ATLANTA, Georgia (CNN) -- One day while driving with her father, Hannah Salwen noticed a Mercedes stopped next to a homeless man sitting on the curb.
The Salwen family of Atlanta, Georgia, has decided to sell this house and give half the proceeds to charity.
"I said to my dad, 'If that guy didn't have such a nice car, then that guy could have a nice meal,' " the 15-year-old from Atlanta, Georgia, recalled.
And so began the tale of what the Salwen family calls "Hannah's Lunchbox." Watch why family wants to give away $800K »
It started as family discussions about what they needed versus what was enough. Hannah's father Kevin, an entrepreneur, is on the board of the Atlanta Habitat for Humanity and is no stranger to community work, but he said this family conversation was life-changing.
"We stopped and paused and thought about what are the things in the world that could really make a difference, a little bit of difference in the world," he said.
They talked about selling their cars or other things, but it was Hannah's mother, Joan, who came up with selling their 6,500-square-foot house, donating half the proceeds and then moving into a house half the size.
For nine years, the family lived in a historic 1912 mansion near downtown Atlanta. It boasts five bedrooms, eight fireplaces, a kitchen that would make any cook jealous and even an elevator. See the new and old houses, side by side »
When Hannah would bring friends over, she said, often their jaws would drop and they'd gasp, "Wow, you live here?"
Like most teens, Hannah loves to shop, and she jammed every space of her massive walk-in closet full of clothes. But she also knows many people are less fortunate; she volunteers at a local community food bank and other relief agencies.
Joan Salwen, a teacher, said the mansion was her dream home.
"It was a challenge," she said of giving up that house. "It was a test, almost, to see: How committed are we? I mean, how serious are these kids about what we should do? And they all nodded and there we were."
So the Salwens put the house up for sale in May 2007 and started figuring out what they would do with half the proceeds, which would amount to more than $800,000.
They spent six months researching charity organizations before deciding on the Hunger Project, an organization dedicated to helping end world hunger through people helping themselves.
Hunger Project Vice President John Coonrod said the family met with organizers in New York and notified them months later that the charity was the winner.
When the Salwen house sells, the money will be channeled through The Community Foundation for Greater Atlanta over a six-year period and end up in Ghana, Coonrod said.
"This will underwrite a process in more than 30 villages to enable people to meet all of their basic needs on a sustainable basis," he said. "They will be able to grow enough food, to build clinics and schools, and the villagers will be doing the lion's share of the work."
Coonrod said he'd never heard of a family donating in this way.
"Hannah's awakening to social injustice, and her family's ability to make a difference in that issue will make a profound difference in the lives of tens of thousands of people," he said, estimating the money could affect more than 20,000 people in Ghana.
Hannah's 13-year-old brother, Joseph, was so impressed with his big sister's ideas that he made a three-minute video of the family's project. Watch Joseph's video
The video won the grand prize in the 2008 "My Home: The American Dream" contest, sponsored by Coldwell Banker and Scholastic Publishing.
In the video, Joseph tells viewers, "We're showing you can redefine the American dream."
But the Salwens' house has sat on the market for more than a year. It's a tough time to sell any house, let alone one with an asking price of nearly $1.8 million.
Real estate agent Sally George said she's shown the house 40 or 50 times, and there have been nibbles but no buyers. See the house's real estate listing
Many people are interested in the house's rich history but often don't know anything about the philanthropic aspects of the family's project.
"I've never handled a house selling for this reason," George said. "I didn't learn about what the family was doing until early this year."
Hannah and Joseph said most of their friends at school don't know about it.
"We didn't do it for the fame or the glory," said Joan Salwen. "This was something Hannah sort of yanked us into."
Even though it was Joan Salwen's idea to sell the house, it has been tough for her to give it up. "I have to admit," she said, "I loved living in this house. Does that make me an evil person? I hope not because it's a beautiful place."
The family recently moved to a house less than half the size of their mansion four blocks away. While Hannah's friends called her old home the "wow house," this one is more ordinary and that's fine with her.
Lately the family has spent a lot of time around the kitchen table talking about an upcoming two-week trip to Ghana. The Salwens will spend six or seven hours a day visiting the villages where their money will be put to work.
Kevin Salwen said the new house is great, it's just smaller.
"We as Americans have so much," said Salwen, a former Wall Street Journal writer. "We love the concept of half. We are going from a house that's 6,000 square feet to a house that's half the size, and we're giving away half the money.
"And we do think everyone can do something if they think through half."