Lisa Gilman doesn't normally discuss her weekend television-watching habits with her boss. But on a recent Monday she was compelled to tell him about "My House Is Worth What?," a show on HGTV in which real-estate agents tour a home and tell its owners how much they could get for it. In the episode Gilman saw, a couple was ecstatic when an agent told them their house was worth … drumroll please … $1.5 million. "These people were jumping up and down," says Gilman, who was dumbfounded by the spectacle. "It floored me that they were so happy. No one had offered them $1.5 million."

For Gilman and her boss, Richard Goulet, talking about the cost of real estate is more than just an idle pastime. Goulet is president of the Appraisers Group in the Boston suburb of Belmont, Mass., and Gilman is a vice president and one of his top residential appraisers.

They work in a profession that brought no end of joy to homeowners during the real-estate boom. During those years a visit from a real-estate appraiser felt like a visit from the tooth fairy. Times have changed, of course. I'd guess my house in Massachusetts has dropped in value by 15 percent over the last two years. Today, when I plug my address into Zillow.com, a home valuation Web site, seeing the number that pops up makes me feel less as if I've been visited by the tooth fairy and more as if I'm about to undergo a dental procedure.

To get a sense of how professional appraisers are calculating these declining values, I arranged a ride-along with Gilman as she figured the toll the housing bust is taking on a five-bedroom Victorian whose value soared during the boom. We hop into her Toyota and drive to the top of a steep driveway in nearby Arlington, Mass. Weeds grow out of broken concrete alongside the decrepit swimming pool. On the porch a decapitated doll head sits amid a pile of debris. The house is empty, foreclosed on by a mortgage company earlier this year.

Outside, I feel somewhat as though I'm caught in a "CSI" episode, as Gilman has me hold the end of her tape measure while she outlines the foundation, confirming the house is 4,308 square feet. Inside she snaps digital photos, sketches the floor plan and reconstructs the story of this house, and what went wrong for its owners.

Back in 1997, the house sold for $370,000. By 2002 it sold again for $715,000, and property records show that the new owners replaced the home's siding, windows and flooring. To pay for all those renovations, Gilman guesses the new owners borrowed heavily.