Olivier Sarkozy has splashed out $6.25 million on a love nest for him and Mary-Kate Olsen.

The 42-year-old banker - the half-brother of former French President Nicolas Sarkozy - has been dating the 25-year-old fashion designer for around five months and now he has bought a New York townhouse so they can spend some quality time together.

According to the New York Post newspaper, Olivier and Mary-Kate fell in love with the 146-year-old property because they ''like that it is old''.

The 4,200-square-foot, five-storey home was designed by architect James Renwick Jr., and comes with nine fireplaces, a Juliet balcony in front and a large rear garden.

It isn't known how much renovation they plan to do, although the plumbing and kitchen need to be completely updated.

Insiders had previously claimed that while Olivier and Mary-Kate won't officially be living together, the former child star will be there most of the time.

A source said: ''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 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''.