No.31 and Attached Railings

No. 31 is a house, now flats, dating to approximately 1790-1793, with alterations in both the 19th and 20th centuries. It was designed by John Palmer and is part of an incomplete development of St James's Square.

The front facade is limestone ashlar, while the rear is constructed of rubble. The building has a double-pile, mansard roof with a coped party wall and two ashlar stacks with early clay pots to the right. A staircase is located to the front. The house is three storeys, attic, and basement, and has a three-window range. The first floor has three six/nine-pane sashes, retaining some crown glass in plain reveals, stone sills, and wrought iron balconettes. The second floor features three six/six-pane sashes in plain reveals with iron guards. The ground floor has three 19th-century plate glass, horned sashes to the right, a front door with reeded and raised and fielded panels and a single glazed upper panel within a pedimented Doric doorcase, with one pennant step and a step to a pennant-paved crossover. A small two-pane window with a guard and two wrought iron bars is to the left of the door. The basement has two 20th-century plate glass sashes and a half-glazed 20th-century door. There are a double and a single raking dormer, the single dormer containing a six/six-sash window, and the double dormer with plate glass and one/six-sashes. Architectural detailing includes a band course over the ground floor, a sill band to the second-floor windows, a frieze, a moulded cornice, and a coped parapet. A lead hopperhead and downpipe are attached to the right. The rear elevation has 19th-century sashes with wrought iron balconettes to the first floor, with the mansard roof extended to the full third floor.

The interior was not inspected during the listing process. Attached to the property are 20th-century railings and a gate on limestone bases. The land was originally leased from Sir Peter Rivers Gay, with the development commencing to the design of John Palmer, and continued later, though not completed, to the design of John Pinch after 1808. The street originally terminated at All Saints' Chapel and was intended to be extended northwest as Regent Place.