21 Union Street, Edinburgh

21 Union Street in Edinburgh is a classical tenement block built in 1806, likely designed by Hugh Cairncross and constructed by John Aitchison. The building features three storeys, an attic, and a basement, with a symmetrical five-bay elevation facing Union Street. The ground floor has smooth V-jointed rustication, while the upper floors are finished in droved ashlar. The basement also uses droved ashlar, and the rear is constructed from random rubble with dressed margins. There are dividing bands between the basement and ground floor, as well as between the ground and first floors, a mutuled eaves cornice, and a blocking course. The windows are regularly spaced.

On the southwest (principal) elevation, the central bay includes a platt that leads to a timber-panelled door set in a round-arched opening, topped with a segmental umbrella design fanlight. This door is framed by a pilastered doorpiece with narrow four-pane margin lights. The roof features two tripartite dormer windows.

The northeast (rear) elevation has three bays, with a bowed right bay. The northwest (side) elevation consists of four bays in droved ashlar, with blind windows on the inner right bay.

The property includes railings at the edge of the basement recess and platt, featuring stone copes on the basement only, topped with spear-head finialled cast iron railings. A random rubble boundary wall at the rear has a flat concrete coping.

The glazing predominantly consists of 12-pane timber sash and case windows, with four-pane glazing in the outer lights of the tripartite dormers. The dormers have timber fascias, grey slate haffits, and piend roofs. The pitched roof is covered with graded grey slates and features stone skews. There is one corniced, droved gablehead stack on the northwest gable, one corniced ashlar ridge stack on the right, and one corniced rubble wallhead stack with droved dressings at the rear, all equipped with circular cans.