The Grade I-listed House of St Barnabas was established in 1846 as a place dedicated to helping the disadvantaged and, until three years ago, it was being used as a hostel for homeless people.
Now the charity is planning to revamp the garden - one of only two in Soho - with a plan from Buckley Design Associates.
As part of the project, people on the House of St Barnabas' Life Skills Programme will learn horticultural skills and have the chance to help with developing the garden.
They have had a first taste of working in horticulture after getting involved with the Eden Project's Key garden at the RHS Chelsea Flower Show in May.
Designer Declan Buckley has come up with a plan for the garden that includes box planters, new paving and fragrant planting including roses and star jasmine.
"The house is steeped in history," he said. "But the garden is looking very tired, dull and in urgent need of attention."
House of St Barnabas chairman David Evans added: "The garden is a special environment - it is a place of quietness and peace for everyone.
"But also, we are trying to help people into employment. They might not know about the disciplines of the world of work, such as turning up on time or being appropriately dressed.
"In the garden there will be a range of tasks that can help those on our programme develop those skills."
In a bid to raise cash for the House of St Barnabas, the charity's leaders have struck a deal with the Quintessentially Foundation charity. The organisation has launched a "pop up" not-for-profit members' club at the house, which is raising funds for both charities.
The organisations hope to be able to raise further funding to carry out the garden project and are looking for firms to donate materials in return for use of the space once it is completed. House of St Barnabas board member Trevor Stanwell, who is leading on the project, said: "The garden will make a huge difference."
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