DEAR BOB: We're having trouble selling our home. It is located on a busy state highway. We've had it listed with two different brokers for more than five months. Any ideas? Singleton A., McLean.

DEAR SINGLETON: Suggest to your real estate agent that he or she advertise your home on a lease with option to purchase later. A six- or 12-month lease, with full credit for rent paid toward the purchase price, is hard for a tenant to resist. The agent's commission is due if the option is exercised.

Lease-options enable many tenants who lack down payment cash to buy their own home. The rent paid credit, in other words, is like a down payment. After paying rent for 12 months, it's almost impossible for a tenant to walk away and not exercise such an attractive purchase option.

DEAR BOB: We own a lot at a summer resort. Last winter a small group of the area's year-round residents changed the zoning on our lot. To get a building permit, we now need six times the size formerly required. What can we do? R.C., Tantallon.

DEAR R. C.: See an attorney. Proposed zoning changes usually require notification of affected property owners. If you weren't properly notified, the zoning change may not be valid as to your property.

DEAR BOB: I want to buy a house that will be an "estate sale." It needs many repairs. Before I make my offer, I want to ask the seller if he will take back a $20,000 mortgage and if I can have an inspector check the house before I make my offer. Do I need a lawyer to do this? G.J., Washington, D.C.

DEAR G.J.: No. Ask for permission to have the house inspected so you can estimate the repair costs. Then make your purchase offer, including the mortgage terms you want. A lawyer can help you write the offer. Don't ask the seller if he will carry the mortgage. Put it in your offer. Negotiate from there if the seller doesn't accept your offer.

DEAR BOB: In 1977 I sold the house in Connecticut where I lived seven months each year. My accountant told me I couldn't use that "over-65 rule" tax break because Connecticut wasn't my voting residence. True? Cecilia J., Upper Marlboro.

DEAR CECILIA: Not always. "Principal residence" tax breaks apply to both the "over-65 rule" and the "residence replacement rule." Many factors are weighed in determining your principal residence (if you own more than one residence), such as voting place, time spent at each residence, years of ownership, and, most improtantly, your state of mind as to your primary home.

All the factors should be weighed when deciding which is your principal residence. Then hope the IRS agrees with your decision.

The 13-chapter Bruss Report "Realty Tax Tips" is available for $1 sent to Robert J. Bruss, P.O. Box 6710, San Francisco, Calif. 94101.