One of the toughest ideas to get through anyone's head today is that the price of the lovely house they're living in may be going down.

The lawn is just as green as it always was. You've kept your home shingled and painted, and the neighborhood is just as nice. Nevertheless, the market value of your home may be falling -- not for anything you did, but because of a general deflation that has been gripping real estate throughout the country.

In six major metropolitan areas, the average resale price of houses dropped over the 12 months ending in March, according to a survey by the National Association of Realtors. The biggest declines came in Milwaukee and Birmingham, both down 4.6 percent. The other unfortunates: Tampa, Fla., down 4.5 percent; Columbus, Ohio, down 2 percent; Orange County, Calif., down 1 percent, and Salt Lake City, down 0.2 percent.

Over other periods, the picture looks much worse. From the third to the fourth quarter of last year, for example, resale home prices plunged in 31 major cities and their suburbs. A year and a half ago in Houston, prices slid a breathtaking 15 percent in just nine months, and still haven't picked themselves up off the floor.

Even if house prices still are rising in your area, their values may be falling behind in "real" terms. In the past year alone, the average increase in the price of an existing home fell below inflation in 13 major metropolitan areas, according to the NAR. Adjusted for inflation, the national median house price is lower today than it was in 1978.

In some individual cases, the losses on homes have been surprisingly large.

A lawyer in a Milwaukee suburb listed his house for sale at $180,000. The tax assessor's office had appraised it the year before at $147,000. It finally sold for $129,000. Before he could close the sale, the lawyer had to get the tax appraisal lowered to cut the buyer's real estate taxes to the proper level.

A stockbroker in Los Angeles tells me he can't get a nibble on his classy condominium at $249,000, although one just like it, on a lower floor, sold for $350,000 a little more than a year ago.

Some people get stuck because they overpay for their houses without realizing it. This often happens when you get a cheap, "buy-down" mortgage through a builder.

With a buy-down, the builder offers you a loan with a very low rate of interest for the first few years. But your cheap loan isn't free; the builder has to pay the lender a lump sum to cover the difference between what you're paying and the market rate. To recoup his costs, the builder adds part or all of that lump sum to the price you pay for the house.

Result: On a $100,000 house with buy-down financing, the house itself may be worth only $90,000. The extra $10,000 represents the value of the cheap loan.

If you put $10,000 down on this deal, you effectively have no equity in the house. Your "down payment" simply reimbursed the builder for arranging the buy-down. If you had to turn around and resell, the house would fetch only $90,000 -- leaving you a built-in, $10,000 loss.

The same thing can happen if you buy from an individual seller who takes back a low-rate mortgage. You may be overpaying for the house, in return for a cheap loan.

In any kind of buy-down, you should be able to learn the true cash value of the house itself by asking the mortgage lender to show you the appraisal. Under standards just established by the American Institute of Real Estate Appraisers, lenders are supposed to get two valuations: one for the house alone and one for the added value of the low-rate financing package. (Another strategy is to ask the seller how much less he'd charge if you paid cash.)

Naturally, there are still some big winners in real estate. Over the past year, average resale prices for homes rose a glorious 24.7 percent in the greater New York City area and 21.5 percent in Boston.

But will it last? In the New York-Long Island and Boston areas, building permits for new single-family homes and condominiums are running at record rates, according to the mortgage banker, Lomas & Nettleton. Sooner or later, the supply of homes in the headiest markets probably will exceed the economic demand -- at which point, these cities easily could catch the Houston disease.