Centex Homes Inc., a Texas-based home building firm, is performing an estimated C $500,000 in major surgery to repair structural defects on nearly 50 houses out of 200 it built three years ago in the Langley Oaks subdivision in McLean.

Carpenters are slicing through four inner walls in about 25 Cape Cod-style "Covington" model houses to extract inadequate wooden support posts and replace them with steel posts, Fairfax County building inspectors said. Workers also are beefing up basement support poles in the $200,000 houses--for a total cost of between $10,000 and $20,000 a job, homeowners in the subdivision estimate. Local Centex spokesmen would not comment on the project.

Building officials in Arlington, Montgomery and Prince George's counties say they have no record of similar problems with Centex homes, and Fairfax building officials say such problems are rare, normally occurring in "much less than one percent" of all new homes.

Although at least one Covington home had cracks in the walls that may have been caused by the post defect, building inspectors said that most of the affected homeowners saw no evidence of the structural defect. The inspectors anticipate that the remaining eight to 10 flawed houses will be retrofitted in two months.

Earlier, Centex workers replaced basement support beams in about 25 colonial-style "Cameron" model houses, correcting a beam misplacement that was causing floors to buckle in some of the homes, inspectors said.

"We were all fairly horrified when we heard about this [building error]," said one owner of a Covington house in Langley Oaks who says she now is "extremely impressed" with the quality of the work done by the repair workers. The crews completely restore the houses, said one homeowner, pointing out that the finishers sometimes have to locate matching wallpaper to replace what they've torn off the walls.

Other Langley Oaks homeowners, though annoyed by the disruption and mess caused by the repair crews, generally say they are pleased with the quality of the rehabilitation.

Centex building companies constructed 8,000 homes nationwide in 1978, the year Langley Oaks was started, according to company officials, who say they will build about 6,000 units this year. Centex has stopped building homes in the Langley Oaks subdivision, but several other companies are constructing houses there, county officials said.

Joseph Bertoni, chief building inspector for Fairfax County, said the Langley Oaks error was discovered by a prospective homeowner who noticed cracks in the wall and called in a private engineer, who found what he suspected was the problem. The engineer notified Bertoni, who notified Centex. Soon after Centex officials hired engineers who pinpointed the deficient support post.

Bertoni said he then issued a building violation notice to Centex, but says it was merely procedural and that the corporation's patch-up effort was voluntary. "Morally, I think that they felt it was the right thing to do," he said.

The complex Covington problem, Bertoni said, focused on four support posts, each of which was designed to be made of eight side-by-side two-by-four timbers, which essentially held up the top floor of the house, Bertoni said. However, he explained, the builders in many cases installed only five or six timbers per post and further, subconstractors weakened some of the posts by drilling holes in them to accommodate plumbing and electrical connections. Although Centex supervised the subcontracting, they would not comment on why the weakening of the timbers went undetected.

The underdesigned wood posts may compress somewhat under heavy loads and result in floor sags and wall cracks, said Richard Lawson, deputy director of inspection services for Fairfax County. "One thing that I do know is none of those houses are in danger of collapse or anything," he said.

Centex officials would not place blame for the design error. Smith said the company has an architectural staff in Texas but sometimes uses outside architects to design homes, which he said vary greatly in style from region to region.

One Langley Oaks homeowner, indicating his belief that the structural errors should have been detected during construction, said, "I could raise some questions about the inspector quality."

Bertoni, however, said the problem essentially was a design error and contended that "you probably really need a trained engineer" to catch the problems. He said the wood post could have caused problems even if it had been built with the called-for number of timbers.

Lawson said county building inspectors are required to examine building plans but said "if they're prepared by a good architect or engineer, we don't look at them really hard." Although he's had a staff meeting to alert employes to errors like those that caused the Langley Oaks problem, Lawson said that at this point "we still do not have plans to hire an engineer."

Lawson recommends that home buyers who are suspicious of major problems either call the county building department for an inspection or engage a private inspection firm, some of which, he said, employ trained engineers.

Fairfax County consumers affairs employes have compiled a list of area home inspection firms, Lawson said.