Building 12 (Officers Mess) South Barracks

Building 12, known as the Officers' Mess, is located in the South Barracks and dates back to 1795, designed by architects J Johnson and J Sanders from the Barrack Department. It has served as Royal Marines barracks since 1869 and underwent alterations in the mid-19th century, with extensions added in the early 20th century. The building is constructed of yellow Flemish bond brick with rubbed brick headers, featuring brick lateral and ridge stacks, and a hipped slate roof. It has a double-depth axial plan and is designed in the late Georgian style, standing three storeys high with an attic and a 19-window range.

The exterior originally had a 13-window range, with a central section of seven windows that projects forward beneath a wide pediment. This pediment includes dentil eaves cornices and a clock set in a semi-circular recess, topped with a square louvred cupola. Mid-19th century pedimented porches at each end feature vermiculated keys and imposts above round-arched doorways, which are fitted with mid-20th century doors and flanked by sash windows. The ground floor has segmental-arched rubbed brick heads above plate-glass sashes, while the first floor alternates between blind windows and 6/6-pane sashes, and the second floor has 3/6-pane sashes, most of which have horns. The front and back are adorned with flat-headed lead-clad dormers.

On either side, early 20th century three-window extensions match the original style and fenestration, each with rendered parapets and flat roofs, and a central lateral stack. The rear elevation mirrors this design and includes four central dormers, with a former kitchen wing extending below that features a tall lantern in one section.

Inside, the building has been largely reconstructed in the mid-20th century, with mouldings and fittings replaced. However, the original transverse dogleg stairs remain, showcasing column newels and stick balusters. Historically, this 18th century range is significant as one of only two surviving officers' quarters from the first English army barrack-building campaign of the 1790s, and it is part of a group that includes the men's accommodation to the right and the former cavalry barracks to the northeast.