A 43-acre Marlborough development site once eyed for a large R&D park is being repositioned for a 475-unit apartment complex.

Post Road Residential of Fairfield, Connecticut proposes a nearly 550,000-square-foot multifamily project called the Green District in two buildings at 107 Simarano Drive.

The property includes 43 acres in Marlborough and six in Southborough, but no development is planned on the Southborough portion. Post Road Residential is seeking special permit approval in Marlborough for the project, which would include 770 parking spaces in surface lots and 16 six-space garages.

In keeping with the project’s resiliency theme, some of the surface parking will be built on reinforced grass pavers to minimize stormwater runoff, according to a filing under the Massachusetts Environmental Policy Act (MEPA). Post Road estimates the project would generate half the traffic as the previously-approved office park.

Sustainability features will include 10 electric vehicle charging stations, rooftop rainwater recycling, resident composting program and garden plots for residents.

Post Road recently completed the Pioneer apartment complex and earlier redeveloped the Charleston Chew factory as the Batch Yard apartments, both in Everett.

Digital Equipment Corp. proposed a 750,000-square-foot R&D park called Crane Meadow on the site in 1986, and a 1995 project change was approved for office use. The current owner, Boston Properties, has agreed to sell the property to Post Road Residential, according to a Middlesex County Registry of Deeds filing.

Demand for office space in the Route 495/Massachusetts Turnpike submarket has remained sluggish, with a 26.7% availability rate in the third quarter and 416,879 square feet of negative absorption since the beginning of 2019, according to CBRE research.