Post Office, 18 Colquhoun Street, Helensburgh

This is a two-storey, L-shaped post office situated on a corner plot, built in 1893 to a Beaux Arts influenced design by Walter Wood Robertson, with a later five-bay extension to the south, likely added in the early 20th century by W T Oldrieve. The building is constructed of cream sandstone ashlar, with a rusticated ground floor and quoins. A base course, cill and lintel courses, a string course separating the ground and first floors, a dentilled eaves course, a cornice, and a parapet contribute to the elaborate detailing. Each block of the building is flanked by rusticated quoins that feature cartouche decoration below the cornice at the first floor level and are surmounted by pedestals with urns at parapet level.

The original block features round-arched, keystoned, and pilastered windows at the first floor, with decorated consoles above the pilaster capitals supporting the cornice.

The north-facing (Princes Street/Entrance) elevation is dominated by a two-storey, two-bay semi-circular entrance tower at the northeast angle. The tower has a lead cupola and a louvered lantern. A modern, fanlit door is located to the left of the entrance, with a window to the right, and pilasters to the outer left and right. A moulded oval ashlar panel above the door and window contains the inscription "Post and Telegraph Office". A floreate panel above the inscription features the royal monogram VR and the date stone 1893, flanked by round-arched windows. A single-storey wing extends to the outer right, with a window flanked by narrower windows. A corbelled ashlar post box sits below the central window, featuring a modern metal face.

The east-facing (Colquhoun Street) elevation has four bays; a bipartite window is centered at ground level, with a window to the right and a doorway to the left. A recessed ashlar panel above displays cartouche and mask decoration. Two closely spaced windows are centered on the first floor, flanked by windows on either side.

The ground floor windows are fixed-pane with top hoppers, and the first floor windows are six-pane with top hoppers. Acroterion detailing is present above the central mullions of the windows. The roof is covered with grey slate, with a rooflight on the east side and corniced ashlar stacks.

The later extension to the east (Colquhoun Street) is a symmetrical five-bay block of two storeys and an attic. It is similarly detailed externally, but features square-headed windows at ground and first floor levels. The windows of this section also have two fixed panes with a top hopper. This section has four flat-roofed, slate-hung dormers with six-pane sash and case windows, and corniced ashlar stacks.