Nicholas Hytner is directing the film, which is an adaptation of Alan Bennett's West End hit, said The Hollywood Reporter.
Smith is reprising her role from the stage version. She will play Miss Mary Shepherd, an eccentric woman of uncertain origins, who "temporarily" parked her broken-down van in Bennett's London driveway - and proceeded to live there for the next 15 years.
In the film version, Alex Jennings, who plays Bennett, stars along with Frances de la Tour, Roger Allam, Dominic Cooper, Jim Broadbent and James Corden.