The 200-room property, which is located next to Westminster Bridge, opened in 1922 as a local government headquarters before first becoming a hotel in 1998.
All guestrooms, including suites, and public spaces will be refurbished in stages, opening from between this month and May.
The redesigned suites will include balconies, lobbies with mosaic flooring and wallpaper featuring an historic map of London. The work is being carried out by RPW Design.
The hotel’s facilities include a health club and spa, a 25-metre swimming pool, various food and drink venues and 13 event and meeting rooms.