The dilapidated Clemens Mansion just north of downtown looks like a horror movie set…. Nightmare on Cass Street. With its faded antebellum columns, collapsing porches and lean-to construction, it seems likely to fall over (or fall in) at any minute.

It’s hard to believe that this decrepit old house is set to become a centerpiece — a “legacy” property — of the $8.1 billion north side redevelopment plan proposed by developer Paul McKee.

Central West End developer and rehabber Bob Wood is partnering with McKee on the project, which will turn the 150-year-old mansion and adjoining chapel into senior apartments and community center.

“We’re talking to the Missouri History Museum housing a museum honoring the heritage of the surrounding neighborhood,” said Dan Holak, who is heading up the project for Wood. “They would occupy part of the chapel building and we’re also talking to the Missouri Botanical Garden about doing some landscaping and putting in an urban garden.”

For years, McKee secretly bought more than 850 properties on the city’s near north side in a two-square-mile area. He announced last year the redevelopment plan and is negotiating nearly $400 million in tax increment financing with the city of St. Louis. He said he plans to turn the mostly vacant properties into 10,000 new homes and millions of square feet of office, warehouse and retail space to the area over 15 years. At the end of last year, the Missouri Department of Economic Development awarded McKee $20 million in tax credits for the project. McKee bought the Clemens mansion in 2005.

Wood is also hoping to finance the $13 million Clemens project through historic and low income tax credits. A model for the development is Wood’s work on the Franklin School at 814 N. 19th Street. A former St. Louis public school, designed by famed school architect William Ittner and opened in 1911, Franklin was closed in 1995. Wood renovated it into 75 senior apartments and reopened it in the growing neighborhood now known as Downtown West in 2007.

“The condition of Franklin School when we started there was pretty nasty,” Holak said. “With funding, we’re confident we can make the Clemens mansion work.”

I met with Holak and architect David Lorentz at the Clemens mansion on a cold December afternoon.

At first, I was content to just walk around the outside of the property, which includes the 55,000-square-foot mansion, an addition on the back of the house, and an adjoining 3,000 square foot chapel building. Wood’s plan, Lorentz said, is to convert the main house into 49 senior apartments. The first floor of the chapel would be residential, and the second floor would be the public museum space.

Holak and Lorentz then pulled out their flashlights and invited me inside the mansion. We stopped first on the front porch and Holak pointed out the distinctive columns. With everything else falling down around, how do they stay up?

“They’re cast iron,” Holak said, and sure enough, I knocked on the two-story high columns and they “clanged” like an old kettle. Maybe this house, built in 1860 and listed as one of the city’s most “endangered” landmarks, might hold up after all.

But then we went inside.

I’ve been inside a lot of old and deteriorated properties, but this one was, well, the creepiest I’d ever seen. We walked a bit down the main hallway and glanced inside dark rooms filled with trash, crumbling walls and exposed wires. I’m not a haunted-house guy, but even this one seemed ripe for a ghost or two as a cold wind rattled through the old walls.

Holak and Lorentz were unfazed. They calmly pointed out the distinctive hardwood floors — “we can save that” — and some of the mouldings and architectural detail that were dirtied but salvageable. Lorentz flashed his light on an elevator shaft that he said would work. Holak said that conditions at Franklin School were just as bad, or worse, and talked about the rehab like any other home improvement project. He added that if financing gets approved, his company would start construction this spring or summer and it would be completed the summer of 2011. As he talked, I could almost imagine a warm and bright historic home filled with “active” seniors. Almost.

Ten minutes was about all I could handle in that place. After we shook hands, I walked to my car and glanced back at the building. I thought about all the people who had lived there — from the Clemens family to Roman Catholic nuns to Buddhist monks. But did Mark Twain? I don’t know, but it would have made a good story.

(Part II: History of the mansion’s inhabitants and the Mark Twain connection.)