North Shore-Long Island Jewish Health System is poised to expand its footprint in a nearly 1.4 million-square-foot complex in Lake Success now that a Massachusetts-based developer has purchased space in the complex, health system executives said Friday.

The builder, Waterstone Retail Development, closed Thursday on its $113.75 million purchase of a portion of the site at 1111 Marcus Ave., with plans to lease much of the space to North Shore-LIJ, health system spokesman Terence Lynam said.

The 940,000-square-foot property purchased on Thursday by Waterstone is occupied by tenants including Hain Celestial Group and Dealertrack Technologies, as well as North Shore-LIJ’s human resources department. Waterstone bought the property from Los Angeles-based Ares Management; the property had been in foreclosure since early this year, Lynam said.

North Shore-LIJ intends to gradually expand its presence in the complex as existing tenants leave. The health system will gain ownership of the property at the end of the 30-year lease.

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The remainder of the complex — 440,000 square feet — is occupied by North Shore-LIJ’s Center for Advanced Medicine. The health system bought that portion of the site in March.

The complex is located across the street from 2000 Marcus Ave., which the health system plans to adopt as its new headquarters in the spring. Headquartered in Great Neck, North Shore-LIJ is leasing 2000 Marcus Ave. from Lake Success-based Lalezarian Properties.