A pending 430,000-square-foot lease by Amazon could kick-start construction of an 18-story office building in Boston’s Seaport District this year, bringing the promise of 2,000 jobs along with a request for a $5 million tax break by landlord WS Development of Newton.

The Amazon offices would be built on WS’s block L4, part of the 7.7-million-square-foot Seaport Square development.

“It’s not HQ2, but it is a significant presence in the Seaport,” Mayor Marty Walsh told WBZ Radio.

WS hopes to break ground on the building, including 81,000 square feet of retail space on the first two floors, in the third quarter and complete it in 2021, according to a filing with the Boston Planning and Development Agency.

A proposed $5 million, 15-year payment in lieu of taxes agreement would require Amazon to create 2,000 jobs in Boston or within a 25-mile radius of the city by 2025. The first building would generate $4.4 million toward affordable housing and $875,000 for job training programs, WS estimates.

The online shopping giant also has an option to lease space for another 2,000 jobs on the nearby L5 parcel by the end of 2020, with a target of an additional 2,000 jobs. The second building also would receive a $5 million property tax reduction. The PILOT agreements are subject to approval by Boston assessors.

Consisting of surface parking lots bordering Congress Street and Pier 4 Boulevard, the property requires costly soil remediation and flood-proofing measures to protect the buildings and underground parking from rising seas, according to WS. The L4 building would be elevated 2 feet above a 500-year floodplain.

Amazon’s job creation estimates would make for the biggest corporate expansion in the booming Seaport since State Street Corp. in 2014 opened its Channel Center offices, which have capacity for more than 3,500 employees. State Street received $11.5 million in tax breaks.

Amazon already is opening new offices for 900 employees in the Seaport this spring after leasing 153,000 square feet at Synergy Investments’ 253 Summer St. last fall. The transactions are separate from the e-commerce leader’s search for 8 million square feet for a second headquarters, with Greater Boston among 20 regions still in the running.

WS bought 12.5 acres in Seaport Square from original master developer Boston Global Investors in 2015 for $359 million. The BPDA approved additional density for the WS parcels in November.