DEAR BOB: Does the rule that an owner doesn't have to report rental income on a vacation home for under 15 days per year also apply if we rent our primary residence for 14 days while we are on a trip -- or must we pay tax on that rental income? Holly S., Kensington.

DEAR HOLLY: If you rent your principal residence for less than 15 days per year, you need not report the rental income on your tax returns. But you can't take any tax deduction for expenses of the rental either. For details, see your tax advisor.

DEAR BOB: In 1968 I bought a lot on St. Thomas in the Virgin Islands for my retirement home. My plans have changed. I've had the lot for sale since 1973, but no luck so far. Any ideas? James N., Chevy Chase.

DEAR JAMES: Your situation shows the folly of buying a lot before you're ready to build your retirement home. Thousands of other buyers made the same mistake; you've got lots of company.

If your realty agent hasn't sold the lot in five years, you need a new broker. The local board of realtors may be able to suggest brokers who have been successful selling similar lots nerby. Write to several brokers to find out if they think they can sell your lot and at what price.

Then list exclusively with the one you think will do the best job. A six-month listing may be necessary since lot sales often take a long time.

DEAR BOB: I figure that a mortgage at 10 percent interest really costs little or nothing since most properties go up in value 10 percent or more per year. Right or wrong? Scotty McG., Annapolis.

DEAR SCOTTY: You're right. Fixed-interest-rate mortgages are a genuine bargain in today's inflationary economy.

DEAR BOB: Recently I read an article saying that the real estate market would collapse. It said that values would plunge 20 to 30 percent. I also read an equally gloomy prediction by an economist who predicted essentially the same thing. What do you think? Morris E., Vienna.

DEAR MORRIS: I think those doom-and-gloom writers are making a fortune from people who pay to hear dire predictions. While the stock market may drop, we may have a recession and there may be other financial hardships, but the real estate market should come through in very good condition.

Real estate is not subject to day-to-day volatility. Its value remains relatively stable. The only realty owners who suffer are those who bought with low down payments, who can't afford to carry their properties and who must sell quickly. Those people should never have bought in the first place.

But if you have staying power, you'll find real estate the best investment in the world as an inflation hedge and a leveraged investment.

DEAR BOB: My sister died in 1975 and the probate of her estate was completed July 20, 1976. Somehow I signed a deed that my brother had drawn up by a lawyer on Jan. 8, 1976.I was in a state of confusion and couldn't see very well. When I asked what I was signing, my brother told me it was just another paper regarding my sister's estate. I didn't own the property yet when I signed. Is my signature on the deed void? I am 86 and would like to sell the property. Can I cancel the first deed? Anxious, Silver Spring.

DEAR ANXIOUS: The general rule is that you can't convey title to something you don't own. However, when you later acquire title to property that you have already conveyed away, it's called "after acquired title" and the earlier deed may become good.

You should immediately consult an attorney familiar with real estate law in the state where your late sister's property is located. He or she will research the title and the legal effect of your deed. If the statute of limitations hasn't expired, it may be possible to go to court, in a quiet title action, to get the earlier deed canceled on the ground that you were tricked or weren't aware of what you were signing.