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When Dr. Mark Shepherd Thomas and his partner bought their home on St. Paul's West Side, they were initially charmed by the century-old trees in the yard and its location near the top of the bluffs.

The couple moved into the home and unearthed some tantalizing clues that this two-story house, then sheathed in drab gray 1950s asbestos shingles, may be connected to one of Minnesota's most famous sons.

"Living in the house and looking at the house from the outside, it was clear to me there was some intelligent design going on," Thomas said.

Thomas spent nearly three decades proving that his home at 402 Hall Av. was an early work of Cass Gilbert, the celebrated architect who later designed the Minnesota State Capitol, the U.S. Supreme Court building and one of the nation's most famous skyscrapers, the Woolworth Building in New York City.

The restored Dwight Henderson and Clara Watson House and Barn, now a striking yellow color with green trim, was listed on the National Register of Historic Places in 2019. The final designation as a St. Paul Heritage Preservation site is pending and could go before the City Council this summer.

The home is an "early, inventive example of Gilbert's residential designs, and a 'wood' prototype of the houses he designed for wealthy clients on Saint Paul's Summit Avenue," according to its application filed with the city. Gilbert went on to design several of the posh residences on historic Summit Avenue.

Christine Boulware, historic preservation specialist with city of St. Paul, lauded Thomas' three-decade odyssey to tell the story of his beloved home.

"Dr. Thomas has an extraordinary commitment to his home, the West Side and preserving its history. This is truly a passion," said Boulware, an architectural historian. "In this case, he didn't even know what he had bought when they purchased it. They loved the house. It was like an onion. They kept peeling back the layers."

Thomas said he wasn't familiar with Cass Gilbert when he and late partner John Neess moved into the home in 1993. The family who had sold them the home did share a few clues, including that the home was originally built in the 1880s for the Watson family.

"That's all I had," Thomas said. "I wasn't really tuned into architectural history at all. But then I went to the Historical Society and put in the name, and I found the patriarch of the family had been a member of the state Constitutional Convention in 1857. I found a picture of him. There was this dashing man. This flipped my switches for some reason. I started doing a lot of research."

Thomas also became more curious about the home commissioned by the Watsons. He spoke to some experts in architectural history and restoration about the property.

In 1997, Thomas R. Blanck, an architect specializing in historical architecture and restoration, knocked on Thomas' door and told the couple he believed their house was a Cass Gilbert design and that documents in New York could potentially prove it.

Thomas flew east to page through Gilbert's papers preserved at the New-York Historical Society. Stashed among them were the architectural plans for 402 Hall Av. The plans were drawn on linen fabric.

"All the details were there," Thomas said. "It was a bonanza."

Thomas said he has spent the ensuing years restoring the home, researching its history and seeking preservation status. He stripped off the home's asbestos siding to reveal and restore the original architectural details underneath.

Gilbert was about 26 years old when he designed the home, Thomas said. He was starting his career and had recently designed a home in St. Paul for his own mother.

"The West Side was just being opened up in 1885. The storm sewers were put in, and all of a sudden there was all this land that was available for development," Thomas said. "The Watsons' lot was basically on the way up the hill. In some ways, I think he used it as advertisement of what he was capable of."

In addition to homes, Gilbert went on to design buildings in downtown St. Paul. The design of the Minnesota State Capitol in the late 1890s solidified his national reputation, and he moved to New York a few years later.

Thomas said he continues to research the Watson family and plans to write a book. He said he's pleased these historic designations will protect the home for future generations,

"This is such a wonderful interlacing of history, ecology, geography and political history," Thomas said.

There are about 100 buildings designed by Gilbert in the Twin Cities, said architectural historian Marjorie Pearson, who is vice president of the nonprofit Cass Gilbert Society. Pearson said she's pleased Thomas has saved another piece of Gilbert's legacy.

"I think it's really commendable to recognize and preserve what was an early example of Cass Gilbert's domestic architecture and bring it back to life in a way we can all appreciate," she said.