It is not unusual these days for the crews with Rand Construction Corp. of Arlington to be watched and followed.

When the crews are working on some projects in Washington, the customer assigns them an escort with a security clearance. If a worker needs to go to a truck to get a hammer, the entire crew, plus escort, trudges down to the truck together. All so no worker is left alone near a classified information site.

Since the Sept. 11, 2001, terrorist attacks, putting up buildings in Washington for government departments or private government contractors has included such an emphasis on security, said Matt Merz, senior project manager with Rand, that "most of the time, we don't even know what the space will be used for."

At least one third of the 14 million square feet of office space now under construction in the Washington area is being built directly for government space or for a government contractor, according to Delta Associates, an Alexandria real estate research firm. Five years ago, about 20 percent of the 16.3 million square feet of office space under construction was government space.

Background checks, escorts, X-ray scanning of trucks, even hiring only U.S. citizens so they can get required security clearances are routine for Rand and other general contractors in the Washington area. While many of the rules have been in place for years, most have been stringently followed only since the terrorist attacks, company officials say.

The renewed emphasis has forced many construction companies, as well as other Washington area businesses, to change the way they hire and do business.

Don Owen, president of P&P Contractors Inc. of Rockville, said he is working on renovations to part of the Pentagon, where his employees have to go through criminal background checks every six months. But he knows he must hire U.S. citizens if he expects to get work in new Homeland Security Department buildings, because the department now often requires clearances for all workers on its sites.

"Like at our headquarters, if you're working in a classified area, you have to have a security clearance," said Katy Mynster, a Homeland Security spokeswoman.

Security clearances are limited to citizens, and an increasing number of sites are requiring clearances at some point in a project, contractors say. That is no easy task for an industry dependent on immigrants and day laborers.

Owen's workers, mostly drywaller installers, are largely from Central America. "A lot of those people have green cards, and they're beginning the process of the American dream, and that's great. But they're not able to work on all this Homeland Security," he said.

Construction workers are required in some cases to be cleared, or to be citizens, because many buildings now have redundant computer systems or separate ventilation systems in case of another terrorist attack. The security clearances would help ensure a terrorist does not learn about or get access to those systems, said Steven C. Smith, acting director for the center of construction and project management at the General Services Administration.

Tight security has always been part of construction in Washington, though in the old days the intent was on keeping out spies, not terrorists. Buildings housing classified materials have long had to be built to exacting specifications for SCIFs (sensitive compartmented information facilities), done by workmen who typically have top-secret security clearances to reduce the chance that bugging devices will be installed.

Employees at the Madison Hotel, several blocks from the White House, have always been subjected to background checks when dignitaries come through, a hotel official said. And places like Capital Party Rentals, which provides the tents, tables and other party equipment for galas around town, requires that only citizens be event site coordinators because some government customers will require that all employees on site have a clearance.

Sigal Construction Corp. employees have had to go through security checks for as long as the company has done construction for the government. But "there are a lot more secure jobs than there used to be," said President Sean O'Day. The policy on clearances is left up to the client agency, depending on whether the construction worker would have access to classified information or sites where it is stored, according to officials at the Defense Security Service, the department that processes clearances.

The Supreme Court is one of those government construction sites with tight security today, according to Smith, of the GSA.

It was "the desire of the court," he said, that only select workers with security clearances be allowed to see the blueprints for the renovation. When contractors work on a justice's personal office, personal papers will have to be cleared off desks, and an escort will be with the workers at all times, Smith added.

Security is also tight at the site of the new Capitol Visitor Center. Last year, 5,000 construction workers and 86,000 vehicles were screened before entering the underground site, according to the project spokesman, Tom Fontana. The footprint of the site is larger than the Capitol, he said, so security is tight for obvious reasons: Trucks, much like the one used in the Oklahoma City bombing, come onto the site daily to deliver materials and workers.

Knowing the more stringent security measures are the norm, businesses are learning to adapt.

When Care of Trees Inc. has a job on Capitol Hill, the tree preservation company prepares to add another hour onto its morning. Workers are run through a security check and wear badges. The company tells Capitol Police before its crews arrive exactly who will be there. In the past couple of years, a round of morning truck X-raying has been added to Care of Trees' agenda. All trucks are scanned at a checkpoint, and then they have to go through another check before they can park and get to work.

"We never had to get trucks through checkpoints down there. It kind of changes everything we do," said Dave Zeitlin, division manager for tree preservation. "Does it impact us? Yeah. I have to spend an extra hour each time we go through a Senate building. But that's part of doing business in Washington, D.C. And we want to do business in Washington, D.C."

Last year, 5,000 construction workers and 86,000 vehicles were screened before being allowed to enter the Capitol Visitors Center construction site, the project's spokesman said.