The 47,000-square-foot building at 25 E. Mason St. in Santa Barbara’s Funk Zone has been sold to an unnamed investor for an undisclosed amount. The property was listed for $21.5 million.

Radius Commercial Real Estate & Investments, which represented the buyer in the deal, said the sale marks the largest transaction involving office and R&D property in Santa Barbara since 2007, when the El Paseo building at 820 State St. changed hands for more than $25 million.

The Mason Street property is the corporate [R&D] headquarters for music equipment maker Sonos, which occupies the entire building and holds a 10-year lease on the property.

Austin Herlihy and Steve Brown of Radius represented the buyer in the all-cash exchange deal. Studley Retail out of Los Angeles was the listing company. “As we saw with the Marshalls building last November, investment properties in Santa Barbara continue to be the most coveted opportunities on the Central Coast,” Herlihy said in a statement. “I have been bullish on this property for the past three years. I’m not sure any property in Santa Barbara has as much development upside and location upside.”

The building last changed hands in January 2012, with East Mason SB LLC paying $9 million, property records show.

The building’s location next to the Entrada Hotel project on State Street was a major attraction for the buyer, he said. After changing hands multiple times, the long-vacant hotel project was purchased by Los Angeles hotel developer Michael Rosenfeld in 2011 and is currently under construction.

“To purchase one of a handful of waterfront properties with ocean views in Santa Barbara, for about $400 per square foot, 100 percent leased for 10 years to one of Santa Barbara’s best tenants, and right next to the Entrada, one of the market’s most anticipated developments in recent years, is in my mind an incredible opportunity if not a steal,” Herlihy said.