DEAR BOB: Last September we bought a home that we thought had a bad furnace and air conditioner. We asked the real estate agent to recommend a company to fix it. She recommended a man who said he would charge us $900 to recondition everything. We were given a five-year warranty. But when the cold weather came, the furnace wouldn't work right. We called the repairman but his phone was disconnected. The real estate agent says she doesn't know where he is, even though this man is the agent's brother-in-law. We had no choice but to call another repair company, which charged us $1,600 to rebuild the furnace. I hate to think about how the air conditioner is going to work when hot weather arrives. Do you think the real estate agent should reimburse us for recommending her relative who did such bad work? -- Ray W.

DEAR RAY: If you had insisted upon receiving a one-year home warranty policy when buying the house, repairs to the heating, plumbing, wiring and built-in appliances would have been covered during the first year.

Your situation shows why real estate agents must be so careful about recommending service personnel. The agent probably has no legal liability to you, even though the man was her brother-in-law. In the future, check out a repairman's reputation more carefully. Consult your attorney for further details.

DEAR BOB: When we rented our house, the landlord said if she ever decided to sell the house we would have the first opportunity to buy. But on Jan. 2 we received notice the house has been sold to the owner's brother and we have 90 days to move out. Do we have any recourse to enforce our right to buy? We have made several valuable improvements to the house, which we would not have made without the promise we could buy it. -- Jerome P.

DEAR JEROME: The Statute of Frauds requires real estate sales contracts to be written if they are to be legally enforced. If your lease contains a right of first refusal or other purchase agreement, then you might be able to enforce its provisions. But without written evidence of the promise to sell to you, it will be hard to overcome the writing requirement.

You could, however, argue that you had relied on the landlord's promise to sell to you by making the valuable improvements to the property. This might take the agreement out of the Statute of Frauds, but it would be a tenuous argument. Consult a real estate attorney for further details.

DEAR BOB: Our home is located in a zone where we can add a second unit to our property. A contractor has given us an estimate of about $40,000 to add a nice apartment, but we are debating whether it would be worthwhile. Several nearby houses have one or two apartments added in the back, but most of the neighboring houses are single-family homes. A real estate agent advises against adding the extra unit. She says it wouldn't add more than $40,000 to the market value and there is more demand for single-family homes than duplexes. What do you advise? -- Helga H.

DEAR HELGA: Your real estate agent has given you sound advice. Unless adding the rental apartment will increase the property's market value by far more than the cost of the construction, it will not be a profitable improvement. She is also correct that single-family homes are usually much easier to sell than duplexes. I vote for not adding the apartment.

DEAR BOB: We sold our home in 1987 because of illness. Our profit is about $89,000. As our 1987 tax return will be due by April 15, 1988, if we are to avoid tax on our profit do we have to buy a replacement home by April 15? -- Kevin L.

DEAR KEVIN: No. There is no need to rush your home replacement decision if you are undecided about buying another house to defer tax on your profit.

Just file Internal Revenue Service form 2119 with your 1987 tax returns. Indicate on that form you plan to buy a qualifying replacement principal residence of equal or greater cost within two years of the sale of your old home. However, if you don't buy such a home within two years then you will owe the tax plus interest. Consult your tax adviser for details.

DEAR BOB: We are trying to buy a home and have noticed that many real estate classified ads say ''principals only.'' Does this mean something is wrong with the property? -- Carla W.

DEAR CARLA: No. There are several reasons to use the words ''principals only'' in a real estate ad. It is just a polite way of saying ''real estate agents, please don't call.''

One reason occurs when a property owner is trying to sell without a real estate agent and doesn't want to be bothered with agents phoning trying to get the owner to list the property for sale. Another reason occurs when an agent has a property listing and doesn't want to share part of the sales commission with another agent.

DEAR BOB: In 1987 we bought a house and received a termite inspection clearance report. However, we have now discovered several of the trees are infested with termites. Shouldn't the termite inspector have told us? -- Ted R.

DEAR TED: No. In most states professional termite inspectors only inspect structures. If you had wanted the trees checked for termites you should have requested that they be checked, too.

DEAR BOB: We paid about $40,000 for our home and it is now worth about $150,000. If we move out and rent the house to tenants, will we have to pay capital gains tax on our profit? -- Walter W.

DEAR WALTER: No. The reason is there is no sale so there is no taxable event.

However, you will be able to depreciate the house when you begin renting it to tenants. Your depreciable basis will be the lower of your adjusted cost basis (usually, purchase price plus any capital improvements) or the property's fair market value on the day of conversion to rental status. Obviously, your $40,000 cost basis is lower. However, you'll need to subtract the nondepreciable land value because only the structure can be depreciated. Consult your tax adviser for full details.

DEAR BOB: Please don't laugh at my question. You often refer to a first mortgage or a second mortgage. What is the difference? Does it depend on the amount? -- James T.

DEAR JAMES: The order of recording determines the priority of a mortgage. A first mortgage is recorded before a second mortgage on the same property. The amounts are irrelevant.

To illustrate, suppose you record a $50,000 mortgage on your house on Feb. 1 and a $75,000 mortgage on the same house on Feb. 2. The $50,000 loan is a first mortgage and the $75,000 loan is a second mortgage, even though the amount of the second mortgage is larger than the first mortgage.

DEAR BOB: We are buying some land on a contract. The developer has an excellent reputation. But it occurred to me, after reading your column about the house that was built on the wrong adjoining lot, that maybe we should record something to show we are buying our lot. What do you advise? -- Ramon F.

DEAR RAMON: For your protection, the land contract, or at least a memorandum of it, should be recorded to cloud the title to the property. You may think the developer has an excellent reputation -- and he probably does -- but even the best sometimes go bad or make mistakes. If the developer refuses to record your land contract, then consult a real estate attorney because something might be wrong.

DEAR BOB: In December we closed the purchase of our first home. But I think we got swindled on the mortgage. The real estate saleswoman steered us to a mortgage broker who had a confusing variety of adjustable and fixed-rate mortgages. Since we wanted a fixed-rate mortgage, the mortgage broker pushed a new type of loan that can be refinanced if interest rates drop. We were all excited about buying the home and, I admit, we didn't read the loan papers very closely. We paid a 2 1/2 percent loan fee to get an 11 percent interest rate mortgage. If interest rates drop to 9 percent in a year, then we can refinance for a fee of 1 percent. At first this seemed like a good deal. Then we realized the chances of interest rates dropping 2 percentage points in a year year are pretty slim. Also, we are paying about one-half percent above the rate on regular fixed-rate home loans. When I confronted the mortgage broker, he said it's too late and we can't change the loan now. Do you think we got ripped off? -- Hamilton R.

DEAR HAMILTON: Yes. Mortgage lenders love to do business with home buyers who have a time deadline for closing the sale. But these lenders hate dealing with homeowners who are refinancing an existing mortgage because refinancing borrowers have all the time in the world to shop and compare mortgage terms.

You were probably so concerned about buying the home that you didn't pay much attention to the mortgage's terms. Not only did you pay a higher-than-normal interest rate, but your loan fee also was not a bargain. As you discovered after signing the loan papers, the new so-called refinanceable loans offered by some lenders are not a good deal for the borrower because chances of mortgage interest rates dropping 2 percentage points in one year are not very great. I suggest you check the Regulation Z disclosure statement you were given to see if it was correctly prepared. If a mistake was made, you can get out of that bad loan. But if there was no error, I'm afraid you're stuck with it. Next time, be more careful to study and think about a loan before you sign.

Readers with questions should write Bruss directly at P.O. Box 6710, San Francisco, Calif., 94101.