The Planning Board on Monday will hold a public hearing on a proposal to build a five-story residential/retail building at 2 Washington St. in the city’s Smart Growth District. The building would have 94 apartments, nine of which would be set aside for low- to moderate-income residents, under the city’s Affordable Housing Incentive Program. The first floor would have three apartments, 2,200 square feet of retail space, and a clubhouse with fitness and media rooms. WP East Development Enterprises, a Marietta, Ga., developer, has applied for site plan review and a special permit. The hearing will be held at 8 p.m. in the mayor’s conference room at City Hall.