The developers of the Olmsted Green development on the old Boston State Hospital along Harvard Street in Mattapan will soon file plans for six new buildings with 265 apartments and condos - including one building that would have 20 apartments for families with foster or adopted kids and 40 apartments for seniors who would agree to help them out.

In a letter of intent filed with the BPDA on Friday, Jerome Rappaport, Jr.'s Lena New Boston II and 2Life communities say their proposal to finish off a 20-year redevelopment at the 52-acre site, now home to some 500 housing units, would also include a new six-story building that would have 127 apartments for senior citizens, a day care, a wellness center and a convenience store, and buildings with 78 condos aimed at moderate-income families.

In their filing, they call the final 10-acre undeveloped piece of the old state-hospital site Olmsted Village, and detail their proposal for the Treehouse at Olmsted Village: "An intergenerational affordable housing community" in partnership with the Treehouse Foundation and Plummer Youth Promise, "two nationally recognized foster care service providers." The plans will include:

40 affordable apartments for seniors committed to providing mentoring and compassionate support to 12 foster/adoptive famiies and 8 youth (18-24 years of age) aging out of the foster care system, and a 5,000 sf community center for both the residents of Treehouse and to the broader Boston foster-adopted community to be developed by 2Life.

The proposed project plans will also include "preservation and restoration" of existing wetlands and woodlands on the site, a shuttle bus to and from the Forest Hills MBTA station and "short term, attractive equity investment opportunities to a socially and culturally diverse group of investors."

Olmsted Village filings and schedule.