The ability to get a mortgage is based largely on income, creditworthiness and the amount of the down payment. But there's another element, over which the buyer has little control: the appraised value of the house.

While appraisals are generally intended to reflect the market value of a house, an overheated real estate market can sometimes lead buyers to offer more for a house than it would be worth in less frenetic times. And that, real estate specialists say, can result in the lender declining to make the loan or offering less of a mortgage than the buyer needs.

"At the end of the day, no lender wants to lend $100,000 on a property that's worth only $75,000," said Steve O'Connor, a senior director of the Mortgage Bankers Association of America, a trade organization based in Washington. "That's why lenders insist on appraisals." Banks will not give a loan for more than the appraised value, minus the down payment, but buyers are free, of course, to pay more out of their own funds.

Mr. O'Connor said that every time a lender makes a loan backed by a pledge of property as security, the lender needs some assurance that the value of the property is sufficient to cover the unpaid balance of the loan. And while a buyer's down payment provides the lender with a cushion of sorts if the property has to be sold at foreclosure, that cushion can quickly evaporate if the buyer overpaid.

"The market, of course, can fluctuate," Mr. O'Connor said, explaining that no appraisal ever comes with a 100 percent guarantee. "And the down payment does help mitigate against the risk of default. But an appraisal assumes a rational buyer."

But even rational people occasionally do irrational things. "The proposed purchase price of a house doesn't necessarily establish the property's true market value," said Wayne Wright, the owner of GW Real Property Analysts, an appraisal company in Wallingford, Conn. "Right now, there's a serious shortage of inventory in most parts of Connecticut. That means that when people really want to buy, the owners start getting multiple offers and the properties start selling for more than they might be worth when things cool down."

Mr. Wright said that a house can sell for more than its true market value for other reasons. For example, he said, a buyer who insists on a house that is in walking distance of a train station would probably be willing to pay more than one who drives to work.

Most lenders insist on getting evidence that the house is worth what the buyer is willing to pay. To do that, the lender orders an appraisal, usually at cost of about $300 to the buyer.

"Generally speaking, we're here to protect the lender's interests," said Jack Geoghan, the owner of Inter-County Appraisers in Bayport, N.Y. Mr. Geoghan said that several tools are used to conduct an appraisal, depending on the property. In an area with similar houses — or condominiums — such as a large tract development, an appraiser can get a good idea of a particular house's worth by examining recent and pending sales.

"In some developments, the houses are so uniform that you have a variation of as little as 2 percent in pricing," he said, adding that in such cases, it might not be necessary for the appraiser to inspect the house. "A drive- by of the property might be all the lender needs."

But in areas with many styles of houses of various ages and condition, he said, it is almost always necessary to inspect the property. "In areas like that, our job is basically to compare apples to oranges," he said.

Among the things the appraiser will want to know, he said, are the location, size and general condition of the property and the age, condition and square footage of the house. Then, he said, the appraiser can take the next step: identifying comparables.

"Most appraisals don't leave this office without at least five or six comparables," Mr. Geoghan said.

John Cirincione, the national chief appraiser of Mortgage Information Services, a provider of title closing and appraisal services based in Cleveland, said that comparables are basically sales of similar houses in the general area of the house being appraised. That information can be obtained from several sources, he said, including county courthouse records and private data vendors who compile the information and sell it to appraisal companies.

An appraiser might also gather recent sale price information, along with data on pending sales, from the local Multiple Listing Service, from area brokers, from other appraisers and even from interviews with property owners.

"The quality and reliability of each piece of information is then weighed by the individual appraiser," he said. The appraiser then looks for "the most similar, most recent sale of a property that would serve as a reasonable substitute for the property being appraised," he said.

Once one or more comparables are found, Mr. Cirincione said, they are compared to the property being appraised, and adjustments in value are made, up or down, where appropriate.

"I might have what looks like a perfect comparable," he said. "But then I go out and look at the house I'm appraising and find out that it backs up to a highway, or maybe it backs up to a real neat lake."

Either way, Mr. Cirincione said, once the appraiser has compiled the information and tabulated the numbers, he should be able to come up with a figure that reasonably reflects the property's market value.

"It might sell for more and it might sell for less," he said. "But appraising is as much an art as it is a science."

Jeffrey Jackson, chairman of Mitchell, Maxwell & Jackson, a Manhattan appraisal company, said that appraising is more challenging when the property is a co-op apartment.

Since the sale of a co-op involves the sale of securities — shares in the co-op — rather than real estate, there is no recorded deed reflecting the purchase price. Moreover, he said, in places without an official Multiple Listing Service — Manhattan being the most prominent, but not the only, example — there is no central repository of recent and pending sales.

And finally, Mr. Jackson said, since buying a co-op apartment basically amounts to buying part of an ongoing business, the financial health of the co-op corporation itself must also be considered.

So, he said, co-op appraisers must rely on a network of brokers, managing agents, lawyers and other appraisers to construct their databases. "Every time we do an appraisal," Mr. Jackson said, "we have one more chunk of data for our database."