The Federal Trade Commission, as part of its campaign to improve real estate compliance with the Truth in Lending Act, said yesterday that one of the nation's largest home builders has agreed to pay $300,000 in civil penalties to settle charges that it violated credit advertising laws.

Nash Phillips/Copus Inc., based in Austin, Tex., agreed to the fine in a consent decree filed in federal court yesterday, the FTC said. Nash Phillips is the nation's 13th-largest home building company and the largest privately owned home builder, the agency said. The consent decree does not constitute an admission of guilt by the company.

To improve voluntary compliance, the FTC said it has monitored newspaper ads in 42 cities, contacting more than 4,000 advertisers to notify them of credit law violations in their ads. As a result, compliance in those cities increased to 88 percent from the initial level of less than 20 percent, the agency said.

In February 1984, the FTC said it would investigate those companies that had been contacted but continued their allegedly illegal advertising practices.

The FTC complaint said Nash Phillips' ads violated the law by offering credit for houses without disclosing required information. For example, the FTC said, one ad stated only a reduced first-year interest rate of 7 1/2 percent, without showing the much higher annual percentage rate over the life of the loan.