Salesforce is in talks to lease additional offices in San Francisco, according to two real estate sources.

The discussions center on a potential lease of about 200,000 square feet — space for more than 1,000 employees — at 45 Fremont St. The 34-story tower is within a block of the software company’s three main buildings at 350 Mission St., 50 Fremont St., and Salesforce Tower at 415 Mission St.

“Salesforce does not comment on rumors or speculation,” the company said in a statement.

The possible deal reflects the seemingly unstoppable expansion of the tech industry that has reshaped the Bay Area economy. Salesforce would potentially occupy a building that has has been home to some of San Francisco’s oldest corporations. San Francisco institutions Wells Fargo and Bechtel have long occupied 45 Fremont St.

The deal could be signed within weeks, according to the sources, who declined to be named because they weren’t authorized to speak about the deal.

Salesforce, which has 8,400 employees in San Francisco, plans to grow to 10,000 employees in the city and has been moving out of some offices as it consolidates. Last year, the company took additional space in Salesforce Tower, where it occupies 881,000 square feet.

Jesse Gundersheim, San Francisco market economist at CoStar, a real estate data firm, said the proximity of 45 Fremont to Salesforce’s other buildings makes it attractive.

“There’s a limited stock of high-quality buildings in the downtown core,” Gundersheim said. “The building’s right next door. It makes sense for them to expand right there.”

Tech growth has led to a severe shortage in large blocks of office space in San Francisco. The vacancy rate in the south Financial District, which includes the Transbay area, is 4.9 percent, according to CoStar.

Only two under-construction office buildings in the city don’t have tenants: Oceanwide Center, at 50 First St., and 633 Folsom St.

Meanwhile, the next wave of large office buildings envisioned in Central South of Market requires a change in zoning to allow taller buildings that could be approved in October. But that proposal is facing opposition from four different groups. Even if the plan is approved, the city could be sued for inadequately studying the zoning change, and projects could be stalled for years.

Roland Li is a Chronicle staff writer. Email: roland.li@sfchronicle.com Twitter: @rolandlisf