The tax bill of 1984 makes several changes for individual taxpayers to keep in mind when preparing this year's tax returns.

Among other provisions that may affect a taxpayer's liability are changes in income averaging and capital gains treatment.

The new law also requires taxpayers to make Individual Retirement Account (IRA) contributions by April 15, even if they have received an extension for filing tax returns.

Next year promises more changes, too. Some provisions in the 1984 bill and earlier legislation become effective then, including indexing. Beyond that, calls for tax reform and the need to cut the deficit down to size suggest that Congress is not likely to leave the tax code alone.

*Income averaging. The law has been changed to make it harder for taxpayers to use income averaging to reduce tax liability. To be eligible to use the technique to offset increased earnings, a taxpayer must be earning more than 140 percent of average income for the previous three years compared with more than 120 percent of a four-year average before.

*Capital gains. Capital investments made after June 23, 1984, may qualify for capital gains treatment by being held for six months, rather than a year. This provision is designed to expire in three years unless Congress extends it.

*Alimony. Transfers of property as a result of divorce will not be treated as taxable losses or gains.

*Medical deduction for lodging. A taxpayer who must travel away from home for specialized or otherwise unavailable medical treatment may deduct lodging expenses of up to $50 a night per eligible individual. This applies even when hospitalization is not necessary.

*Business use of automobiles and other equipment. Taxpayers are eligible for business-related tax benefits for automobiles, personal computers and other equipment only if they can demonstrate that they use the equipment for business at least 50 percent of the time and if the equipment is required for the convenience of the employer and as a condition of employment.

In addition, tax benefits are available for only approximately $16,500 in value for cars.

*Real estate depreciation. Writing off real estate investment through depreciation will take longer -- 18 years instead of 15, except for low-income housing. These changes are generally effective for property placed in service by the taxpayer after March 15, 1984.

*Estimated taxes. The law has been changed to require estimated tax payments for the alternative minimum tax.

In addition, the method for computing an underpayment penalty has been changed.

It is now based on the lesser of 80 percent of the tax shown on the return, 100 percent of the tax shown on the preceding year's return or 80 percent of the current year's tax computed on annualized income.

Two exceptions to the estimated-tax penalty have been repealed.

But the new law also allows the IRS to waive the estimated-tax penalty in the event of a casualty, disaster or other unusual circumstances.

It also may be waived for reasonable cause during the first two years after the taxpayer retires upon reaching age 62 or because of disability.

*Charitable gifts of property. The law requires that the donor obtain written independent appraisal of the property's value by a qualified, independent appraiser for gifts of property with a value of more than $5,000 for a single item or $10,000 for stock.

Both the donor and the recipient are required to provide more information than in the past, and the penalty for overvaluing a property by more than 50 percent is set at 30 percent of the resulting underpayment of taxes.

*Foreign earned income exclusion. Scheduled increases in the foreign earned income exclusion are postponed, with the exclusion remaining at $80,000 until 1988.

In addition, Social Security recipients with earnings of $25,000 filing individually and of $32,000 filing jointly will have to pay federal income tax on half of their benefits. The major change for tax year 1985 is tax indexing, designed to prevent taxpayers from being pushed into higher tax brackets when their income has done no more than keep pace with inflation. The indexing plan, enacted in 1981, means that the typical family of four with $30,120 in income in 1984 will save $84 in federal income taxes in 1985.

The standard deduction, the personal exemption and the cutoff figures for each tax bracket will be indexed by 4.08 percent. As a result, the standard deduction will increase from $2,300 for an individual to $2,390.

The deduction for families will rise from $3,400 to $3,540, and the personal exemption for each family member will rise to $1,040.

The 1984 tax bill will make additional changes in 1985, including an increase in the earned income credit.

The net interest exclusion, a tax feature that was scheduled to begin in 1985, was repealed before it ever took effect.