The present castle was built as a residence in 1808–1809 at the order of John Buchanan of Ardoch (1761–1839), who had bought the estate in 1800. Buchanan was a Glasgow merchant, the son of Thomas Buchanan of Ardoch (who died in 1789). John Buchanan was involved in his father's hat-making business, and also like his father was a partner in the Ship Bank, Glasgow's oldest banking house, founded in 1749. He represented Dunbartonshire in parliament from 1821–1826.
The architect of the Tudor Gothic building was London-based Robert Lugar, who had designed the nearby Tullichewan Castle in 1792. The building's turrets and crenellations are purely decorative with no defensive value. The lancet windows, tracery, hoodmoulds and blind arrow-slits are all borrowings from earlier building styles. Although an unimaginative designer, at Balloch Lugar helped to introduce the asymmetrical, "picturesque" form of castellated house into Scotland. The new castle may incorporate stone from the earlier castle, or the fabric of an earlier building on the site. Buchanan established the parkland around the castle, including the planting of ornamental trees and imported shrubs.