DEAR BOB: There is a rental cottage at the back of our property. It has been vacant for three months. Can we take any tax deduction for the lost rent? For depreciation? It's pretty run down and we just don't have the money to fix it up now. That's why it's vacant. Sarah Mae W.

DEAR SARAH MAE: I'm surprised at how often your question is asked. You cannot take any deduction for the rent you've lost during the vacancy. But you can continue taking depreciation deductions for the cottage if it is available for rent. Your tax adviser has full details.

DEAR BOB: I have terminal cancer. My wife is also sick but not in danger of dying before I do. We have a lovely daughter to whom we eventually want our property to go. Fortunately, money is not a problem. Our lawyer suggested I set up my wife (all the property is in my name alone) so that she receives a life estate in the properties. When she dies, my daughter is to inherit them. Does this sound like a good plan? Martin M., Laurel.

DEAR MARTIN: Yes. It sounds like an excellent estate plan. By giving your wife only a life estate, that will avoid an estate tax when she dies. You are smart to consult an estate planning attorney to minimize the estate tax and to maximize the assets your family receives from your estate.

DEAR BOB: My brother's will gave me all his property. He owned some land in joint tenancy with survivorship. The other joint tenant is claiming that land. Shouldn't I get it too? John G., Vienna.

DEAR JOHN: No. A will has no effect on property held in joint tenancy with right of survivorship. See your attorney for full details.

A Bruss report, "How the 1978 Tax Act Affects Homeowners and Property Investors," is available for 25 cents plus a self-addressed stamped envelope sent to Robert J. Bruss, Box 6710, San Francisco 94101.