Domoney says the upflift in viewing figures was down to "comfort" with the format after six seasons, a move from Tuesday to Wednesday evenings, more core gardening information, and more human interest stories behind the makeovers.
He says the show, which also features Alan Titchmarsh, always attracted 3m viewers, making it 50% more popular than BBC Gardeners' World.
Landscapers working on the show are JP Landscapes for northern makeovers and Berry Landscapes for southern makeovers.
Meanwhile, Domoney says he has yet to submit a Chelsea 2018 after entries opened on 8 August.
Domoney had success designing the Commonwealth War Graves Commission garden at Chelsea 2017. M&G have returned as sponsor and are therefore confirmed to have a garden. Show garden numbers halved in 2017 after Brexit-led nervousness, but the RHS says it has had more interest ahead of 2018.