The couple who own one of the few structures atop Munjoy Hill to survive the Great Fire of 1866 are seeking Portland’s landmark status for the house, even as they sell it.

The brick house at 147 Congress St. is “a particularly high-style example of transitional Greek Revival/Italianate architecture,” according to a city report prepared for the Portland City Council, which is to vote Monday night on whether to grant the landmark designation.

Karen Rasmussen and Manuel Pena, the couple who own the house, could not be reached for comment, but the architect who prepared the nomination for the landmark designation said they are moving because an older family member started living with them and having a two-story home presented difficulties.

The architect, Julie Larry of ttl-architects, said the landmark designation would ensure that most exterior changes to the house would have to be approved by Portland’s Historic Preservation Board. Additions, window replacements, masonry repair, wood trim and work on the porches would have to be reviewed, she said.

It would also protect the building from demolition unless the owner can prove an economic hardship.

Interior changes wouldn’t need to be reviewed, Larry said, She noted that the couple had to do a lot of work inside when they bought the house in 2000.

“I believe Karen said it was a rooming house when she purchased it and they restored it back to a single-family,” Larry said in an email.

She said the structure, known as the Ann Freeman House for its first owner, is under contract and the potential new owners are aware of, and support, the landmark designation.

Larry said she didn’t know the sale price on the house. City property records say it’s assessed at $307,400.

The local designation doesn’t provide any tax breaks, Larry said. Owners of buildings on the National Register of Historic Places can get tax credits for restoration work, she said, but the Freeman house isn’t on the list and hasn’t been nominated.

The city report said the house, built in 1857, was one of a handful of buildings on the east end of Congress Street to survive the 1866 fire. One of the others is the Portland Observatory, which sits across the street from the Freeman House.

The request for landmark status contrasts with growing tensions between economic development and historic preservation in Portland as the city becomes a hot real estate market.

Last week, the preservation board recommended that House Island be designated as a historic district after a lengthy debate between those hoping to strengthen historic protections and critics who viewed the effort as a last-ditch attempt to prevent improvements on the private island.

Situated between Peaks Island and South Portland, House Island occupies a prominent location in the city’s harbor and an important place in Portland history. Fort Scammel on the island’s southern tip is the only fort along Maine’s coast to see action during the War of 1812, when the fort exchanged fire with a British privateer. And in the early 1900s, House Island became Maine’s version of Ellis Island and served as a federal immigration station and quarantine facility.

Michael Scarks, a Portland developer who has restored several properties in the city, purchased House Island last spring with plans to build several new houses there while preserving Fort Scammel. The nonprofit Greater Portland Landmarks requested that the island be designated a historic district in the summer, and several preservation board members agreed to consider the proposal.

Designation would not prohibit development, but means new structures would be subject to additional review by the preservation board or planning staff. Additionally, changes to historic structures on the property would have to be approved in a process that board members have described as cooperative, not confrontational.

The preservation board’s recommendation will go to the Portland Planning Board and then to the City Council for consideration.

Another Portland site in line for development also has led to tensions between preservation and development. Portland Landmarks has asked the preservation board to designate the Portland Co. site at 58 Fore St. as a historic district.

Located at the base of Munjoy Hill within walking distance of the Old Port, the Portland Co. complex has both enormous development potential and significant historical features.

The Portland Co. began operating at the Fore Street site in 1847 and ceased operations in 1982. The more than 600 locomotives built by the Portland Co. were used by railroads throughout Maine and eastern Canada, as well as the Northern Pacific Railroad and the Panama Railroad. Hundreds of ships built in Maine’s many shipyards were outfitted with marine engines built in Portland, including the Civil War gunship Agawam and the 280-foot passenger steamer the Portland.

The 10-acre site was purchased by Yarmouth-based developer CPB2 LLC in 2013. CPB2 has not released specific plans for the site, but one of the CPB2 principals, Casey Prentice, has said the company envisions a combination of residential, commercial and office space while maintaining the marina and historic elements of the property.

Portland Landmarks requested the historic district designation last month after CPB2 filed an application in August to rezone the property as a first step toward its redevelopment. A historic designation would subject any development plans to greater scrutiny. It also could force the property’s owners to alter plans for some of the former manufacturing buildings on the site.

“The Portland Company complex currently lacks any protection of its historic resources, which are at risk as development pressure increases on Portland’s peninsula,” Hilary Bassett, executive director of Portland Landmarks, wrote in a letter to Deb Andrews, manager of the city’s Historic Preservation Program.

Prentice has said the company knew the organization would likely request landmark status for individual buildings, but did not expect the broader request for a historic district.