The hotel company, which owns Alexander House hotel in West Sussex, Langshott Manor in Surrey and Rowhill Grange in Kent, has bought the 58-bedroom property with 19 conference rooms with plans to turn it into an 'iconic hotel'.
Renovation work has already started on the Queen Anne-style property, which was owned and used by the Red Cross and St John War organisation as a convalescent hospital before becoming a conference venue.
Alexander Hotels and Utopia Spas owners Peter and Deborah Hinchcliffe said while they would be renovating the bedroom and public areas they were keen to maintain the property's historic charm.
They said: "The moment we first saw Barnett Hill, we knew this property would work for us, it showcases considerable potential to be an award winning, iconic hotel in Surrey and the UK and we are truly delighted to start a new journey with this property.
"We have recently celebrated 21-years of Alexander Hotels and our passion for our properties and the industry continues to grow. Our three current hotels have achieved tremendous success and the time is now right for us to embark on a new adventure; somewhere we can continue to make our mark and add to our Alexander Hotels family."
The venue will remain fully functional while refurbishment work is carried out with general manager Tim Hall remaining in his role and director of operations Ian Heath overseeing renovations.