DEAR SAM: We expect to put in a new fireplace with "Heatilator" this summer. Our one-story house stands on a concrete slab. The chimney will be freestanding for approximately half of its 20 feet. With a raised hearth, we will have an ash pit and outside cleanout door. Do you have any suggestions for a trouble-free fireplace?

ANSWER: Ask your local Heatilator dealer for a brochure showing the full details and dimensions, sizes and construction requirements. Ask the dealer for the names of a few masonry contractors experienced with this type of installation.

You may need to check with your local building department about the zoning requirements of your locality, to make sure you have sufficient distance from the lot line. You will also be required to obtain a building permit through your contractor.

You should also determine whether the exposed brick for the new fireplace will match that of the house.

With a raised hearth over the slab, the mason may have a problem of having sufficient pitch for the ash pit and cleanout, since there will be little space below for the accumulation of the ash. He may want to connect a piece of flue lining (8x12 inches) directly from the ash pit to the outside door.

Your chimney should rise at least two feet above the roof ridge. Also, the height of adjoining roof tops should be considered for satisfactory draft conditions. I am sure that an 8x12-inch flue lining will provide the correct draft for a wood-burning fireplace.

If you need further information about the operation and construction of the heatilator, write to the manufacturer at Mount Pleasant, Iowa 52641.

DEAR SAM: I am planning a 16x16-foot addition to the master bedroom that will be partially brick and partially frame to harmonize with the existing style. There will be a footing, crawl space and gable roof, all to tie in with the existing structure. Windows and masonry below windows will be removed for access from the old bedroom to the new space.

Since I want the contractor to provide only the roughing, roofing, framing and outside finish, including one Thermopane window, what's a reasonable price per square foot? I plan to do all the finishing inside, including insulation and walls, plumbing, electrical, heating and painting. It appears to me that $10 per square foot is plenty!

ANSWER: You are actually asking for the foundation and shell of the addition - a three-wall extension to your house. Although this space may be only 256 square feet, the heavy equipment for excavation cannot be proportionate as for a larger job. The same would be applicable for the foundation forms, the trucking away of excess fill, etc.

Another factor is the tie into the existing wall of brick and frame, not only with handwork for the flashing, but the removal of the windows for reuse. Will you be able to secure matching brick or do you plan to salvage the existing bricks, after cleaning them, for the front?

Since you will be required to submit a "plan of addition" to your local building department, this should be prepared first. An architect would be the person to see after you have prepared a small sketch of your proposed addition. Incidentally, I hope you have checked with the building department to make sure your proposed addition will not violate your lot line requirement. You may need 10 to 20 feet from building to lot line.

Finally, even though the architect may help you with a close estimate of the "shell cost," the going rate of construction can only be determined by obtaining several estimates.