has a tall window on its north face with a flat semi-circular concrete lintel above it. The east side of the tower features two casements at the top, while the west side has one window. A corner chimney projects from the building and has three recessed channels. The tower has separate access on its south side. Both the east and west sides of the main building have five triple metal casements, with a projecting brick stringcourse below the windows. The south end has two separate entrances.

Inside, the northern part of the building contains toilets on either side of the entrance door, a small room on the east side, and a larger room to the west with a triangular shelf and a double wall opposite that likely served as an airlock, featuring two cupboards with shelves for clothing. There is a long room supported by three concrete roof girders, leading to two separately accessed rooms at the south end, each with outside access. This long room likely had partitions originally. A wooden parquet floor, probably added later, is present. The layout of the gas de-contamination centre would have included an undressing area, eye douche, showers, drying rooms, and dressing rooms.