SAN JOSE — A million-square-foot mega complex of offices, homes and retail would sprout in downtown San Jose under new plans that have been proposed by one of the city’s most active real estate entrepreneurs, adding enough space for 6,000 office workers.

The proposal, if approved by the city, could be built on a choice downtown property that’s come to be known as the Valley Title site at 300 S. First St.

The mixed-use development would consist of 1.2 million square feet of offices, 50,000 square feet of retail and restaurants, 332 residential units and 645 parking spaces, according to preliminary plans submitted to San Jose officials.

The 2.8-acre site property was bought for $64 million in August 2018 by a venture led by Gary Dillabough. The site fronts on South First, East San Carlos and South Second streets and is directly across from the popular Original Joe’s restaurant.

This is deemed to be one of the most important development sites in downtown San Jose, primarily due to its size and location in the up-and-coming South First Area, or SoFA district.

“It’s going to be a great project,” said Geri Wong, a senior managing director with commercial realty firm Newmark Knight Frank. “It will help activate the SoFA district, and will create more connections between SoFA and the rest of the downtown.”

SoFA is an emerging downtown arts, entertainment and nightlife neighborhood, and local officials and realty entrepreneurs are seeking ways to make the area even more active.

The site also would help connect San Jose State University to other parts of downtown.

“Getting 1.2 million square feet in the SoFA area is really exciting,” said Bob Staedler, principal executive with San Jose-based Silicon Valley Synergy, a land use and planning consultancy. “This kind of activity in this part of the downtown will really help SoFA take off.”

Dillabough has become one of the most active investors in downtown San Jose properties, acquiring sites that in several cases are poised to be transformed into dramatic redevelopment ventures.

Among the high-profile ventures being undertaken by Dillabough in downtown San Jose: the Museum Place office, restaurant and retail project at the old Parkside Hall; renovation of the iconic Bank of Italy office tower; redevelopment of the Bank of the West building across the street; and the historic Moir Building next to St. James Park; along with other nearby property purchases, including a parking lot on South Second Street.

Adding to the heightened interest are high-profile enterprises such as Google’s proposal in the area of the Diridon train station for a transit-oriented village of offices, homes, restaurants, shops and open spaces where 20,000 to 25,000 could work, including 15,000 to 20,000 tech workers employed by the search giant.

San Jose-based Adobe plans to dramatically expand the size of its three-building downtown headquarters by constructing an adjacent fourth office tower where 4,000 would work.

Over a one-year period that ended in September 2018, investors, tech titans and developers spent $1.43 billion buying properties in downtown San Jose — a shopping spree that was three times larger than the $484 million that was spent downtown during a similar 12-month period that ended in September 2017.

And in recent weeks, Boston Properties formally teamed up with TMG Partners and Valley Oak Partners to launch development of the Platform 16 site near Julian Street and Autumn Park, a 1.1-million-square-foot project.

A few days after those plans were revealed, Boston Properties proposed a vast 1.8-million-square-foot campus on South Almaden Boulevard across the street from the San Jose convention center, a project that features striking structures that would follow the contour of the Guadalupe as it meanders past the Children’s Discovery Museum.

If these three projects that have just been proposed were all built, that would be enough office space for more than 20,000 workers.

“These proposals show how infill projects and transit-oriented development are the new thing and are really catching on,” Staedler said. “These big office projects that could attract a large tenant are what tech companies are really looking for.”