The estate, which is near Largs and is owned by The Earl of Glasgow, Patrick Boyle, has invested to add an indoor pavilion and caravan park. It is also undertaking major restoration work on the castle, parts of which date back to the 13th Century.
Restoration work on the castle will include removal of world-famous graffiti painted by Brazilian artists about six years ago on the outside of the castle. Lord Glasgow has given the go-ahead for the removal of the graffiti after engineers discovered the early-1950s cement coating on which it is painted is causing damage to the original castle walls.
Kelburn Castle and Country Centre is developing camping facilities and highlighted its plans to add yurts and teepees, as well as log cabins.
On top of targeting holidaymakers, the estate is seeking to host corporate and private functions, including weddings, in the pavilion and in the newly-upgraded castle.
The pavilion function suite, which will be opened officially on September 19 at an evening event to which guests from the tourism and wider business community have been invited, includes a self-contained kitchen and will be able to cater for parties of up to 250.
Kelburn Castle and Country Centre highlighted the fact that its investment programme had been funded by the sale of 18 acres of land at the rear of the neighbouring village of Fairlie to Dawn, a developer which has plans to build 100 houses.
It added that the housing plans had been approved by North Ayrshire Council with the provision that revenue raised by the sale was reinvested in the Kelburn estate.
Kelburn Castle and Country Centre also noted that a trust, whose members include two local councillors, was responsible for overseeing the investment plan.
Kelburn, which has facilities including a "Secret Forest", an adventure course, a falconry centre, an animal park, miles of woodland walks and one of Scotland's biggest stables, attracts up to 100,000 visitors each year.
Lord Glasgow, a Liberal Democrat peer, said the investment programme would allow the estate to diversify its offering. He added that the provision of indoor facilities would make the estate less dependent on the weather.
Lord Glasgow said: "This is the biggest investment in Kelburn for a century and will consolidate its status as a significant tourism and hospitality facility."
The graffiti project, commissioned by the Earl's son and daughter, David and Alice, in 2007 at a cost of £20,000, created a mural which adorns the exterior of the keep and features a psychedelic series of interwoven cartoons depicting surreal urban culture. It has been hailed as one of the world's best examples of street art.
However, engineers found the cement render underneath had created a build-up of moisture. This is causing the original stonework beneath it to deteriorate.
The timing of the removal has yet to be finalised by the project management team, but is expected to be within the next year.