The Beatles legend asked the US millionaires - who he has been dating for almost a year - to move into Peasmarsh, his East Sussex estate, after she began spending almost every weekend there.
A family friend said: "Paul and Nancy are as solid a couple as you can get after just a short time together."
"She always seems to be at Peasmarsh anyway so it made sense to him just to ask her to move in."
"She'll still travel between the UK and her base in the US but is setting up home with Paul."
The 66-year-old musician asked Beatrice, his four-year-old daughter with ex-wife Heather Mills, for her approval before asking Nancy to live with him.
Paul's fashion designer daughter Stella - who had a well-publicized feud with Heather - has also given the move her blessing, although she is said to be urging her father not to rush into marriage.
The source added to Britain's The Sun newspaper: "Paul asked Beatrice if she liked Nancy and got a resounding 'Yes,' which meant the world to him."
Paul bought the secluded Peasmarsh estate as a wedding gift for his first wife Linda in 1969.
