A Portland housing complex for seniors plans to replace a Goodwill store on Forest Avenue with a new $18 million addition containing 55 one- and two-bedroom apartments.

Portland’s Planning Board unanimously approved the expansion plan on Tuesday.

The Park Danforth now includes 124 dwelling units in a seven-story building located just behind the secondhand store and facing Stevens Avenue. The existing complex includes market rate and subsidized housing, and some assisted living units. The units in the new addition will all be offered at market rates and would not include any assisted living apartments.

It’s not clear what the new housing will cost, but one-bedroom units in the existing building rent for between $2,200 and $3,446 a month, according to The Park Danforth’s website.

The new wing would be marketed to seniors who want to live independently but also want to be part of a community with on-site amenities and services, including a restaurant-style dining room. The target population is 75 years of age and older, according to plans presented to the city.

The addition will be attached to the existing building by a one-story connector that also would serve as a main entrance and a common area with seating, a solarium, a piano room and a media room. The expansion also will include two new interior courtyard gardens and a larger central courtyard with open space.

As a separate project, the owners have told the city they also plan to convert the first floor of the existing building into a “Main Street” with a boutique, ice cream parlor, bistro and wellness center.

The Park Danforth is a nonprofit that dates back to 1881. It was originally located at the corner of Park and Danforth streets in the city’s West End and moved to its current location in 1985.