DEAR BOB: Should I refinance my mortgage or add a second mortgage to finance some improvements to my home? Mr. J.R., Washington.

DEAR MR. J.R.: There is no easy answer. Only by shopping among local savings associations, banks and mortgage brokers can you compare costs both ways. After you've found each lender's terms for refinancing and home improvement mortgages will you know which alternative is best for you.

DEAR BOB: Would you recommend getting into the real estate sales profession today? Will the volume of sales decrease because many potential home buyers are being priced out of the market due to increasing interest rates and rising home prices? Steve P., Silver Spring.

DEAR STEVE: Real estate sales is the greatest job in the world. Each realty agent is his or her own boss - with no earnings limits, no fixed working hours and unlimited opportunities for service to others.

Although many potential home buyers think they can't afford to buy houses, they can if they try. But they need the help of good realty agents to show them how to find and finance the right homes. There are always plenty of opportunities for good realty agents.

But many real estate agents drop out of the realty business every year because they can't stand the uncertainty of no fixed salary. Good times and bad, the top realty agents prosper. If you decide to become a realty agent, make up your mind to be the best and to never stop learning. Do your best to serve your customers in the way would want to be treated. If you do, you'll soon join the ranks of successful agents who never have to worry about being unemployed.

DEAR BOB: My son and I sold a rental property. He says we have 18 months to reinvest without paying capital gain tax on our profit. My tax man says we have to pay tax on our profit this year even if I invest in a home. Who is right? Clarence B., Bethesda.

DEAR CLARENCE: Your tax man is right. No tax deferral is allowed only if you sell your principal residence and buy a more expensive replacement within 18 months before or after the sale.

DEAR BOB: I own a house that I rent to tenants. It is located out of state. Occasionally I make trips there to rent it and handle other matters. Are expenses for such trips tax deductible? If so, how? Felix T., Annandale.

DEAR FELIX: Yes, your reasonable travel expenses to inspect and rent your investment property are tax deductible. Deduct the costs as an operating expense for the property. This is reported on Schedule E of your IRS Form 1040 income tax return. Your tax advisor can explain further.

DEAR BOB: I am a widow, age 55, with my only income from a $90,000 bank savings certificate. I am thinking of buying a house as a hedge against inflation. Anything decent will cost at least $50,000. If I made a $20,000 down payment, I will have to dip into my principal for about $3,500 a year until I am old enough to collect social security. Would you advise taking the chance that the house will increase in value to offset my loss? Joan F., Alexandria.

DEAR JOAN: If you buy a sound, well-located house or condominium, it should appreciate in value at least as much as the inflation rate. In 1978, the average home appreciated in value 14 percent; 1979 may be even better.

I can think of no better place to invest some of your money than in your own home. Look at it as a forced savings account paying interest at least as much as the inflation rate. Even if your planned $50,000 home purchase only appreciates 10 percent this year, that $5,000 will be a 25 percent return on the $20,000 you plan to invest in it. If you don't buy a house soon, and continue renting, inflation will continue to outpace the bank interest paid on your $90,000.

DEAR BOB: Recently you advised an elderly couple to see an estate-planning attorney about disposing of their property after their deaths. Why didn't you suggest that they use a living trust? Most attorneys discourage them because they avoid probate, thus saving attorney's fees at death. Edwin W., McLean.

DEAR EDWIN: Yes, living trusts can save substantial probate costs. In addition, they assure that the intended beneficiary receives the property.

While living trusts may save death taxes, that is not their primary purpose. Many estate palnning attorneys recommend living trusts for their client's major assets, such as reat estate. But some don't suggest living trusts either because they don't understand them or they want to handle the legal work on as large as estate as possible.

For more information on living trusts, read Norman Dacey's book, "How to Avoid Probate," which is available at larger libraries and book-stores. While I don't recommend his do-it-yourself approach to living trusts, he does have some excellent ideas for laymen and attorneys.

DEAR BOB: I own a lot on which I want to build my home. If I handle the subcontracting of the foundation, plumbing, wiring, framing, etc., how much will I save? Barry N., Fredericksburg, Va.

DEAR BARRY: The average home-builder's profit and overhead, according to the National Association of Home Builders, is about 12 percent.

While you might save 12 percent of the cost of your new home, you could lose far more than this if you make one costly mistake. Don't risk it. Unless you have many years of experience as a contractor, it's usually cheaper and easier to turn the job over to a general contractor.

To get the best price, ask for bids on your plans from at least three builders. By the way, you'll find that many banks and savings associations won't finance a do-it-yourself home building project as they know there are too many pitfalls when amateurs try it.

Readers who want a copy of "How to Avoid Tax and Put Cash in Your Pocket When Selling Your Residence" should send 25 cents plus a STAMPED self-addressed envelope to Robert J. Bruss, P. O. Box 6710, San Francisco, Calif. 94101.