Boutique multi-family offices are outgrowing bank-affiliated organisations worldwide, according to a Bloomberg Markets study.

Of the top 10 fastest-growing firms ranked, only HSBC Private Wealth Solutions was part of a big bank with the other nine on the list boutiques.

The boutiques operate with generally smaller margins but offer a range of services from tax advice and governance to managing bill payments.

By size alone however, the family office units of banks still dominate with nine of the top 10 bank-affiliated.

HSBC Private Wealth Solutions comes in at number 1 in terms of total assets under advice with US$126bn as at December 31.

In second place is Chicago-based Northern Trust Corporation with US$90bn followed by BNY Mellon Wealth Management with US$64.5bn.

According to Bloomberg, boutique family offices question both the big banks' level of service and their motivation, claiming banks are often too eager to sell their own hedge and mutual funds creating an inherent conflict.

Last year a US federal judge ordered Citigroup to pay two clients US$54.1 million for losses in a hedge fund that borrowed billions in an effort to extract higher yields from municipal bonds.

The fund had been pitched to Citi's private-banking clients as a safe bond alternative.

Bloomberg quotes the Harvard Business School Families in Business program chair as saying multifamily offices have been trying to figure out a profitable business model for a couple of decades and are now seeing the limits of providing a lot of services.

In the US however boutique family offices are already attracting buyers, with asset manager Affiliated Managers Group recently buying a stake in a Pennsylvania-based family office firm for an undisclosed sum.