That big drop in mortgage rates during the past four months is on the verge of having measurable effects on the most important asset of millions of American families: the value of their home.

It's still too early to break out the champagne, according to top real estate appraisal experts around the country, but the outlook for housing resale values has improved significantly, thanks to lower interest rates. For the first time in nearly two years, appraisers say, homeowners can look forward to net gains in the capital values of their real estate above the rate of inflation.

Those gains won't be uniform nationwide, appraisers caution. Nor will they hark back to the zesty double-digit jumps of the real estate go-go years of the late 1970s. But in economically sound areas of the country -- the Sun Belt cities, much of the West and big metropolitan markets such as Chicago, Boston, Washington and Philadelphia -- home values could rise 5 to 10 percent next year if rates stay on their present course.

"I don't want to overstate the case," said Dick Nichols, head of Richard E. Nichols Associates, a major appraisal firm based in Indianapolis. "But the fact is that we're already experiencing small net gains here. Sellers this fall are able to walk away from the table with more dollars in their pockets than they would have a year ago."

Prices are up only fractionally in the Indianapolis housing market over November 1981 levels, according to Nichols, but sellers no longer have to subsidize their buyers' financing to the degree they did during much of the past 18 months. There's less need for below-market, seller-assisted, deferred loans and less need to discount prices sharply to produce qualified buyers.

Using the "cash equivalency" analysis that most sophisticated real estate appraisers now employ to determine home values, Nichols said "there is no question the market has moved up slightly" since August.

With 12 percent FHA/VA rates and 13 percent conventional mortgage rates pulling consumers back into the marketplace, "I'd say the outlook on values is better today than it has been since 1979." If rates decline into the 11 or 12 percent range and stay there (or go lower) for an extended period, values could move up impressively in some areas, according to Nichols' projections.

In the Washington metropolitan area, appraisers confirm Nichols' view of where real estate values are likely to be headed. Alfred Jarchow, who surveys housing price movements in the Montgomery suburbs, says this fall's rate drops should produce an increase in real values "very shortly." The initial increases "won't be beyond the single digit rate range" on an annualized basis, according to Jarchow, "but any increase at all should be seen as very good news."

The cash equivalency approach now used by most appraisers factors financing conditions into the true market value of a home. When money costs are prohibitive and sellers are forced to provide cut-rate concessionary financing to consumers, the cash value of that financing has to be subtracted from the selling price of the house to arrive at its true value.

A $100,000 contract price, in other words, may not represent the true cash value of the property. If the seller had to provide a $50,000 second mortgage for 15 years at a low rate, the real price may be $90,000 or less, depending on the interest-rate factors involved.

Cash-equivalent values and creative-financing costs have been bad news for most American homeowners since 1980. The reverse could be true in the months ahead.

"I think you're going to see a lot less creative financing by individual sellers and builders--and pricing that is much easier to understand," said Dr. Richard Hewitt III, chief district appraiser for the Federal Home Loan Bank Board in Fort Lauderdale, Fla.

"There's almost no incentive to buy down subsidize rates once they get to a level where the majority of consumers can afford them anyway," Hewitt said. "I think we're nearly there."

As a result, cash-equivalent values should be much closer to actual selling prices in 1983--provided rates remain moderate and consumers bargain hard on prices.

That final point merits the special attention of anyone who's getting ready to plunge into the market. Asking prices are going to be heading up, but some of them will be padded to cover financing concessions that no longer exist.

Take the case of a town house builder who late last spring offered a model for $90,000 with 12 percent fixed-rate mortgages arranged with a local bank. That special loan money would have cost him several thousand dollars per unit back then, when conventional rates were still in the 16 1/2 percent range. The builder tacked that several thousand dollars onto the selling price of the unit, just like any other expense.

If his price is still the same and he's now offering 12 percent FHA/VA, there's little or no special financing cost to him. At $90,000, the unit's price has unnecessary fat on it.

A sharp buyer ought to negotiate the fat away before signing on the dotted line.