The tennis star bought the 15-room mansion, near his hometown of Dunblane, in 2013 for £1.8m with the aim of transforming it in to a ‘leading luxury’ hotel.
Murray, who was knighted in the New Year’s Honours, said he wanted to ‘give something back’ to the community he grew up in.
Now the hotel has requested planning permission to build an extra 30 bedrooms, spa treatment rooms, 200-capacity events area and a gym designed by Murray himself in its 34-acre grounds.
The plans also include a new breakfast and afternoon tea area overseen by chef Albert Roux, who is behind the hotel’s existing Chez Roux restaurant.
Graeme Green, general manager at Cromlix, said: “Since we re-opened in spring 2014 Cromlix has been a real success story, with both the hotel and Chez Roux restaurant far exceeding expectations and often operating at capacity.
“We are now hoping to build on this success by extending the existing buildings to create additional hotel bedrooms, banqueting facilities and staff accommodation as well as growing the team.”
The plans will create around 40 local jobs and include the development of on-site accommodation for hotel staff.
Construction will begin in 2018 with the extension expected to open in 2020 if planning permission is granted.
Green said: “It is still early days however if these plans do progress we will discuss our proposals with the local council and community and look forward to receiving their feedback.”
The Cromlix was named the AA Hotel of the Year in Scotland in 2016 and Chez Roux holds three AA Rosettes.
The grounds of the hotel also feature a tennis court and practice wall finished in the famous Wimbledon colours.
Cromlix is managed by Inverlochy Castle Management International (ICMI), the group behind Crossbasket Castle and the upcoming Ury House hotel development in Aberdeenshire.