The 68-year-old musician proposed to girlfriend Nancy Shevell earlier this month with a $650,000 ring but the American heiress - who has been in a relationship with the singer for four years - insists they will not be having a lavish wedding.
She said: "It will be small. Just our families. I don`t know the exact date. And don`t ask what I`ll wear because how dressy do you get to stand before a Justice of the Peace in his chambers, which is exactly what we`re going to do."
Nancy also revealed Paul`s "surprise" proposal took place in California and she was thrilled with the five carat ring he presented her with.
She described the band, saying: "A vintage 1925 Cartier engagement solitaire diamond from Tate Galleries,"
Nancy - who said the nuptials will take place "soon" - admitted she would prefer to remain in New York, where she is vice president for her family`s transport conglomerate, but she will relocate to her fiance`s native England and make regular trips back to the US.
She told the New York Post newspaper: "I`d love to live here but it`s probably England. I still have a job here so I`ll commute once a month."
The wedding will be `Yesterday` hitmaker Paul`s third, after he was married to Linda McCartney from 1969 until her death from breast cancer in 1998 and then to former model Heather Mills from 2002. Although he didn`t officially divorce Heather until 2008, they separated in 2006.
Nancy has been married once before, to Bruce Blakeman.
The 51-year-old businesswoman is also said to have been a big hit with the `Drive my Car` singer`s five children, Heather, 48, Mary, 41, Stella, 39, James, 33, and Beatrice, seven.