Google’s huge Seattle campus being built on Amazon’s backyard of South Lake Union is about to get a lot bigger.

Paul Allen’s Vulcan Real Estate said today it is developing a third block for Google in addition to the more than 607,000 square feet currently under construction. Work will begin on this block, which will feature a 12-story office building for Google with 23,000 square feet of retail space, in the fourth quarter of 2019, and it will be ready for Google in 2021.

The site is directly across an alley from one of Amazon’s original office buildings in the neighborhood. Vulcan would not say how many square feet the new building will be, but previous permit filings with the city of Seattle showed plans for a 12-story, 322,000-square-foot structure there. A new building of that size would bring the future Google campus size to nearly 930,000 square feet, which could mean room for upwards of 4,500 to 6,200 people.

Construction began last year on the first two blocks, which will feature four buildings for Google and a combined 149 residential units. Those two blocks, first announced in 2016, will be complete early next year.

Flinn Ferguson and CBRE represented Google in the deal, while Vulcan represented itself. NBBJ is designing the project and GLY Construction is the general contractor.

The expansion of this already huge campus is another example of Google’s insatiable appetite for office space in the Seattle area. The tech giant has been scooping up every available piece of real estate in the Fremont neighborhood, including taking space from other major tech companies like Tableau. Google also has a campus in the Seattle suburb of Kirkland, and it is reportedly eying expanding there as well.

Google employed 3,000 people in the Seattle area as of March, up from 1,900 two years ago. Google hasn’t said how much of its current space it will hold on to once the Seattle campus is built.

Amazon has long been the main player in South Lake Union. But Google’s entrance into the neighborhood just down the road, as well as a rapidly growing presence for Facebook, should make for plenty of competition for top talent.

Last year at the GeekWire Cloud Tech Summit, a Google executive said the company’s cloud division will anchor the South Lake Union campus.