Dutch firm Verweij and Partners entered negotiations with Alderney States over the Victorian fortification in 2007.
Frans Verweij said the restoration of the existing building would be first followed by work on new buildings with work expected to take about a year.
Plans for the site released in 2010 included a hotel and conference centre.
Mr Verweij said the Â£20m hotel would have "about 70 rooms, a nice restaurant overlooking both sides of the sea, a swimming pool and a spa".
He said the company was also approaching airlines about possible new routes to and from the island in the hope of bringing in more tourists.
Mr Verweij said Biggin Hill and Oxford were among the possible new destinations.
Fort Tourgis dates back to the 1850s. An earlier plan to convert it into flats fell through when the proposed investor pulled out in 2006.
Developers behind plans for Alderney's Fort Tourgis have confirmed redevelopment work on the site will begin by summer 2012.