The opening of the newest residential building in the Mattapan Heights campus will be celebrated with a block party planned for this Saturday, July 20. The new apartment building, containing 60 affordable housing units, is called Phase 5A. In late 2011, Trinity Financial Inc, the Boston-based development company that has been working on Mattapan Heights since 1999, received approximately $8.75 million in grants and loans towards the construction project. It was completed at the end of 2012, and is now fully occupied by tenants.

Phase 5A brings the total number of affordable housing units on the campus to approximately 332, according to Trinity’s Mattapan Heights project manager Eva Erlich. With the completion of Phase 5A, over $100 million in public and private has been spent on the development of the campus.

The Mattapan Heights campus is located on River Street at the former site of the Boston Specialty and Rehabilitation Hospital, which closed in 1996. In 1997, the city put the Boston Public Health Commission in charge of the site, and the BPHC has worked with Trinity Financial Inc. and the Mattapan Community Advisory Board to redevelop historic buildings and construct new ones to create residential units.

Though there are no current plans for further development of Mattapan Heights, Erlich said that she hesitates to call Phase 5A the “final phase” of the campus redevelopment process.

“There are always other sites on the campus to improve,” she said. Erlich added that it is important to the campus developers to keep the Mattapan community involved in any planned changes to Mattapan Heights. According to Erlich, the developers’ original plan was to create more single-family homes in the space now occupied by Phase 5A; however, the community wasn’t in favor of this plan, so the developers changed course and designed the multi-family 5A complex instead.

Erlich said that the 52-acre campus, a “city gem,” is especially appealing to people because of the atmosphere.

“Phase 5A came out of the high demand to live on campus,” she said, citing the waiting list that aspiring Mattapan Heights residents have to stay on. “You don’t always see all that green space in more urban residential areas. It doesn’t seem like the middle of the city.” Erlich said that the community’s support of each campus development phase is vital to the success of the project.

“We’ve always kept the larger community involved as we designed and moved forward with each phase,” said Erlich.

This year, the campus is also celebrating the 10th anniversary of the Foley Building, a historic structure once part of the Boston Specialty and Rehabilitation Hospital. The Foley Building was the first phase of the project to be completed, and is now the site of affordable senior housing units.

According to Erlich, some of the seniors who live in the Foley Build are former employees of the Boston Specialty and Rehabilitation Hospital, showcasing the uniqueness of the campus community.

A ribbon cutting ceremony will commence at 11 a.m. on Saturday at the Upper Campus Courtyard at 229 River Street. It will feature a variety of speakers, including Mayor Tom Menino. The campus party will last from 12 to 3p.m. and will feature performances, food, and games.

To RSVP to the block party on July 20, email reception@trinityfinancial.comor call 617-720-8400 ext. 1520.