Basin Street Properties, the Petaluma-born development firm that helped establish the city as a telecom hub and transformed downtown before moving its headquarters to Reno two years ago, is reinvesting in Petaluma.

In the past four months, Basin Street has spent $21 million to reacquire eight Redwood Business Park buildings it sold in 2005, including three it is purchasing today.

Last year, Basin Street acquired the Marina Business Center, which consists of 116,000 square feet in three office buildings.

The deals mark a significant reinvestment into Petaluma's struggling real estate market, said Matt White, Basin Street's president.

"We've always been very committed to Sonoma County," he said. "Even at that time (2009), we left about half of our employees in the Petaluma office. We have been adding people in the Petaluma office over the last year and with this acquisition, we should be adding another person."

The eight buildings include five purchased in March for $8 million on North McDowell Boulevard and Redwood Way, and three in today's deal worth about $13 million, according to the company.

The newest acquisitions include 210,000 square feet in three properties at 1400 and 1420 N. McDowell and 5341 Old Redwood Highway.

The office buildings purchased in March were 40 percent occupied, with major tenants including Wells Fargo Bank, the U.S. Department of Agriculture and Force 10, formerly known as Turin Networks.

Each of the three new buildings are vacant. They formerly housed Alcatel and Digilock.

Signing on as the first tenant in the new buildings is Enphase Energy, a Petaluma startup that makes solar components and recently announced plans to go public with a stock offering worth up to $100 million.

Later this year, it will relocate its headquarters from a small space in Basin Street's Theatre District on First Street to 96,000 square feet of space in the business park under a 10-year lease.

Enphase has been a tenant of Basin Street since the early days of the solar company. Enphase started with 1,000 square feet and a handful of employees and now has about 20 percent of the residential solar inverter market in California, according to the company.

Enphase, founded in 2006, now has about 200 employees worldwide with most in Petaluma.

The moves show renewed confidence in Petaluma's market.

"It's definitely a vote of confidence," said Tom Birdsall, a Basin Street partner.

He and White said the new acquisitions accommodate Enphase's need to expand into larger space as it prepares to increase its operations.

Matt White's father, Bill, founded the company in 1974 and started building Redwood Business Park in 1982, offering discounted rent to several high-tech startups that blossomed, creating Telecom Valley.

Matt White took over the firm as president in 2001, and took the enterprise beyond its business park beginnings with developments like the $100 million Theatre District that helped revitalize Petaluma's downtown with apartments, offices, small businesses and a cinema.

In 2005, Basin Street sold about 50 percent of its holdings in terms of value in a $263 million sale to Equity Office Properties, one of the largest real estate transactions in the history of the North Bay.

Four years later, in a move to diversify its portfolio and be closer to Matt White's home, Basin Street moved most of its operations to Nevada. About 20 employees remained in Petaluma.

The 2005 sale involved 1.43 million square feet of office space, including 20 buildings in Santa Rosa and three in Novato, in addition to the McDowell Corporate Campus and 13 buildings in Petaluma's Redwood Business Park.

All eight of the reacquired buildings were purchased from banks, which had taken over ownership from Equity Office Properties.

White said they were purchased for 28 cents on the dollar for what Basin Street sold them for six years ago.

The business park offices on North McDowell surround a pond with landscaping, picnic areas and volleyball and basketball courts. Basin Street said it will make some immediate upgrades, including to the lobby, grounds and parking lot, and install more energy-efficient systems.