Before and after renderings show The Row Hotel in Assembly Square’s proposed dining area and rooftop deck. (Images: Somerville Planning Board)

The Row Hotel in Assembly Square is approved to get a rooftop deck, but operators promised Somerville’s Planning Board on Thursday that it wouldn’t become a party spot.

“This will not be a late-night bar. This is not a late-night destination,” said Patrick McMahon, a senior vice president at Federal Realty, developer of the 5 million-square-foot complex of homes and businesses called Assembly Row. “If one wants to find that, there are plenty of options in The Seaport.”

The addition would relocate a dining area and lounge from behind the lobby bar, allowing the hotel to expand its meeting and conference space to meet demand. There has also been growing demand for food service, and the hotel expects to serve more people at one time and add service into the evening with a menu of small plates, McMahon said. The hotel will also explore adding brunch. The expansions are expected to add 10 to 15 permanent jobs.

The 5,500-square-foot dining space and a 1,700-square-foot outdoor terrace would go atop the ground-floor lobby and a second-floor ballroom – becoming a third floor that, because the lobby and ballroom have high ceilings, is connected to the fourth floor of rooms in the 158-key hotel and not far from some of the residents in the connected 122-condominium tower.

The hotel has been open for eight years at 360 Foley St., a roughly two-minute walk from the Assembly Square orange line T stop.

Operators expect to close the new terrace at 9 or 10 p.m., said Leo Xarras of XSS Hotels, which also runs the AC Hotel near Alewife in Cambridge and Fairfield Inn & Suites in East Cambridge. The Row Hotel “has a very distinct customer who is not looking for loud, and they want this nice getaway space,” Xarras said. “We don’t have a lot of good months here, but the months we do have, we want them to get outside and enjoy it.”

There was concern from the Planning Board that moving dining off the first floor would damage Assembly Row’s street-level vibrancy, but McMahon noted that the dining area is now down an alley and serves only breakfast. In terms of visible use by people, the conference space “will likely be used all day and maybe even into the evening” near what will “continue to be a very active, lit, dynamic hotel lobby.”

Board members approved the proposal. Hotel staff said the new dining area and terrace was expected to open next year after construction that is set to begin late this year. There were no specific dates.