12-20 Northumberland Square including 17A (No 12 includes No 11)

This is a terrace of eight houses, numbered 12 to 20 Northumberland Square, including number 17A (number 12 also incorporates number 11). They were likely built before 1811. The houses are constructed of sandstone ashlar, with a plinth and quoins. The roofs are of Lakeland slate (on numbers 13, 16 and 19), with the other roofs having renewed Welsh slate coverings. Brick chimneys are present.

The terrace presents a symmetrical appearance with two storeys and a total of 33 bays. The houses range from three to five bays wide. The entrances consist of six-panelled doors with fanlights, set within classical doorcases; some doorcases have open pediments, while others have engaged columns and entablatures, all topped with pediments. Number 17 has been subdivided and now has four-panelled doors. The windows are sash windows, mostly with glazing bars and plain reveals. A moulded plinth, sill bands, and a gutter cornice are also present. The central three bays and two bays at each end project, with the latter constructed in a bow shape; quoins and coped blocking courses are visible on the central and end features. The overall roof is hipped, with chimneys running along the ridge.