Ballentine, Finn & Company, one of the fastest-growing independent fee-only wealth management companies in the nation, has opened up its services to a broader range of investors.

The firm has announced the launch of Mill Street Investment Management, which will focus on providing financial planning and investment management services to investors with portfolios of at least $3 million.

Mill Street was created two years ago as an internal division of Ballentine Finn that was designed to serve friends and family of core clients who weren't a good fit for Ballentine Finn's multifamily office services.

Ballentine Finn, with about $2 billion in asset under management, normally provides a range of multifamily office services for clients with at least $20 million in assets. The average Ballentine client has a portfolio worth about $50 million.

"In developing Mill Street, we originally built a dedicated team of experienced, accessible individuals to address the needs of our clients' extended family and friends," firm President Roy Ballentine said in a prepared statement.

As Ballentine has become more well known in recent years-it was ranked #21 on the Forbes Top 50 Fee-Only Advisors list in 2008-there has been a demand for its services from a broader audience of investors.

"The demand from this new client base for the services offered by Mill Street has exceeded our most optimistic projections and we are now expanding our client base to a broader range of investors," Ballentine said.

While Mill Street clients don't have access to Ballentine Finn's multifamily office services, they do share in the same research as the firm's ultra-high-net-worth clients, as well as much of the same investment approach, according to the firm.

Mill Street currently has close to 30 clients with assets under management of about $150 million, said Tom Bullitt, a 30-year financial planning and investment management veteran who joined Ballentine Finn two years ago to head Mill Street operations.

The division's client base is a mix that includes family trusts, corporate CEOs and retired executives, he said. These clients share in the same investment management techniques as Ballentine's ultra-high-net-worth clients, he said.

"I sit on the investment committee with the same advisors who are advising the ultra-high-net-worth families," Bullitt said. "The investment strategies we use are similar."

One key difference is that Mill Street clients will typically have investments that are more liquid than ultra-high-net-worth investors, with less utilization of limited partnership type investments, such as hedge funds and private equity, Bullitt said.