Social media giant Facebook Inc. plans to add to its Austin presence by leasing an entire new office tower at the Domain, according to multiple local real estate industry sources.

Facebook will lease all of Domain 12, a 17-story office building under construction at the Domain mixed-use project in North Austin, according to commercial real estate brokers who spoke to the American-Statesman on the condition of anonymity.

Facebook currently occupies three floors in Domain 8, another office building at the Domain, and has offices in downtown Austin, where it employs about 700 people.

It’s unclear how many workers Facebook might be planning to add in Austin. Facebook executives declined to comment. "We don’t have anything to share at this time," Kyle Gerstenschlager, a Facebook spokesman, said via email.

Domain 12 is being developed by Dallas-based TIER REIT and Austin-based Endeavor Real Estate Group.

TIER REIT and Endeavor declined to confirm that Facebook would be the tenant for Domain 12, citing confidentiality agreements. TIER REIT issued a statement conforming only that it has completed a lease deal "with a Fortune 100 company" for the entire building, which will be about 320,000 square feet. The building, which will be the tallest at the Domain to date, is due to be completed in the fourth quarter of 2019, the statement said.

Facebook’s newest lease comes on the heels of a large lease transaction in downtown Austin last year — 231,507 square feet of space the company plans to occupy in a 29-story office tower called Third + Shoal that is under construction at the site of the former Austin Music Hall. Facebook also occupies space in another downtown Austin office building called 300 West Sixth.

"Facebook’s continued growth in Austin is an affirmation of our exceptional work force and quality of life," said Diana Holford, senior vice president in Austin with JLL, a global commercial real estate services firm. "Facebook joins Indeed, HomeAway (Expedia) and Amazon in providing both a central and suburban option for their employees."

Last year, Katherine Shappley, head of Facebook’s Austin office, said the Third + Shoal lease would "expand our presence in the Austin area as part of our continued commitment to this thriving technology community. We’re delighted to be here and excited for what the future holds."

Shappley at that time did not disclose Facebook’s hiring projections, what type of positions would be housed in Third + Shoal, or the projected move-in timetable.

Facebook among a number of big tech companies — including Apple, Google, Oracle and Amazon— that have been expanding their workforces in the Austin region. The companies are gravitating to the Austin area in part due to its tech talent pool and a lower cost of living for the jobs they need to fill.

Local real estate brokers said the Third + Shoal lease was the largest office deal signed for downtown last year, and one of the largest ever for a new lease in the city’s central business district.

The biggest office deal in all of Austin for 2017 was for Domain Tower, an 11-story office building under construction at the Domain. Indeed signed a lease for the entire 310,000-square-foot tower.

Stonelake Capital Partners and HPI Real Estate Services & Investments are the developers of Domain Tower. Stonelake Capital Partners and HPI also plan to build two additional towers next to Domain Tower. One tower would be 19 stories with about 345,000 square feet of office space, and the other is planned for 23 stories with 300,000 square feet of space. One of the two buildings is expected to be under construction in 12 months.

"Major tech companies such as Facebook, Indeed, Google and others seem to be broadening and expanding their footprint across downtown Austin and the Domain," said Mike Kennedy, principal with Avison Young, a commercial real estate services firm. "It does not appear to be slowing down as talent competition continues and Austin attracts employees in waiting."

In addition to announcing that Domain 12 was fully leased, TIER REIT said that it plans to start construction on another office building, Domain 10, in October. Domain 10 will have 300,000 square feet of space.

Holford said that "employees, and therefore employers, continue to migrate to the Domain" for its mix of uses.

"Workers love the restaurant, bars and shopping just outside their workplace door," Holford said. " Companies are attracted to its mix of hotels for out of town guests and prestigious address. The apartments within the development allow many employees to walk to work. Plus, the Domain is closer to more affordable housing choices."