Big oil companies and tax-shelter investors aren't th e only ones who stand to benefit from costly give-aways and dubious-sounding tax breaks when Congress heads another election campaign.

The lawmakers already are putting the finishing touches on a $400 home-insulation tax credit - designed to reward taxpayers for buying items that will save them money on fuel bills - and a costly tuition tax credit bill.

Now, a House-Senate conference committee is rushing toward approval of what some critics say could be one of the bigger tax-break ripoffs going - a larger tax exemption for homeowners who sell their houses and don't re-invest.

The House has approved legislation that would allow homesellers of any age or income status to claim the first $100,000 of their profits tax-free, even if they don't roll the money over to buy a new more costly home.

And the Senate has voted to raise the present $35,000 exclusion for over-65 homesellers and raise it to $100,000 and make the tax break available to homeowners aged 55 or over. Some compromise version seems certain to pass.

Both provisions are costly: The House-passed measure woudl drain almost $1 billion in Treasury revenues, and the Senate version woudl cost $300 million or more. Even a modest compromise would take half a billion dollars.

To hear the lawmakers tell it, the proposal is an emergency measure. Unlike those rich, fat eat investors, Joe and Martha constitutent have only their home as a money-maker. Now, on their retirement, Uncle Sam wants it all.

In fact, however, by the time a homeowner is ready to retire, he already has reaped tax breaks and economic benefits that are far out of proportion to those that would accrue from almost any other form of investment.

Indeed, a good many tax experts make the case that even the existing tax breaks most likely are overly generous. Why shouldn't a person have to pay taxes on the profits he makes from selling his home?

There are these considerations: Homeowners, already get a big break simply by not having to keep up with soaring recent levels. Even with sharply rising property taxes, home payments don't shoot up nearly as rapidly as rent.

If you bought a $32,000 house in 1971, the likelihood is your payments now are about $300 a month - a rise of only about $50 from seven years ago. But rentals, in many areas, have leaped to twice that or more.

Homeowners already are entitled to a big tax deduction not allowed to renters - the writeoff for the interest and taxes they apy. Renters pay their share as part of their monthly rent, but can't claim a writeoff.

Homeowners who sell their homes already can escape taxes entirely on any profit they make simply by using the money to buy another house that is either the same price or more costly. And the profits build up as equity.

Present law allows a homeseller to avoid taxes on his profits if he re-invests the money in a new home within 18 months. It's only those who sell their houses and move into retirement homes or apartments who pay.

When a homeowner does sell his house without buying a new one, the profit is considered a capital gain, and only half of it is subject to regular income tax - at tax rates the same as for wage and salary income.

Of the rest, the first $10,000 is tax-free and the remainder is subject to a 15 percent "minimum" tax, which now appears likely to be diluted by Congress. For those over 65, there's the additional $35,000 exclusion.

If a homeseller does reap a truly big windfall, he can reduce his already-modest tax liability even further by using the "income-average" method of figuring his taxes, which can trim his taxable income quite substantially.

Put it all together, and the homeowner emergers as one of the nation's most-pampered categories of taxpayer. And all this for an investment that, during times of rapid inflation, produces higher profits than almost any other.

There also is the question of whethr these big tax breaks for homeowners are good social policy. The writeoffs were enacted year ago, when Congress wanted to encourage home ownership as a national policy.

But today, especially in crowded urban areas, there simply isn't the supply of available homes. Economists argue that bigger tax breaks simply heighten demand for homes. If anything, we should be encouraging renters.

They why the big push in Congress to enact bigger tax breaks for homeowners, instead of using the $300 million-to-$1 billion for more general tax relief for Americans?

Politics, of course. As much as the oil industry is, home ownership still is a sacred cow in the United States, and homeowners make the most vocal evoters - as electioneering House and Senate members well understand.

There's also the I-worked-all-my-life-to-build-up-equity-and-now they're-going-to-tax-it-away argument. Congress has been so generous to homeowners over the years that some now seem to believe tax-free status is a right.

Among them are some of the same taxpayers who, in the very next breath scream bloody murder when big business reaps a tax windfall, or when tax-shelter investors exploit so-called "loophols."

In the end, however, the question is one of budget room. If the compromise version of the homeowners provision comes out at between $500 million and $700 million, it would add significantly to budget costs.

Put that together with the home-insulation credit and the tuition tax credit, and the total for the three would be almost $3 billion. If Congress gave that much to big oil, the voters would be madder than a wet hen.