The study covered 77 multi-family offices with $621 billion under management as of year-end 2015, representing approximately 90 per cent of the industry's estimated assets. Average size of the firms was $8.1 billion, with an average asset growth rate of 8.7 per cent during 2015. 

Buoyancy in equities was a factor driving AuM higher; MFOs have also won market share from other entities, such as large brokerages.

One source of growth has been at trend of single-family offices closing and consolidating their forces with other SFOs, or joining an existing MFO structure, the report said.

"We're getting a lot of business from single-family offices," said one chief executive interviewed for the study. "One thing is they decide to close down their family office, and we serve as an outsource solution for them. The other is single-family offices outsourcing their chief investment officer function. Those trends have been very positive for us."

As the Baby Boom ages and intergenerational wealth transfer looms larger as a concern, rising complexity around areas such as complying with tax laws, investment ideas and structures of ownership are encouraging families to examine the merits of MFOs, the report said.

The report said MFOs are bulking up to reach the scale required to operate effectively, with a growing share of them holding $5 billion or more in assets. 

A comparison of year-end data from 2010 and 2015 shows that 15 per cent of firms topped $5 billion in assets in 2010 while five years later the figure was 35 per cent. Only 39 per cent of firms exceeded $2 billion in 2010, while last year it was 71 per cent. The data suggests the level of optimal scale to make an MFO viable is now around at least $5 billion, the report said.

The report lists the top MFOs by assets

Top Ten Multifamily Offices by Assets

                                                                                                            Assets under
      Company                                            Headquarters           Management *
1.    Bessemer Trust Co.                     New York, NY                      113,700.00 
2.    Northern Trust                              Chicago, IL                          65,000.00 
3.    Abbott Downing                            Minneapolis, MN                 38,000.00 
4.    Silvercrest Asset Management         New York, NY                      31,199.16 
5.    Hawthorn, PNC Family Wealth         Philadelphia, PA                  30,500.00 
6.    Atlantic Trust                                     Atlanta  GA                          26,800.00 
7.    SCS Financial                                   Boston, MA                         16,084.77 
8.    Oxford Financial Group                    Carmel, IN                            13,576.98 
9.    Plante Moran Financial Advisors      Southfield, MI                      12,400.12 
10.    CTC Consulting/Harris myCFO      Portland, OR                        11,328.86