4 Mellen Street (formerly known as 1627 Massachusetts Avenue) is a 14,400 square foot site located on the corner of Mellen Street and Massachusetts Avenue, between Harvard Square and Porter Square in the Baldwin neighborhood. The parcel consists of a 6,500 square foot historic building, as well as a parking lot at the rear of the site with access off of Mellen Street. The existing building, known as the Charles Saunders House, has been deemed historically significant and is on the National Register of Historic places and was designated a historic landmark by the Cambridge Historical  Commission (CHC) in 2023.

Homeowners Rehab, Inc. (HRI) will create affordable housing on the site under the provisions of the Affordable Housing Overlay (AHO). HRI purchased the property from Lesley University in August 2022 and intents on preserving and converting the existing building into affordable housing and adding a building at the rear of the site for more affordable housing. The new rear addition will meet Passive House standards and will include features to ensure maximum energy efficiency such as robust insulation, highly efficient HVAC systems, and eco‐friendly materials and will be entirely electric. In addition to the residential component, the new building will contain a 700 sq.ft. ground floor amenity space. The front yard will be retained, and additional site improvements are being made along the edges of the site, including small sitting areas, improved landscaping and lighting, and passive open space.