The project – set to be completed in 2016 - includes the renovation of 100 rooms and the installation of a new kitchen at the hotel’s AA Rosette wining Grill Room restaurant.
A recently completed ‘ultra-modern’ Eden Spa includes a hydrotherapy pool, mood-lit sauna, steam room and four new treatment rooms.
The next phase of the refurbishment will take place in stages, with rooms being completed floor-by-floor to ensure the hotel remains operational.
Andrew Oxley, general manager of Down Hall, said: “Huge investment is being made to develop both our leisure and corporate facilities to reinforce our position as a four star superior hotel.
"The combination of experienced staff, premium products and therapeutic treatments will ensure guests are provided with a first rate leisure experience.”
Down Hall dates back to 1322 and sits among 110 acres of woodland.