QI bought my first house in 1950 for $45,000 and sold it in 1990 for $500,000. My basis for tax purposes was $50,000. I took advantage of the once-in-a-lifetime exclusion of capital-gains tax on $125,000, which was then in effect. I paid capital-gains tax on $25,000 of profit and rolled over the balance by purchasing a condominium apartment for $300,000. The property is now appraised at $450,000, and I would like to sell it. How much tax will I have to pay, if any? Both properties were my principal residence.

AYou certainly have done well on your real estate ventures.

Before responding to your question, I have to remind readers -- especially the younger ones -- of the tax rules that applied to real estate before the law changed in 1997.

Currently, the tax laws are quite favorable for people who own and sell their own home. If you are single, you can exclude from capital-gains tax up to $250,000 of gain from the sale of your house, on the condition that you have lived in the property two out of the five years before the property is sold. If you are married and file a joint return, the exclusion goes up to $500,000.

However, before the 1997 tax law change, there were two important tax rules:

* Rollover. If you bought a principal residence either within two years before or two years after you sold your principal residence, and if the new property was equal or greater in value compared with the selling price of your older home, you did not have to pay capital-gains tax on the profit you made on the sale. This was only a deferral, not an avoidance, of the tax. You rolled over your gain to the new property. An oversimplified example: If you made a profit of $100,000 when you sold your first home for $200,000 and rolled over into a home that cost $400,000, the $100,000 gain was used to reduce your tax basis on the new home. Thus, the $400,000 house you purchased had a tax basis of $300,000. If you later sold the house for $500,000, even though you really only made a profit of $100,000 ($500,000 minus $400,000), for tax purposes you made a profit of $200,000 ($500,000 minus $300,000).

* Once-in-a-lifetime exclusion. If you were 55 years of age or older when you sold your principal residence, you were allowed by the tax law to exclude up to $125,000 of your gain. Thus, in our example, although you made $200,000 in profit, after excluding the $125,000, you would have to pay tax only on $75,000 ($200,000 minus $125,000).

Many readers will no doubt ask: Why is he writing about this old law? Can't we just take up to $250,000 (or $500,000) when we sell our principal residence? Isn't that what the new law says?

The answer is that the new tax law allows the principal home seller to exclude up to $250,000 of gain ($500,000 if married and filing a joint tax return). But what is "gain"? Gain is the profit you have made, taking into consideration the tax basis of your property.

Now let's go to the original question. When the writer sold her property in 1990, she made a profit of $450,000 ($500,000 minus $50,000). She took advantage of the once-in-a-lifetime exclusion of $125,000, leaving her with a profit of $325,000. However, she rolled over this profit by purchasing a condominium that cost her $300,000. As noted, she paid capital-gains tax on the $25,000 difference.

Now she wants to sell the condominium for $450,000. At first blush, one would think that because she purchased it for $300,000, she would make $150,000 on the sale, and thus under the new law would be able to exclude all the profit.

Unfortunately, that's not the case. Although she purchased the condominium for $300,000, keep in mind that this was all "rolled over" dollars. She earlier avoided paying capital-gains tax on the $300,000 profit she had made. Thus, despite the purchase price, her basis in the condominium is zero ($300,000 minus $300,000). If she sells the property for $450,000, all of this is gain. If she is not married (or files a separate tax return) she can exclude only up to $250,000 of this gain and will thus have to pay capital-gains tax on the $200,000 difference.

It should be noted that for this discussion I have ignored various closing costs and real estate commissions that should be included in the calculations, so as to reduce her actual capital-gains tax obligation. It should also be noted that currently, the capital-gains rate is 20 percent, whereas in 1990, when she sold her first property, the rate was 28 percent.

Thus, while our writer will have to pay approximately $40,000 in capital-gains tax to the IRS, there is some consolation. Had the older laws not existed, she would have paid a lot more back in 1990 and probably would not have been able to afford the new condominium -- on which she also made a nice profit.

However, if you took advantage of the old rollover (and even the once-in-a-lifetime exclusion) years ago, it is important to dig out your old tax returns and calculate exactly what your current tax basis is. If you do not report the true tax basis of your home, you risk a letter from the IRS telling you that you owe a lot of money on the sale of your principal residence.

Benny L. Kass is a Washington lawyer. For a free copy of the booklet "A Guide to Settlement on Your New Home," send a self-addressed, stamped envelope to Benny L. Kass, Suite 1100, 1050 17th St. NW, Washington, D.C. 20036. Readers may also send questions to him at that address.