A tiny flame from a candle touched papers that melted vinyl siding and set off a fire that raced unnoticed up three floors. The blaze torched 18 condominiums, left three people dead and forced a man on fire to leap from a third-floor balcony.

Beyond the devastating personal tragedy, the fire in Fairfax County last weekend also highlighted a little-known danger in the Washington region and across the county, fire safety officials say: Houses are built too close together. Radiant heat from the fire in the Kingstowne section of the county nearly set ablaze another building 34 feet away.

"It was about ready to go over there, very close," said Fairfax fire and rescue's Peter J. Michel, lead investigator in the fatal blaze. "I'm surprised we only lost the three the other day."

Michel and other fire officials fear that entire blocks of houses could erupt in flames in a serious fire. National building codes allow single-family houses to be built just six feet apart. Increasing development pressure and the scarcity of land in metropolitan areas are resulting in more -- and larger -- houses built to minimum spacing standards.

Building suburban-style houses at an urban density could cause conflagrations that could devastate whole neighborhoods, fire officials warn.

"It's just a matter of time before we have big fires here," Michel said. "They're building them right on top of each other."

Fire officials in Virginia, alarmed by recent blazes that have moved swiftly from one house to the next, have been at the forefront of efforts to strengthen national standards by forcing houses to be built farther apart or requiring such additional protections as fire walls. They are seeking to make the International Residential Code, used in the District, Virginia, Maryland and 41 other states, at least as strong as regulations adopted recently in Virginia.

The National Institute of Standards and Technology also is concerned and today will conduct a series of test burns at the federal agency's facility in Gaithersburg. The agency hopes to use the data to create computer models to help fire officials decide how close to one another houses can be built safely.

"If one house has a fire, it shouldn't affect the neighbor's house. It certainly shouldn't affect the neighbor's neighbor's house," said Battalion Chief L. Ray Scott of Prince William County. That is what happened in Loudoun County in 1994, when three houses -- each six feet apart -- went up in flames before the first firetruck arrived. In January, a Gainesville house fire spread quickly to the house next door, 11.4 feet away.

Scott pushed successfully for new rules from the Virginia Board of Housing that will force developers to build houses 10 feet apart or build a fire wall. The rules will go into effect this year, after they are published in the Virginia Registry and after a public comment period.

Some fire experts say planners and political leaders in U.S. suburbs have been lulled into complacency by decreasing numbers of fires and fire deaths. The success is largely a result of smoke detectors, sprinklers, safer electrical wiring and better stairwell and exit design.

Industry groups say there are no data to support the contention that building houses farther apart would prevent the spread of fires. They say that houses today are safer than ever. Indeed, the number of fire fatalities in the United States has dropped from 3,825 in 1993 to 2,695 in 2002, according to the U.S. Fire Administration.

But reliance on newer materials and technology is resulting in buildings that some say are less sturdy than houses of the past. Using vinyl siding instead of stucco or masonry, smaller and drier timber, and home furnishings made of plastics and chemicals that create a toxic smoke when burned make newer houses more dangerous, some experts say.

"We are building more and more combustible buildings and building them closer together," said Vincent Brannigan, a professor at the University of Maryland's department of fire protection engineering. "We are pushing all of the envelopes at the same time."

With house and land prices continuing to rise, cluster houses on smaller lots often are more affordable. Linda Canadiate, for example, said she loves her house in a new subdivision in Woodbridge so much that she treasures a framed photo of herself and her family next to the "sold" sign. She enjoys sitting in her rocking chair in the setting sun and saying hello to her neighbors.

She doesn't have to raise her voice. Next door is just 11.9 feet away.

"Kissin' close," Canadiate said.

She said she doesn't smoke and takes other precautions against fires, such as storing flammable materials in the garage. But she also has to worry about whether her neighbors are equally careful.

"They're building them too fast and too close," she said.

The home-building industry emphasized that improvements are costly. "We can build a house that will never burn down," said Jeff Inks, who monitors housing codes for the National Association of Homebuilders. "Maybe Donald Trump or the average billionaire could afford it, but the average guy couldn't."

Cozy neighborhoods where houses are closer together also are part of the "new urbanism" movement that aims to build suburban villages where people can walk instead of drive.

"People are interested in having less yard and living in a more compact environment," said Sherman Patrick, Prince William's zoning administrator. "It's a desire to provide an alternative for townhouses and a new urban design that people like. The challenge is how do we allow and encourage new designs for usable open space and at the same time not run afoul of the building code?"

In some cases, townhouses are safer than close-together single-family houses, because attached townhouses are required to be built with firewalls separating the units. "We'd be better off with two townhouses because you don't have that air and space separation," said W. Keith Brower Jr., deputy fire chief in Loudoun County.

How close is too close is a question without a scientific answer. But the subject is of interest to federal officials. For the fire test scheduled for today, National Institute of Standards and Technology scientists in Gaithersburg have built two average "homes" six feet apart. Each has a wood frame covered with a weather wrap, such as Tyvek, and is finished with vinyl siding. One will be furnished with a sofa, table, armchair, wall paneling and carpet.

"Then we're going to start an ordinary living room fire, which will break out through the window, and then we're going to measure the time needed to get the second unit involved," said David D. Evans, an institute engineer.

It shouldn't take long. A 1988 Canadian study of two similar structures 5.9 feet apart showed that it took 4 minutes and 50 seconds for the second house to ignite. Thirty seconds later, the second house was fully engulfed.

Evans has studied the fire response at the World Trade Center on Sept. 11, 2001, and has researched the behavior of wildfires when they hit residential communities. He said it is unlikely that a single "safe" distance between houses will be determined.

"That's a sharp line, and physics is rarely that sharply defined," Evans said. Variables come into play, including materials that compose the house, its design and how long it takes a firetruck to reach the house.

Scott, also a former Prince William fire marshal, said he has been told that he is scaring people. But he said that is not important.

"All I want is for them to live safely," Scott said, "and for the people who are building these things to know the risk.''