Champneys-owned Eastwell Manor in Kent has submitted a planning application for a £1.6m, 28-bedroom extension to the hotel.
The property in Ashford, Kent, was bought by spa resort firm Champneys in October last year when plans to refurbish the 62-bedroom hotel were announced.
The hotel, with origins dating back to the Norman Conquest, currently includes two restaurants, a spa and leisure facilities.
Champneys intends to create a 28-bedroom annexe as well as additional parking as part of the scheme.
It will operate as Champneys at Eastwell when the refurbishment programme is complete.
The planning application was submitted to Ashford Borough Council.