Philadelphia was largely spared severe damage from the thunderstorms that hit the region Sunday. Or so it seemed.

Yesterday, word spread that the city had suffered a major loss.

The historic Garrett-Dunn House, a summer cottage in Mount Airy built in Greek-revival style by the 19th-century architect Thomas U. Walter, was struck by lightning Sunday morning and destroyed. The building was vacant.

Yesterday, a black skeleton remained of the two-story structure, with two chimneys towering over the ruins and only the arched front door intact. Scattered over the front yard were burnt timbers and bricks.

James Gwaltney, 45, who lives next door, said he was looking out his window on Sunday morning, watching the rain, when "I saw the lightning strike the mansion - and heard it; it was very loud."

Gwaltney, an automation engineer, said he watched for about five minutes to see if the building had caught fire. "Then I went to the kitchen, and when I came back, it was in flames."

Jordan Dillard, 34, another witness, described chunks of ash falling two blocks away. "You could feel the heat from the flames standing across the street," Dillard said.

Neighbors said two women had lived in the home for decades but moved out when the property was purchased by a developer.

"It's a horrible tragedy," Elise Vider, deputy director of the Preservation Alliance of Greater Philadelphia, said yesterday. "Garrett-Dunn House was a landmark for the neighborhood and the city, really one of those places that make Philadelphia so special."

Walter, who was born in Philadelphia, built Founders Hall at Girard College and the Biddle Estate in Andalusia. He also designed the dome of the Capitol and contributed to the reconstruction of the Library of Congress. "But there's little left of his residential work," Vider explained.

The Garrett-Dunn House, 7048 Germantown Ave., was a summer home for George Howell Garrett, a farmer who had made a fortune in the tobacco trade.

According to Vider, the historical importance of the property was only discovered in recent years. Community activists and several organizations, the Preservation Alliance among them, had joined in efforts to preserve the fragile building.

They succeeded in getting it listed on both the national and the Philadelphia Registers of Historic Places.

Nevertheless, the structure was in bad shape. In October, the city filed a complaint against owner John Capoferri - a developer who planned to turn the property into condominiums - for failure to protect the historic site.

According to the Preservation Alliance, the walls were deteriorated, and one had collapsed. A judge ordered the mansion to be secured and stabilized. As a result, the doors and windows were sealed.

"It's pretty ironic, a sad twist of fate," Vider said. "We had managed to secure the house against the elements, we were extremely pleased with the way it survived the winter. And now this."

Yesterday, preservation architect Justin Detweiler walked past the Garrett-Dunn House and lamented its fate.

"What a shame, what a loss," he kept repeating. "I have a feeling that it will cost too much, but I would love to see this place rebuilt."

Contact staff writer Roman Deininger at 215-854-5015 or rdeininger@phillynews.com.