PCA has been working with The Mount Vernon Company on the design of a five story, 74-unit apartment building and a separate, four-story building with nine condos across the street within Allston-Brighton’s Boston Landing. The apartment units will be priced for the middle market, providing housing for people often left behind in the booming Boston housing market. A Letter of Intent has been submitted to the Boston Planning & Development Agency for permitting review, as noted in a recent Boston Herald article…