Twitter leased space from Shorenstein Properties, a real estate firm based in San Francisco, known for its blue-chip office towers in the Financial District here. Shorenstein bought an 11-story building in 2011 fronting Mid-Market that had been vacant for five years. For them, it made sense to buy the undervalued Art Deco landmark built in 1937, which had some of the most spacious floor plans in the city at a time when office space was tight. Twitter signed a lease until 2021 for 295,000 square feet in the building and could expand that as its work force grows. “In our gut, we believed if we changed it, they would come. We thought it would be a real catalyst for the neighborhood,” said Charles W. Malet, chief investment officer for Shorenstein Properties.

Now 15 other companies, like Spotify, Square and Yammer, emboldened by Twitter’s move and a city tax incentive that largely exempts them from city payroll taxes if they relocate to the Mid-Market, have committed to take 1.3 million square feet in the area, which the city has renamed Central Market. Apartment towers with 5,500 units are in the works, and arts groups, chefs, retailers and even a venture capital firm have taken up residence.

“You had a once vacant and blighted area that is now a gravitational center for some of the most innovative companies in the world,” said Todd Rufo, director of the San Francisco Office of Economic and Workforce Development.

Not to be outdone, the rest of San Francisco is in the middle of an impressive building boom. Developers are building office towers downtown for the first time in five years, many confident enough to build without signed leases for the space. Other buildings are undergoing extensive renovation. Branches of technology companies, old and new, like Google, Amazon, Microsoft and Yahoo, are expanding or moving to the city and now make up more than half of demand for office space. (During the tech bubble in the late 1990s, tech accounted for only a quarter of demand.)

“People tend to build when they can lease and make money,” said Meade N. Boutwell, a senior vice president at commercial real estate brokerage C.B.R.E. in San Francisco.