Forthlin Road is a quiet street in Allerton, in the south of Liverpool, lined with the modest two-storey houses that the local authority built in 1949 as part of the postwar drive to rehouse Liverpudlians displaced by the Blitz and the slum clearances. The McCartney family moved in in 1955, when Paul was at secondary school and his brother Mike was a few years younger. Their father Jim was a cotton salesman with a sideline as a bandleader, who played piano and trumpet at local dances. Their mother Mary was a midwife. Mary died of cancer the year after they moved in, when Paul was fourteen, and her absence shaped the household profoundly. Jim raised the two boys alone in the small council house, working long days and coming home to encourage their music.

The National Trust acquired the property in 1995 and has marketed it since as "the birthplace of the Beatles", a phrase that is part advertising and part accurate. It was at Forthlin Road, more than anywhere else, that the earliest Lennon-McCartney songs were composed and rehearsed. John would cycle down from his Aunt Mimi's house at 251 Menlove Avenue, a mile and a half north, and the two of them would sit in the front room playing guitars while Jim McCartney was at work. The list of songs at least partly written or worked out in this house is long. I Saw Her Standing There is the most often cited, but Love Me Do, When I'm Sixty-Four, and many others have origins here. The Beatles recorded their early demos in front of the fireplace.

Paul's brother Mike McCartney is part of the building's history in his own right. He became a musician under the stage name Mike McGear and was a founding member of the Liverpool comic trio The Scaffold, which had hits including Lily the Pink in 1968 and Thank U Very Much in 1967. The Scaffold was effectively born inside Forthlin Road. Mike also became a respected photographer, and his shots of his older brother from this period, the band rehearsing in the front room, Paul at the kitchen table, are among the most intimate images of the early Beatles ever published. The house had two musicians, two writers, two performers, in a household that could not afford much beyond the rent. Jim McCartney's encouragement of both his sons' artistic interests was unusual for a working-class father of his generation.

In 1965 Paul bought his father Jim a house in Heswall, a wealthy part of the Wirral, and the family left Forthlin Road for good. The Beatles were by then the most successful band in the world, and Paul's gift to his father was the kind of gesture that working-class sons make when they have made it. Jim took the move; he had spent his whole working life in Liverpool, and Heswall was a step into a different sort of life. The council house at Forthlin Road went back into the local authority's stock and continued to be rented out to other families for the next three decades. It might easily have been demolished or sold off without ceremony, the way thousands of similar postwar council houses have been. It was the National Trust's purchase in 1995 that locked it in.

In February 2012, both 20 Forthlin Road and John Lennon's childhood home at 251 Menlove Avenue were Grade II listed by Historic England. The listing was unusual; Grade II protection normally requires architectural or historic significance of a more conventional kind, but the cultural importance of the houses had become undeniable. Forthlin Road still does not have an English Heritage blue plaque, because English Heritage requires its subjects to have been dead for twenty years or to have passed the centenary of their birth, neither of which currently applies to Paul. In 2018, James Corden brought Paul back to the house for an episode of Carpool Karaoke. McCartney walked into the front room and said it was his first visit since he had left as a teenager. He sat down at the piano in the corner and played. The Trust runs minibus tours of the house and Lennon's Mendips together. They are usually booked out weeks in advance.