U.S. corporations, nervous about possible changes in tax laws, plan to reduce real spending on modernization and expansion by 1 percent next year, the sharpest drop in capital spending plans since the 1982 recession, the Commerce Department reported yesterday.

The announcement was another indication that congressional consideration of tax revision, capped this week by passage of a bill in the House, is crimping spending and investment plans.

While uncertainty about the tax bill is causing some consumers and companies to save money they had planned to spend, it's also prompting others to spend money they might have saved, depending on whether they would be better or worse off under the revamped tax code should it become law. Either way, the total effect of their decisions is expected to distort economic activity, economists said.

"Uncertainty over the tax provisions or any kind of uncertainty is no kind of beneficial treatment for capital spending," said Commerce Department chief economist Robert Ortner. Increased capital spending is important for both short- and long-term economic growth, Ortner said.

The House tax plan would eliminate or curtail some of the most widely used business tax writeoffs. It would eliminate the investment tax credit, which subsidizes up to 10 percent of the cost of spending on new equipment, and lengthen the time periods over which investments in real estate and equipment can be written off, or depreciated. The effect of a longer write-off period is to reduce the deduction in any one year.

Those and other limitations in the bill in the area of non-mortgage interest payments, tax shelters and tax-exempt bonds also would affect many types of investment by individuals.

Many of the changes would be effective as of Jan. 1, a disconcerting consideration for investors going into the new year without knowing whether the Senate will pass a bill or, if it does, how much it will resemble the House measure.

"The business community has gone through a year of uncertainty [on tax overhaul]. We can't make them go through another year of uncertainty," said Sen. Robert W. Kasten Jr. (R-Wisc.). "We gotta tell people, 'Go ahead and buy that new refrigerator or that new assembly plant.' "

Both houses of Congress have passed nonbinding resolutions calling for postponing implementation of portions of the bill until Jan. 1, 1987. But the resolutions not only do not have the force of law, they are not even the same, hardly a guarantee to worried investors.

The Senate resolution, passed yesterday with the strong support of Majority Leader Robert J. Dole (R-Kan.), calls for postponing implementation of the whole bill until 1987. The House measure approved earlier in the week said only that ranking congressional tax writers should issue a joint statement with Treasury Secretary James A. Baker III before the end of the year suggesting postponement of those provisions likely to affect investment.

Even Senate Finance Committee Chairman Bob Packwood (R-Ore.) said yesterday that businesses should not make investment decisions based on any nonbinding resolutions passed by Congress.

Meanwhile, the Commerce Department reported that among those firms expecting to spend less next year on capital equipment were manufacturers, which plan to reduce real spending about 4 percent.

Many manufacturers say they will be hurt by the tax revision plan, and Ortner said that uncertainty over the tax bill played a part in the reduction in spending plans in addition to the slowdown in economic growth in the first half of this year, as well as the reduction in corporate earnings and cash flow.

However, the recent rallies in the bond and stock markets are making businesses' financing costs lower, so spending plans could later be revised upward, Ortner said.

The capital spending and investment issue is important because, in the short run, spending by businesses to modernize and expand contributes directly to gross national product -- the nation's output of goods and services -- and increases the capacity for the economy to grow in the long run.

Investment in labor-saving equipment also increases productivity and reduces costs to businesses, and thus keeps prices down.

The concern about added costs to their investments because of possible changes in the tax code have frightened industries ranging from construction to auto production. However, some businesses -- largely high-tech firms and service providers -- say they will benefit.

"It's going to affect our businesses immediately," said Scott Slesinger, a spokesman for the National Apartment Association. "People have to assume something that may not happen."

"We are reviewing all of our investment plans in light of the Ways and Means plan going to the Senate," said a spokesman for GTE Corp., the giant telecommunications company.

Although uncertainty over the tax plan is not the only consideration in Ford Motor Co.'s investment decisions, it is a factor, said Robert Waite, a Ford spokesman. "It means that our numbers crunchers have to crunch two sets of numbers," Waite said. "We have to look at scenarios. That slows things up."

Most of Ford's major investment in new lines has already been done under the current tax system.

The worries of all these companies are understandable, but tough to prevent, said Assistant Treasury Secretary Ronald A. Pearlman. The problem in writing tax bills is this: If the effective dates are set far in advance, people and businesses work overtime to take advantage of existing provisions. If they are set to be effective on the day they are enacted in committee, as Ways and Means did in terminating the investment tax credit, companies get caught in the middle.

"It's always unfortunate to have uncertainty in the tax legislative process. But it's a fact of life," Pearlman said.

His point was underscored yesterday as Congress struggled during its late-night closing session to wrestle with another uncertainty issue: how to deal with 13 provisions of the current tax code that will expire at the end of this year unless they are extended.

Among those provisions were the tax credit for increases in research spending, the credit for residential energy expenditures, the credit for hiring disadvantaged workers, and the tax exemptions for legal services provided by an employer and employer-paid educational expenses. The House was debating a proposal to extend and modify the provisions, while the Senate plan would simply extend them all for six months.

The Commerce survey of businesses showed that the decline in spending next year compares with a 5.6 percent increase in investment spending planned for this year. That rise is smaller than the 15.3 percent gain in 1984, the largest in 20 years.

According to the Commerce report, manufacturers said their spending on new plant and equipment, adjusted for inflation, would drop from $68.59 billion this year to $65.83 billion next year. Spending by durable-goods industries, which make products lasting more than three years, would drop from $35.26 billion to $33.08 billion in 1986.

Spending by nondurable goods industries would fall from $33.33 billion this year to $32.75 billion in 1986, Commerce said.

However, nonmanufacturing spending is projected to increase slightly, from $109.52 billion to $110.53 billion.