DEAR BOB: A few weeks ago I read a newspaper article about a financial innovation, called a "reverse annuity mortgage," which allows a person with a large equity in a home to borrow against it from a savings and loan association. The lender will then pay a monthly annuity to the homeowner, with repayment coming when the home is sold, either by the owner or his heirs. I am very interested in this type of mortgage as we are on a fixed income and this would solve our problem of rising expenses. I called ever lender in town, but with no succes.. They've never heard of such a loan. Where can I get one? Mrs. Herman S., Alexandria.

DEAR MRS. HERMAN S.: Unless your property is in Cleveland near the Broadview Svaings and Loan Association, you can't get a reverse annuity mortgage. Braodview is the only lender now making usch loans.

Last January the Federal Home Loan Bank Board authorized federal savings and loan associations to make reverse annuity home mortgages. So far, not one new lender has started making them.

Why? Because savings associations have strong loan demand for traditional mortgages, and they won't waste their time or money developing the reverse kind. Perhaps if senior citizens and others pressure lenders to make such loans, one or two will lead the way and the others will follow.

DEAR BOB: We want to buy a vacation cabin on which the seller has accepted our purchase offer. But the agent has been unable to find any mortgage lender who loans on such properties. Any ideas? Betty L., McLean.

DEAR BETTY: Rural and vaction properties can be extremeley difficult to finance. The seller is your best financing source. Your purchase offer should have contained the terms of the mortgage you want the seller to carry to finance your purchase.

Perhaps if you beg the seller, by bowing and scraping, he may agree to take back a first or second mortgage. If he refuses, you may lose your deposit since your apparently failed to include a mortgage finance contingency in your purchase offer.

In the future, never fail to include a finance clause in your purchase offer. If you can't get the desired financing, your deposit is refunded. Without such a finance clause, you lose your deposit to the seller.

DEAR BOB: We don't own any real estate but are interested in buying. Would you recommend buying a house or income property, such as apartments, first? Myron A., Suitland.

DEAR MYRON: Buy your house first: If you want to combine your house with apartments, buy an apartment house and live in one apartment. Then you'll have the best of two worlds.

DEAR BOB: If I buy single-family houses for rental, do they qualify for depreciation tax deductions? Roy A., Washington.