Potentially half of the homes sold in the United States and virtually all second mortgage and home equity line transactions will escape stiff new federal appraisal requirements that go into effect in August.

In an unexpected move, four of the five federal agencies that regulate the nation's thrifts, banks and credit unions have agreed to exempt residential transactions of less than $100,000 from the scrutiny of a licensed or certified appraiser, according to a recent Federal Reserve Board memo, although lenders on their own still may require appraisals.

All five of the regulators, according to the memo, also have decided to allow licensed appraisers, as opposed to more-qualified certified appraisers, to perform appraisals on uncomplicated residential transactions in the $100,000 to $1 million price range.

Critics claim the decisions impede efforts to clean up the appraisal industry. Supporters, including the National Association of Realtors, say the rules will head off an anticipated appraiser shortage caused by the reforms and hold down the cost of appraisals to consumers.

The financial regulators are charged with implementing a federal law designed to curb fraudulent and incompetent appraisal practices. The law, among other things, defines acceptable appraisal practices, sets competency levels for appraisers and places appraisers under government supervision.

The $100,000 cutoff and narrower certified appraiser role "undermine the essential purpose" of the law, said Rep. Doug Barnard Jr. (D-Ga.), the author of the federal appraisal reform legislation passed in August.

"What astounds us is that no one has learned anything from the thrift crisis," said Jeffrey Tassy, counsel to the House Government Operations subcommittee on commerce, consumer and monetary affairs chaired by Barnard.

The subcommittee estimates that 10 percent of the total S&L losses, which likely will reach $300 billion over 10 years, stem from residential appraisal problems.

So far the Fed is the only one of the regulators to issue the final version of its rules, although the other regulators are expected to release very similar rules within the next few weeks.

The only holdout, the Office of Thrift Supervision, which oversees the S&L industry, is under pressure to abandon a more conservative exemption cutoff of $15,000 and adopt the $100,000 figure to present a united regulatory front, said Mary C. Short, an agency official.

Barnard and the appraisal industry now are trying desperately to persuade the regulators to change their minds.

Ritch LeGrand, president of the Society of Real Estate Appraisers, said that exempting houses less than $100,000 from appraisals and liberalizing use of licensed appraisers "rolls back all the principles of appraisal reform we have fought so hard for."

Passage of the appraisal reform law spawned a massive push to recast the way appraisals are performed in this country.

It culminates a year from now when each of the 50 states and the District of Columbia are expected to begin licensing and certifying appraisers. To prepare for that eventuality, the states must adopt their own appraisal laws and regulations, prepare appraiser courses, formulate testing procedures and set disciplinary standards.

Until now, the states and appraisers have proceeded without benefit of the regulators' final interpretation of the appraisal standard regulations, which go into effect Aug. 9.

The $100,000 and under exemption comes as a particular surprise because the draft proposal circulated by the regulators in February proposed a $15,000 cutoff, said Peter Barash, a consultant for the American Society of Appraisers.

Barash said: "If you're going to throw out {requiring certified appraisals} for $100,000 and below, then why all the fuss" to set up the elaborate certification and licensing process?

The $100,000 cutoff is less likely to have an impact on the Washington market than other parts of the country. In 1989, with a local median sales price of $144,400, 17 percent of existing home sales here fell to less than $100,000, according to National Association of Realtors data. By comparison, the U.S. median sales price last year was $93,100 and 54 percent of homes nationally sold for less than $100,000.

At worst, the cutoff will lead lenders to conclude appraisals no longer are necessary on properties worth less than $100,000, Barash said. The appraisal reform forces also worry that the door is once again open to moonlighting real estate agents and others untrained in the business to present themselves as qualified appraisers.

A Fed official, who requested anonymity, denied that the exemption means lenders will forgo appraisals altogether. "It is pretty unlikely that a transaction of $100,000 would be one that did not require some sort of collateral evaluation," the official said.

Citicorp, one of the country's largest mortgage lenders, confirmed through spokeswoman Betsy Martin that it plans to continue requiring a valuation performed by an appraiser credentialed with one of the professional appraisal societies. Next summer, the company will restrict its work to state-licensed or state-certified appraisers when that part of the reform law takes effect, "regardless of the purchase price or loan amount," Martin added.

Moreover, officials at the two major secondary mortgage market companies that buy home loans from lenders said that they do not intend to drop their requirements for professionally performed appraisals. Between them, the Federal National Mortgage Association and the Federal Home Loan Mortgage Corp. influence the lending policies behind about two-thirds of mortgages made today.

The American Bankers Association, which lobbied for the higher $100,000 cutoff, is pleased with the change, said spokeswoman Tara Little. Many commercial banks prefer to use their own staff to perform appraisals, she said.

Home equity and second mortgage lenders are less likely to embrace full-blown appraisals now that it is not a requirement, said Fritz Elmendorf, a spokesman for the Consumer Bankers Association. Many lenders, he said, rely on a visual inspection of the outside of a property or the assessment reflected in tax records to set a property's value.

Much of the business done by the association's members will fall within the exemption, Elmendorf said. In 1989 the average home equity credit line approved was $33,305 while the average second mortgage amounted to $24,463.

The nod given licensed appraisers to perform residential appraisals from $100,000 to $1 million also caught appraisal reform proponents off-guard. The only time a certified appraiser is required is if a residential transaction exceeds $250,000 and is considered "complex" or a commercial real estate deal exceeds $250,000.

"Congress strongly favors the use of certified appraisers in most transactions, but what {the regulators} have done is a 180 degrees from that," Barash said.