The development team for a sprawling $57 million mixed-use project in Mattapan Square submitted a Letter of Intent to the city’s planning agency on Thursday.

A residential, commercial, and retail project is planned for the the underutilized 2.57-acre MBTA parking lot by the Mattapan terminus of the high-speed trolley, according to the letter the Preservation of Affordable Housing (POAH) and the Nuestra Comunidad Development Corporation filed with the Boston Planning and Development Agency (BPDA).

POAH and Nuestra Comunidad expect to build 135 units of mixed-income rental apartment housing in one building, along with 10,000 square feet of commercial/retail space, and a 2,000-square foot community room on the ground floor. A future second building would include nine homeownership units. The buildings would total about 166,750 gross square feet of usable space.

“This innovative transit-oriented development project will provide critically-needed affordable and market-rate housing in the Mattapan community,” they wrote in the letter. “Just as importantly, residents of the complex will have a variety of transportation options, including public transit, ride-share, and bike-share, along with direct access to the Neponset River Greenway, which will substantially enhance their quality of life with the provision of passive and active recreational opportunities.

The first building would stand at five stories along River Street, rising to six floors at its southern elevation nearer to the bus and trolley station. It includes two town houses with ground-floor entrances, as well as 133 multi-family units on the top four floors. “A range of income levels” will be included in the residential housing, which is composed of studios, one-, two-, and three-bedroom units.

Boston’s Inclusionary Development Policy requires that larger developments dedicate at least 15 percent of on-site units to affordable housing, or 18 percent off-site. POAH-Nuestra is pledging 50 percent of the units in their project will be affordable, a topic that dominated a July community meeting.

A four-story residential condominium building -- nine units and about 10,500 square feet -- would front River Street.

People can access the community space from an entrance near the Neponset River Greenway, according to the letter.

“The commercial/retail space along River Street will include a mix of small businesses and retail stores intended to meet the needs of Mattapan Station residents and MBTA commuters, along with residents of the broader Mattapan community,” the development team wrote. They anticipate greater foot traffic along the way with the new commercial components and a subsequent economic boost for the broader Mattapan Square area.

The project is subject to the BPDA’s Article 80 Large Project Review process. POAH and Nuestra Comunidad will later file an Expanded Project Notification Form with more detail.

Jennifer Smith can be reached at jennifer.smith@dotnews.com, or follow her on Twitter at @JennDotSmith