BANGOR, Maine — The building sits opposite Cascade Park. Plywood covers its windows, red paint peels from its shingling, and a sign that reads, “Danger: Restricted Area,” hangs on its front door.

The 125-year-old house has a warp nearly a foot tall through the center of its ground floor. Without lights or electricity, it’s gloomy and dark. Crumbled white plaster is everywhere, and paint flakes from door moldings. Yellow traffic tape blocks a door to a small room where a building inspector fell through the floor a year before, rather abruptly ending the inspection.

The place would smell much worse of mold and pigeon droppings, except the plywood doesn’t keep out enough of the frosty wind to make the odors intolerable.

“I think Ellen is being optimistic when she says it’s gonna take a few weeks to get it done,” Bob Kelly says.

“Six months is more like it,” J.P. Kelleher answers.

It’s Tuesday, and the building restorers are touring what for decades has been called the engineer’s house. It accommodated the family that ran the old Bangor waterworks, which are nearby but now converted into low-income apartments.

Kelly’s company, House Revivers and Ames Associates LLC, an architectural firm that restores historic buildings, announced the day before that they had signed a purchase and sale agreement to buy the building for $1 if they get their building permits by March 31. Ames hopes to relocate from Broadway next winter.

Ellen Angel, a senior architect at Ames, said the engineer’s house was of considerable historic value because it was designed by architect Wilfred E. Mansur, in 1892. Arguably the pre-eminent local architect of his time, Mansur designed dozens of buildings in Bangor but the house is unique among his portfolio for its gambrel-roofed, wooden-shingle styling.

Besides its historic value, Kelly says, the building will be an excellent demonstration of the companies’ abilities when it’s finished, hopefully this winter. The model will be pricey: Angel estimated that the restoration will cost $350,000 to $450,000, unless Kelly’s inspection reveals problems worse than those already known.

Gloomy wisecracks aside — “This place would make a good basis for a Stephen King novel,” says David Kelly, Bob’s son — Kelly and Kelleher are quite pleased with the good shape that they say the engineer’s house is in.

They estimate that about 75 percent of it is salvageable.

In historic restoration terms, that means that workers from House Revivers and Kelleher’s company, J.P. Kelleher Building and Remodeling, will remove about 75 percent of the building’s features, mark them so they know where they go, and restore them elsewhere before putting them back into the house.

The restoration work, they say, will include sanding and repainting doors, moulding and other individual features taken from the dwelling. Parts that crack or break will be glued together or replaced. Hardwood flooring will replace the warped floor, though the upstairs floor looks salvageable, says Rick Martin, one of Kelleher’s restorers. New electrical wiring, lighting and plumbing also are needed and many if not all windows will be rebuilt.

It’s a lot of work but not as bad as it could be, Kelleher says.

“It’s going to be a fun job,” Kelleher says. “The woodworking is all in the original layout and they haven’t added layers and layers of carpet” or other modern clutter that would have to be peeled away and junked before they can begin the restoration.

Kelly is particularly fascinated with the horizontal 1¼-inch laths that start directly above the wainscoting and continue up the walls and cover the ceiling. Somebody, he says, did a great job of removing the plaster from the walls and ceilings that the laths supported and keeping the laths intact. Rarer still, the laths are in good shape — few of them are cracked or rotted despite the place having had a leaky roof for at least a decade.

“It’s strange to see all the plaster removed but the wood intact like this,” Kelly says. “Very unusual.”

But the laths, Kelly says, will be removed to make way for drywall. Plaster, like the house was built with, is simply too expensive today. And even if today’s plaster were affordable, it too closely resembles drywall to make its use worthwhile.

Kelly says the house, which is 2,400 square feet, not counting its basement and attic, has several features unique to its time. It’s stairways feel narrower than those in modern buildings, and double doors center the wall between what was likely its living and dining rooms. It has a 3-foot-by-5-foot horizontal accent window only a few feet above the floor of the stairway landing between the first and second floors. Its placement puzzles Kelly.

Despite their age, the shingles that dominate the house’s exterior will be salvaged. Many are in good condition, but not for a reason anybody would support today. Their ruddy red paint is lead-based and, therefore, poisonous, Kelleher says.

“That’s why houses rot so much faster today,” he says. “There’s no lead in the paint anymore.”

“It’s like asbestos. It did its job well,” Martin says, “but it killed people.”

The house is also in a unique location. Farther down the Penobscot River, thick hunks of snow-covered ice pile irregularly, but directly behind the house, water whitecaps as it flows over rocks. Traffic on State Street, in front of the house, flows a lot slower.

The house’s previous owner, Shaw House, did the prospective buyers a huge favor when it put the building atop a new foundation about 15 feet from the old one a dozen years ago, Kelly said.

“That’s often a weak area in old houses. This gives us a real leg up, and it was well done with the brick facing, which was probably what the original had,” Kelly said.

The new roof Shaw put on isn’t much of a blessing. The firms plan to replace it with architectural shingles more aesthetically pleasing. They also will likely need the city Historic Preservation Commission’s permission to restore the chimney through the center of the roof, Kelly said.

The house’s ruggedness, might come from its possibly being built by old-growth pine, Kelleher says. Kelly and Kelleher say they look forward to starting renovations and seeing what secrets the house will yield.

“We love old houses like this,” Kelly said.