HBO’s Seattle engineering office has grown to 55 people since opening last year, and the company is planning to double the size of the operation by the end of this year — taking over a second floor in its existing building to accommodate the expansion.

The growth of HBO’s engineering office in the city “certainly has exceeded my expectations,” said Otto Berkes, HBO’s chief technology officer, confirming the expansion plans in an interview with GeekWire this morning.

The office, which focuses on the HBO Go service and other R&D projects, is the latest example of big media and technology brands growing rapidly in the Pacific Northwest. Facebook, which just two years ago expanded into new offices in Seattle, is also on the hunt for more office space in the region, according to reports that surfaced last week.

“Seattle is an incredible source of very strong software technology talent,” said Berkes, who was one of the original Xbox leaders during a long tenure at Microsoft. “That has really fueled our ability bring in the people we need to work on our future technologies.”

Another Microsoft veteran, Drew Angeloff, leads the HBO Seattle engineering office.

HBO has hired engineers from a variety of companies and engineering outposts in the Seattle region, including Microsoft, Amazon and Zynga, which went against the trend by closing down its Seattle office in January, as part of broader companywide cutbacks.

Both Facebook and HBO are based in the Metropolitan Park complex near Interstate 5. A real estate source told GeekWire last week that Facebook was eyeing the top two floors of the Macy’s building in downtown Seattle, with enough room for about 500 workers. Separately, HBO declined to comment on rumors that it’s considering moving to a new office building under construction near downtown Seattle.