Millbrook managing director Tammy Woodhouse said: "It’s great to have Sandstone on board. It has been designing, landscaping, and making bespoke timber framed buildings in Kent and Sussex since 1998.
"We look forward to working with them in the future to give our customers even more inspiration in their gardens."
Sandstone Design Solutions employs 10 full-time landscapers and four office-based personnel.
Owner Ali Dempster said: "The past year has seen an unprecedented increase in the amount of design work and landscaping projects. This continuing success, which culminated in a BALI award in 2011, led to the employment of three further full-time landscapers.
"The move to the new offices will offer opportunities to ensure that our expansion can continue.
"While we remain totally independent from Millbrook Garden Centre this is a great re-location for the company and I feel that the move will bring additional customers via the retail space provided. Being based on the Millbrook site should be mutually beneficial to both our core businesses."