DEAR BOB: Almost every week you tell people to go out and buy a home. Well, we've been trying to do that, without much success. It seems that the houses we can afford are just a bunch of junk. The really nice ones are beyond our budget. Do you think house prices and mortgage interest rates will come down in the next few months? How much should a couple, earning about $28,000 a year, spend for a house? Do houses appreciate faster than condominiums? Mary Ann G., Bethesda.

DEAR MARY ANN: Prices and rates won't be coming down. According to the Bureau of Labor Statistics, since 1963, home prices have gone up 8 percent a year. But those are only averages. In the San Francisco real estate market, which is similiar to yours, home value increases in 1978 ranged from 12.4 percent to 20 percent, depending upon neighborhood. I see no end in sight to rising home prices.

Similiarly, until the inflation rate declines, don't expect much drop in mortgage interest rates. If you wait to buy, any drop in mortgage rates will be more than offset by the continuing rise in home prices.

The historical criteria for home buying has been to spend 2 1/2 to three times your family's gross annual income. That's $70,000 to $84,000 in your case. Try to keep the monthly payments at 25 to 30 percent of your takehome pay.

Until recetly, single-family homes appreciated faster in value than did condominiums, due mainly to greater buyer demand. But in the last two or three years, appreciation in good condos has rivaled that of single-family homes. The sooner you buy, the sooner you'll start profiting.

DEAR BOB: What are the pros and cons of buying and selling a real estate contract prior to settlement? The potential for a quick gain with no risk appears great. Al F., Fairfax.

DEAR AL: Yes, it's possible to make a quick profit by offering to purchase a property and then "reselling" that property at a higher price before you close the purchase. However, be sure the purchase contract is assignable because if it isn't, you may have difficulty with the seller.

Of course, to make much of a profit, you'll have to buy at a bargin price below the market. You'll be gambling that you can find another buyer to pay more for the property than the seller gareed to sell at. That can be hard to do unless the local market is rising fast or unless the colsing settlement is six months or more away.

Another problem is that if you can't find a buyer, you lose your earnest money deposit if you can't close the sale on the agreed date. As long as you understand the risks, you might make good profits buying and selling property purchase contracts.

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 mortgage are a genuine bargin in today's inflationary economy.

DEAR BOB: Is it true that if we sell our summer home and buy a vacation condo in Florida we can defer paying our profit tax? Michael L., Takoma Park.

DEAR MICHAEL: No The tax degerral benefits of the "residence replacement rule" apply only when you sell your principal residence and buy one of equal or greater cost within 18 months before or after the sale.This tax deferral rule does not apply to any other type of property. Ask your tax advisor for details.

DEAR BOB: You recently said that it's better to buy directly from a defaulting owner rather than at a foreclosure sale. How? Mr. H.M., Washington.

DEAR MR. H.M.: Notices of mortgage default are usually recorded with a local country official, such as the county recorder. Ask the official if he publishes a list of default notices or if you can buy such a list from a private source. If you still can't find such a list, ask a local title insurance or real estate firm how they learn of mortgage defaults.

One way or another, you can find which borrowers have defaulted. Then visit them to see if you can buy out the equity in their property, often for just several thousand dollars, and take over the existing mortgage. Ask your attorney to help with your first few purchases as the title must be checked to be sure there are no outstanding lines.

For the Robert Bruss report, How to Get the Greatest Possible Tax Savings From Your Home, send 25 cents and a stamped, self-addresssed envelope to P.O. Box 6710, 711 Commercial St., San Francisco, Calif. 94101.