One of the nation's largest residential apartment developers agreed yesterday to survey and, if needed, retrofit thousands of apartments in 71 buildings across the country to settle charges that its properties are not accessible to the physically handicapped.

Plaintiffs said the settlement, reached in U.S. District Court in Baltimore, is the most far-reaching to date in any case alleging that construction defects violate the rights of the disabled. They said 12,000 apartments would have to be surveyed, and they predicted the cost of the modifications would exceed $20 million.

The company, Archstone-Smith, said it had not determined the cost of that work. The company said in a statement that it would try to recover a substantial portion of that expense from architects, engineers and builders it relied on for assurances that building design and construction complied with legal requirements.

Archstone-Smith also agreed to pay $1.4 million to the three disability organizations that filed the lawsuit in December: the Equal Rights Center, the American Association of People With Disabilities and the United Spinal Association.

The company, incorporated in Maryland but headquartered in Colorado, is the nation's seventh-largest apartment owner, with more than 79,000 units in 231 complexes across the country.

Archstone-Smith was formed in a 2001 merger in which Archstone Communities Trust acquired Charles E. Smith Residential Realty, which was perhaps best known for building the office and residential towers of Crystal City. Among the local apartment complexes covered in the agreement reached yesterday: Archstone Reston Landing and Arlington Courthouse Place in Virginia, Archstone Bowie Town Center in Maryland, and the Park Connecticut in the District.

The lawsuit resulted from an investigation by the Equal Rights Center into Archstone-Smith's complexes in five states, including Maryland and Virginia, as well as the District. According to the lawsuit, the organization found doorways that were too narrow, kitchens with insufficient turning space and other defects in 29 of 30 properties examined.

Sara K. Pratt, a fair housing consultant and former director of enforcement at the Department of Housing and Urban Development, said the case was closely watched in the building industry.

"The scope of the relief is what's significant, how many properties, how many units are affected," she said. "This settlement will result in increasing the stock of accessible housing for people with disabilities across the country significantly."

The housing rights of the disabled are protected in the Fair Housing Act, amended in 1988, and in the Americans with Disabilities Act, which was passed in 1990. Pratt said as many as 60 percent of properties built since 1991 do not comply with the law.

"I think builders just don't have it on their radar screen that they need to comply," said Isabelle M. Thabault, a lawyer for the plaintiffs and director of the Fair Housing Project at the Washington Lawyers' Committee for Civil Rights and Urban Affairs.

Roderic V.O. Boggs, executive director of the lawyers' committee and co-counsel in the litigation, called the deal "historic both in the sheer number of apartment complexes and apartments covered, but also in the monetary damages the defendant will pay to correct the wrong that has been done in building inaccessible housing."

Richard Ben-Veniste, a lawyer for Archstone-Smith, said the settlement was "a fair one which allows Archstone three years to make the kinds of modifications reflected in the parties' agreement, and does not preclude Archstone from selling" any of the properties.

He said the number of units that will require work is "very unlikely to be" equal to the number surveyed. "That's only the theoretical number," he said of the 12,000.

The lawsuit alleged that Archstone-Smith had discriminated against the disabled in more than 100 apartment complexes in 18 states, including Virginia and Maryland, and in the District. When the suit was filed, Joseph M. Sellers, another plaintiffs' lawyer, said the company had "perpetrated a cruel hoax" by erecting dozens of buildings that were inaccessible to people with disabilities.

Yesterday, Sellers praised the company for "agreeing so promptly and fully" to correct the construction defects.