25, 27 William Street, Helensburgh

25 and 27 William Street in Helensburgh is an earlier 19th-century, two-storey and attic block of flats featuring four bays. The building is constructed of red sandstone rubble, which is stugged, squared, and coursed on the east elevation, with ashlar margins. It has base and eaves courses, quoin strips, and raised margins.

On the east elevation, which faces William Street, there are two closely spaced bays at the center. There is an architraved and corniced doorway slightly off-center to the left, featuring a deep-set modern fanlit door. To the right at the first floor, there is a blind window, and the remaining bays have regular fenestration.

The south side elevation includes a window at both the ground and first floors on the outer right. There is a small modern window in the center at ground level and a boarded door on the outer left.

The west rear elevation features a projecting circular stair tower off-center to the right, topped with a cat-slide roof. There is a door by the re-entrant angle to the right and a window at the first floor. To the left of the stair tower, there is a fanlit door flanked by windows, with another window on the outer right. A narrow window is located to the left of the stair tower at the first floor, along with windows in the outer bays. The building showcases a variety of glazing patterns, predominantly 12-pane sash and casement windows on the east elevation, with some modern replacements. The roof is covered with grey slate and includes two slate-hung bipartite dormers with piended roofs on the east, two rooflights in between, and six small rooflights on the west. The building has ashlar coped skews and corniced squared and coursed red sandstone chimney stacks.