Mary-Kate Olsen is moving in with boyfriend Olivier Sarkozy.

The Fashion designer has been dating Olivier - the half-brother of former French President Nicolas Sarkozy - for around four months and now the couple are set to spend a lot more time together.

Olivier, 42, is buying a townhouse in New York and although they won't officially be living together, 25-year-old Mary-Kate will be there most of the time.

A source told the New York Post newspaper: ''They are really, really happy. And Olivier is buying a townhouse below 14th Street, to be with Mary-Kate. We are told they're not officially moving in together - they both think it's too early for that. But he's telling friends that Mary-Kate is the best thing that has happened to him in a long time.''

Mary-Kate previously dated artist Nate Lowman for over two years before splitting in 2010, while Olivier romanced actress Stella Schnabel for a year before their break-up in March.

Olivier was previously married to wife Charlotte and they have two children together.

Charlotte has since called the coupling ''grotesque'' and ''not right''.