The absence of sprinklers in the L'Enfant Plaza building that burned this week causing about $100 million in damage to the U.S. Postal Service headquarters and injuring 25 firefighters, raises serious questionns for consumers:

Should your residence be equipped with a sprinkler system? How much would it cost to install? How effective would it be?

Fire safety officials say sprinklers could reduce losses from home fires that kill about 6,000 people every year and cause an estimated $1.1 billion in damage. But the home-building industry, while acknowledging that test results sound good, is waiting for a cost-benefit analysis before taking a position on whether sprinklers should be routinely built into new homes.

"Without that [information], the picture is somewhat incomplete," said Bowen (Chip) Hosford, an analyst with the National Association of Home Builders' research foundation.

The thinking is that housing already is so expensive that it is beyond the reach of many consumers. Higher prices would exacerbate the problem.

But the U.S. Fire Administration, a federal agency that now plans to spend $2.5 million in fiscal 1985 promoting the use of sprinklers in one- and two-family residences, contends that costs should decline as the sprinkler industry grows.

At present, only a handful of U.S. residences have sprinkler systems. In San Clemente, Calif., sprinklers are required in all new construction. Cobb County, Ga., is also at the forefront in developing residential sprinklers.

Each sprinkler head, typically installed in the ceiling, is soldered to hold back the water until it is needed. When there is a fire and the temperature reaches 160 degrees Fahrenheit, the heat melts the solder on the sprinkler head and releases the water. This occurs automatically.

USFA estimates that a sprinkler system adds about 1 percent to the price of a new house or $1,000 for a $100,000 home. The cost of adding a sprinkler system to an existing house is about three times that much. The USFA said that retrofitting the same $100,000 house with a sprinkler system would cost $3,000.

That compares to less than $100 for several smoke detectors that provide warning alarms, but no water.

Still, fire officials view the residential sprinkler as a "life-safety system."

Harry Walsh, director of the USFA office of fire prevention and office control, said that sprinklers would be better for "the old, the young, the incapacitated, those who might be on medication" or when "the fire may have been started by someone under the influence of alcohol."

Walsh said a sprinkler system may extinguish the fire before the flames and poisonous gases have a chance to spread. Moreover, he said, the sprinkler system, by controlling the fire, provides more escape time.

To find out what difference a sprinkler can make, fire officials in Hartford, Conn. this week built two identical structures, each measuring 20 by 14 feet, furnished them as living rooms and started fires in wastebaskets that were part of the decor. One structure had a sprinkler system; the other did not.

In the structure with the sprinkler, the fire caused a minimum amount of damage. The sprinkler was activated in 97 seconds and completely extinguished the flames. Temperatures in the room increased to only 75 degrees, up from 72 degrees before the fire. Carbon monoxide reached a concentration of 300 parts per million.

In the structure with no sprinkler, flames quickly engulfed the furnishings. Temperatures reached 650 degrees in six minutes and carbon monoxide concentration hit a lethal 5,000 parts per million, officials said.