For centuries, monarchs have let associates live rent-free in royal residences.Prince Edward, Duke of Kent, outside his 'grace-and-favour' home, Wren House, in 2024.Credit: Jordan Pettitt - WPA Pool/Getty ImagesThe GistSince the 1700s, monarchs have offered government-owned residences rent-free to relatives and household staff.Grace-and-favour properties, as they’re known, include Ivy Cottage and apartments at St. James’s Palace.The practice has…