NOVATO -- National residential real estate brokerage giant NRT, LLC, on Tuesday said it has acquired Novato-based Frank Howard Allen Realtors, dividing the company's 17 offices between two of its subsidiaries.

Four Frank Howard Allen offices in Sonoma, Glen Ellen and Napa will become Sotheby's, and 13 remaining offices are now operating as Coldwell Banker. Frank Howard Allen has about 300 agents, which will continue to operate under the new ownership.

Financial details of the deal weren't disclosed.

Founded in 1910, Frank Howard Allen was the second-largest residential real estate brokerage in the North Bay in the number of agents, behind Bradley Real Estate, according to the 2013 Book of Lists. Frank Howard Allen had $1.92 billion in sales volume in 2012.

"It's business as usual for the Frank Howard Allen agents,” said Rick Turley, San Francisco area president for Coldwell Banker. He described the firm as Coldwell Banker's greatest competition in Marin County and a strong player in the North Bay in general.

"Joining with them easily gives us the No. 1 spot in the North Bay,” he said.

Frank Howard Allen's chairman and chief executive officer, Larry Brackett, will join Coldwell Banker as an adviser. President and Chief Operating Officer Noreen Smith will join the senior management team of Coldwell Banker as a regional senior vice president.

"The Coldwell Banker Residential Brokerage sales team is extremely professional and experienced, especially in the luxury market. Coldwell Banker Residential Brokerage is part of a national brokerage, and the company's offices are locally managed with local leadership. Like us, they take great pride in giving back to their communities," said Mr. Brackett, in the announcement.

Prior to the acquisition, Coldwell Banker had 42 Northern California offices with $17.7 billion in sales volume in 2012, according to the announcement and Mr. Turley. There are currently no plans to close offices that exist in overlapping areas, he said.

Sotheby's previously operated two Wine Country offices, located in Sonoma and Napa, and 28 sales associates. The acquisition will add 16 agents to their roster in the North Bay.

The acquisition is the third for NRT in two weeks, following the acquisition of Greenwich Fine Properties LLC in Greenwich, Conn. and Coldwell Banker Cocoa Beach Realty in Cocoa Beach, Fla.