The King Edward VII Hospital, which opened in 1906, and its Gertrude Jekyll gardens are being renovated as part of a redevelopment of the site near Midhurst, West Sussex.
Land Use Consultants (LUC) has been working on the project to revive the gardens and return them to the original Jekyll vision.
"It is very exciting because we look at her plans and realise it is a chance to bring the gardens back to life," said senior landscape architect Liz Jackson, who has been carrying out research into Jekyll's designs for the site. "The gardens are not too bad but have become impoverished because it hasn't had adequate care.
"It would have been very colourful, with the grand herbaceous borders that she's famous for."
Two mature magnolias will remain in the "rosemary courtyards" of the early 20th-century plans but some planting - such as two later pear trees - will be removed to restore the historical landscape.
"We don't have to slavishly reproduce everything that was there," explained Jackson. "But there's no reason why we can't get certain areas, like the south garden, to look better than when Jekyll created it because things like the Wisteria and Magnolia she planted are more mature and look fantastic."
Dry-stone walls feature throughout the site, with plants embedded in between the walls and tumbling over the tops.
"Foxgloves, hollyhocks and snap dragons were used a lot around the walls," added Jackson.
Part of the original design of the site as a hospital for tuberculosis patients was a series of "measured walks" that patients could use for exercise and to improve their health. LUC plans to restore the walks, along with the reintroduction of croquet lawns and a golf course.
LUC associate and King Edward VII gardens project manager Shona Williams said the health aspect was an important influence. "Recreation and rehabilitation are quite a bit of the site and we are looking to recreate that in a modern context.
"The entrances to these measured walks are punctuated by Jekyll's rhododendrons or a splash of Azalea."
Work is expected to start on site in January next year, with the hospital buildings being developed by Lincoln Holdings. The gardens will be open under the National Gardens Scheme and the cricket pitch is to be used by the local team.