Other shortlisted firms were: CMInternational, a Hong Kong-based construction management specialist; Cadweb, which invented an online management system; housebuilder Gusto Construction; architect Piercy Conner Fairman and contractor Integra.
Unite, which carries out everything from design and construction to maintenance, has a £30m turnover, employs 370 staff and manages 24,000 beds.
In awarding Unite the top prize, the judges said: "It had some great ideas and spotted several gaps in the market. That's one great characteristic of a successful entrepreneur. The other is to see those ideas through and emerge as market leader. Unite has now done that, too. That is a true success story."
The judges included Sir Frank Lampl, life president of Bovis Lend Lease, and John Morgan, chairman of Morgan Sindall. The competition attracted more than 50 entries from around the world.