The Portland City Council voted unanimously Monday night to grant historic status to a Casco Bay island that was once used for immigration processing but is now eyed for development.

Greater Portland Landmarks put House Island on its “Places in Peril” list in 2012, according to the group’s executive director, Hilary Bassett.

Bassett, one of six residents to speak in support of the designation, said the site meets all of the criteria for being historically, culturally and architecturally significant.

“House Island is especially significant to the city on multiple levels,” she said. “(It) has tremendous significance to the history of Portland and to the overall character of Portland Harbor and needs to be protected for future generations.”

Situated between Peaks Island and South Portland, House Island has a prominent location in the harbor and an important place in Portland history.

Fort Scammel, on the island’s southern tip, was the only fort along Maine’s coast to see action during the War of 1812, when it exchanged fire with a British privateer. For centuries, fishermen used the island for curing fish and processing lobster. And in the early 1900s, House Island became Maine’s version of Ellis Island, serving as a federal immigration station and quarantine facility.

Michael Scarks, who did not attend Monday night’s meeting, bought the 24-acre island in May and plans to build a few homes while preserving the fort. In October, he sold the northern part of the island to Christina and Vincent “Cap” Mona, who plan to restore three buildings that date to 1907 and were once part of the immigration station.

The designation does not prevent development on the island, but it limits the type of development that is allowed and increases the scrutiny during the planning and application process.

Although no one spoke against the proposal, several supporters noted the importance of protecting the entire 24-acre island, rather than the structures.

“It’s a whole neighborhood where different parts were significant during different periods of time,” said Peaks Island resident Arthur Fink. “It’s important to protect one piece of it. It’s important to protect the whole.”

Also Monday, the council postponed a vote on a liquor license application for a movie-themed bar at 10 Exchange St., the former site of the Movies on Exchange. Councilor Edward Suslovic said the postponement was due to a “slight discrepancy” in the age of applicant Josh Soley, who is 20. A person must be at least 21 to hold a liquor license. The council will take up the application in June, after Soley turns 21.

The council also approved the sale of a roughly 3½-acre site in the envisioned Portland Technology Park, which is located off Exit 47 of the Maine Turnpike.

Patrons Oxford Insurance will pay $625,000 in cash for the site, where it will build a 20,000-square-foot building so it can relocate from Auburn and expand its 40-employee firm, according to Greg Mitchell, the city’s economic development director.

Currently, the park consists of a road and utilities that cost $1.3 million to build.

At a ribbon-cutting ceremony in September 2013 for the 950-foot road, city officials touted the park as an opportunity to create a bio-technology cluster.

On Monday, Mayor Michael Brennan said the sale of the first of three development sites was a “watershed moment.”

“It’s the type of jobs we want in the city of Portland,” he said.

The council also hired Adam Lee as associate corporation counsel. He will earn $80,800 a year.