Amazon.com Inc., which agreed to buy its Seattle headquarters for $1.16 billion, plans to start construction next year on the first of three new buildings that would add another 3 million square feet (278,700 square meters) as technology companies expand and invest in real estate.

Apple Inc.'s blueprint for a new 175-acre campus in Cupertino, California, features a futuristic ring-shaped main building of about 2.8 million square feet designed by British architect Norman Foster's firm. Facebook Inc. will start on an expansion to its Menlo Park, California, headquarters that would add a single-story building designed by Frank Gehry with about 430,000 square feet, or enough for about 2,800 engineers.

Expansion by technology companies is helping drive a nascent rebound in U.S. office development from the lowest in more than five decades as default rates on construction mortgages decline from a 2010 peak. Tech companies are expanding workforces and many also want to centralize employees in custom offices with amenities existing buildings don't always offer.