Sources tell us that billionaire Julia Koch has brought in star designer Peter Marino to work on a $40 million Upper East Side townhouse, which she bought in 2018, a year before husband David died.

We hear she’s redecorating the 15,000-square-foot home that previously belonged to real-estate mogul Joseph Chetrit.

Marino’s clients have reportedly included David Geffen, Giorgio Armani and Gisele Bündchen and Tom Brady, as well as Chanel, Dior and Fendi. He’s also worked on a previous Koch apartment.

We hear Marino’s providing “interior design touch-ups” for Koch’s manse, which has a gym and a spa that are being left intact. Sources said Koch likes the place because it provides more privacy for her family.

She’s recently been splitting time between New York and Palm Beach, Fla., where she’s also made some tweaks to her home, spies told us. Another source said of New York’s richest woman: “Julia is remaining very low-key with her family … focusing on them and charitable work.”

She’s made recent contributions to charities including St. John’s Bread & Life in Bed-Stuy and East Harlem’s Little Sisters of the Assumption Family Health Service, we hear.