Image courtesy of Arcove Architects

A Portsmouth developer wants to take a former department store in the city’s downtown and give it a third act.

Once home to the J.J. Newbury department store, the upper floors of 15 Congress St. were converted to offices once the store left. Now, local developer Mark McNab has filed a plan to turn the 3-story building into so-called “co-living” space, where small apartments are combined in suites with extensive common areas, sometimes including shared kitchens, laundry areas and living rooms.

It’s a type of housing more common in big cities like Boston with plenty of recent college graduates more used to dorm life – and in some cases more interested in exploring their new city.

“The proposed residential use of the upper floors of J.J. Newberry is high density co-living, which will require a zoning change to allow for that use,” project engineer John Chagnon, of Ambit Engineering, wrote in project filings. “In the event the applicant is not successful with creating a Co-Living Use, then the proposed use of the upper floors of J.J. Newberry will be a mixture of micro apartments, studios, one-bedrooms and a boarding house, all permitted by the zoning ordinance.”

Amenities detailed in the plans include an extensive roof deck, and plans filed with the city’s Historic District Commission show the addition of bay windows and landscaping along an alleyway behind the building. Plans filed with the city suggest the property’s existing retail tenants would remain.

The project will need a conditional use permit to eliminate parking requirements for the project and would need Portsmouth to adopt changes to its zoning to allow for co-living in its downtown.

McNab has already received approval for a project next to 15 Congress St.