Share story is in final negotiations for big leases at two more Seattle office buildings, Blanchard Plaza and 5th & Bell, according to people familiar with the discussions.

If the Seattle-based company signs both leases as expected, its footprint of leased existing space in downtown Seattle would grow by about 275,000 square feet, or almost 20 percent.

The online giant currently leases just over 1.5 million square feet, in addition to the 1.7 million square feet it occupies in its South Lake Union campus.

Amazon has signed a letter of intent and is expected to finalize a lease for at least one of the buildings before the year’s end, said a person familiar with the negotiations.

Real-estate brokers have suspected Amazon’s interest in Blanchard Plaza, at 2201 Sixth Ave., and 5th & Bell, at 2301 Fifth Ave., for months.

“We’ve heard there’s a large tech user that’s very close to leasing those spaces,” said Brian Hayden, a principal at Flinn Ferguson, a leading tenant-representation firm in Seattle. “And it seems like it would make sense that it would be Amazon.”

People familiar with the discussions said Amazon could occupy about 100,000 square feet at 5th & Bell and more than 175,000 square feet at Blanchard Plaza.

Neither Houston-based Hines, the owner of 5th & Bell, nor Shorenstein Properties of San Francisco, which owns Blanchard Plaza, would comment. Amazon also had no comment.

The company has become the juggernaut of Seattle’s commercial real-estate market: Since 2010, Amazon’s Seattle footprint has grown by a net 140,000 square feet each quarter, according to a recent presentation by commercial real-estate brokerage CBRE.

Amazon doesn’t disclose how many employees it has here, but real-estate experts estimate it’s more than 18,000
. That assumes about 170 square feet per worker, which facility managers say is typical for high-tech firms.

Blanchard Plaza and 5th & Bell, which are where Denny Triangle and Belltown meet, offer Amazon proximity to the company’s future 3.3 million-square-foot high-rise campus: Construction already has begun on the first million square feet, which could be ready for occupancy as early as 2015.

The two office buildings are also about a half-dozen blocks from where Vulcan is building four structures that will add about 980,000 square feet to Amazon’s office space in Seattle.

The pending leases suggest Amazon is growing too fast to wait for those new buildings to be completed.

In September, Blanchard Plaza’s largest tenant, U.S. Health and Human Services, announced plans to move from the 15-story building in April, when its lease expires. Health and Human Services’ Seattle regional office occupies about 107,000 square feet, or more than 40 percent of Blanchard Plaza.

Additional space in Blanchard Plaza is available or will be next year as tenant leases expire.

Cochran, the electrical contractor that renovated Amazon’s leased space in three other office towers — 1800 Ninth Ave., 1220 Howell St., and 1260 Mercer St. — last month obtained a permit to install equipment on floors two through 15 of Blanchard Plaza.
The client isn’t disclosed.

In the six-story 5th & Bell building, meanwhile, about half the space — or 100,000 square feet — is available, according to Office­

The University of Washington’s Institute for Health Metrics and Evaluation occupies about 46,000 square feet there.

Amazon’s leases in Seattle generally have terms of 10 years. It’s unclear how long it might lease the Blanchard and 5th & Bell buildings, especially with so much new space under construction.

Vulcan broke ground earlier this year on Amazon’s “Phase 6” — two six-story office buildings totaling 377,000 square feet on the block bounded by Mercer and Republican streets on Ninth Avenue.

The two other buildings that Vulcan plans to build for Amazon — 12-story structures between Thomas and Republican streets on Ninth Avenue — may be ready for occupancy in late 2015.

The developer plans to break ground on both structures in the next two months.

Sanjay Bhatt: 206-464-3103 or On Twitter @sbhatt