Image courtesy Elkus Manfredi Architects

Repositioning of the Tufts Health Plan offices in Watertown will create 481,000 square feet of lab space in one of Greater Boston’s most sought-after life science development markets.

Spear Street Capital is seeking approval to transform the 11.3-acre campus at 705 Mount Auburn St. with a 2-story addition to the existing complex, revamped building infrastructure to accommodate life science users and new open space and amenities.

Designs by Elkus Manfredi Architects call for comprehensive improvements to building systems including new electrical service, high-efficiency mechanical systems including heat recovery and new elevators. A 2-story, 27,000-square-foot addition is planned with rooftop mechanicals, according to a site plan review application filed with the Watertown Planning Department.

In addition, a new signature open space known as “The Grove” would replace the existing triangular courtyard and add public seating and an employee patio garden connecting to a cafe.

The San Francisco-based developer bought the former health plan offices for $307 million in September after Tufts opted to relocate to the former Reebok headquarters in Canton as part of its merger with Harvard Pilgrim Health Care.

The project is the latest in a series of life science conversions that have made Watertown a favored landing spot for expanding biopharma companies and prompted a series of acquisitions for life science conversions by developers including Alexandria Real Estate Equities, Westbrook Partners and Boston Development Group.

The property was originally developed in the 1930s for New England Telephone and Telegraph and expanded in 1958.