If you're one of the estimated 7 million American homeowners who supplement their incomes with a part-time business operating out of your house or condominium apartment, good news is on the way. The U.S. Treasury Department is about to back down from its strict, controversial "office-in-the-home" tax-deduction rules.

For the first time since 1976, the Internal Revenue Service is going to allow you to write off expenses on a work place in your home, even if your principal job or business is somewhere else. That's a major change for the government, and it's potentially worth thousands of dollars to you next April.

Within the next three to four weeks, the IRS plans to issue proposed regulations for deductions on businesses or trades operated from residences.

Unlike regulations proposed during the Carter administration, however, the new rules will permit deductions on your federal tax returns for home real estate expenses in connection with "secondary" businesses. That means, for instance, if you work full-time for an employer, but run a part-time, profit-making office in your home, you may qualify for significant new tax relief.

You may be able to take deductions for part of your home utility bills, as well as certain costs for repair, maintenance and depreciation.

The deductions would be limited solely to that portion of your home that you use exclusively and "on a regular basis" to conduct your sideline business ventures. The room or space you use would have to be the "principal place of business" for your secondary income, and your total deductions in any one year could not exceed your income from the sideline venture.

The IRS's standards for "offices-in-the-home" during the past several years have been far tougher than these. In regulations proposed last year, the IRS banned any deductions whatsoever for space in the home that wasn't the principal place of business for the taxpayer.

The practical effect was to deny deductions to virtually anyone but the self-employed, home-based entrepreneur. Taxpayers who spent the majority of their time working for someone else in a downtown office building, but earned a large minority of their income working part-time out of their home, were hit hard.

Homeowners who ran sideline consulting, sales, mail-order, freelance writing, investing, accounting and similar businesses found themselves unable to take the deductions they had counted on. Part of the reason for that change was political, part of it had to do with a key U.S. Tax Court case the IRS lost on the issue last year.

The case, known as Curphy vs. Commissioner, involved a doctor whose primary work was at a hospital. The doctor also maintained a room in his condominium apartment, however, exclusively to manage investment real estate.

Under the IRS's tough office-in-the-home standards, the government disallowed Dr. Curphy's business deductions for the room. Real estate investment was not his principal source of income, the IRS argued, and therefore Dr. Curphy's write-offs on his secondary business couldn't qualify.

The U.S. Tax Court overturned the IRS position, and at least indirectly helped convince the Reagan administration not to push the issue any further. The court held that individuals can and do have multiple business activities. If a legitimate business is conducted primarily from a part of a taxpayer's home set aside exclusively for that purposes, the enterprise should qualify for deductions, the court said.

The new regulations, which will be published for comment late in November, are all but certain to be adopted early in 1982 by the IRS. They'll apply to tax year 1981, so watch for them if you think you qualify.

Keep your eye out, too, for a closely related rule change coming from Treasury on personal use of vacation homes. The IRS plans to issue draft regulations that loosen up on tax treatment of "maintenance and repair" visits by owners to vacation rental properties.

Under standards proposed during the final months of the Carter administration, the IRS said it would count any visit to a property as "personal use" unless the owner spent a full working day repairing or maintaining it. If the owner brought along a relative or young child to the unit and they didn't work on the cottage or condo, the owner would be charged with a day of "personal use" -- even if he himself spent eight hours at repair work.

(Personal use is a key issue to vacation property owners because their allowable deductions are severely limited by the U.S. tax code beyond certain thresholds. Once an owner spends more than 14 days a year at his or her unit, or more than 10 percent of the time the unit is rented out, deductions for depreciation, repairs and maintenance are curtailed.)

The revised IRS rules, according to a top Treasury official, will not count owners' trips to vacation units as personal "if the principal purpose of the visit was maintenance or repair."