The Trapezoidal Building was built in the Hadrianic period (c. 128-130 AD; opus latericium and reticulatum). Outside the building, to the right of the entrance, is a large pit. Water could be taken to it through a hole in the side wall (of the building). In the north-east corner of the building is a shop.

Originally there were two doors in the centre of the south-west wall. These were later blocked by a staircase that was set against the south-west facade. The building has a wide entrance at the north side with a travertine threshold and large travertine blocks in the lower part of the jambs. It leads to a wide corridor or courtyard, without a roof. This area was paved with basalt blocks. On either side is a large, covered room. In the back part is a basin that was open on one side.

This must have been a stable for the horses of the guests in the hotel in the neighbouring House of the Painted Vaults, and, in view of the size of the building, surely also the stable of the affluent inhabitants of the Garden Houses. On the first floor there will have been accomodation for the servants of the hotel guests. The basin inside the building may have been a trough or provided water for the grooming of the horses. The pit outside the building was a manure pit.