An adjustable desk apparatus includes an auxiliary work surface that is movable between a sitting configuration and a standing configuration. When in the sitting configuration, the auxiliary work surface is located adjacent to a primary work surface of an existing desk so as to accommodate working while sitting. When in the standing configuration, the auxiliary work surface is upwardly displaced from the primary work surface of the existing desk so as to accommodate working while standing. The adjustable desk apparatus also includes a base member that interfaces with the existing desk. An adjustment assembly includes at least one counterweight and corresponding counterweight sprockets configured to selectively urge the auxiliary work surface toward the standing configuration when actuated by a user. A pin couples the auxiliary work surface to the base member so as to hold the auxiliary work surface at a desired location.