Housing and Urban Development Secretary Samuel R. Pierce Jr., citing "exceptional circumstances," gave himself special permission to fly first class on 19 foreign and domestic trips over the last two years.

Pierce's decision to fly first class on more than a third of his trips has cost the government thousands of dollars in higher fares. The first-class flights include visits to Europe and Asia, as well as Los Angeles, San Francisco, Las Vegas, Hilton Head, S.C., and Anchorage, Alaska.

In addition, Pierce's hotel and travel expenses often have been paid by groups of bankers, builders and municipal officials who do business with HUD or are regulated by the department. Several other senior HUD officials also have allowed trade groups to pay some of their travel expenses on official trips.

Pierce signed a statement each time he exempted himself from the rule that federal employes generally must travel in coach class. He cited "exceptional circumstances to ensure successful performance of agency mission as determined by the secretary. Review of official confidential and sensitive material in preparation for meetings which must be accomplished en route and cannot effectively be done in coach accommodations."

One first-class flight to Paris, Bonn, Copenhagen and Sweden cost the agency $2,709. Another first-class trip last summer to China, Japan, Hong Kong, Egypt, Singapore, Austria and Germany cost $4,435. And Pierce's first-class domestic travel included a $1,278 flight to San Francisco and a $684 visit to Dallas.

Leonard Burchman, a spokesman for Pierce, said the roomier accommodations often are necessary. "The man carries bundles of materials that he reviews," he said. "He spreads himself out. He needs that extra space. And you have a little more security in first class."

Burchman said Pierce is an attorney who could command a far higher salary in private practice. He added that Pierce "is a workaholic. He never gets on a plane and just sits and rests. He's always answering correspondence and making notes. He very rarely gets out of his hotel room."

A spot check of federal agencies found that other Cabinet secretaries rarely travel first class. Education Secretary Terrel H. Bell, for example, always flies coach, according to a spokesman.

One exception is Attorney General William French Smith, who generally travels first class. A spokesman said Smith's FBI protective detail had requested this for security reasons.

Pierce's frequent trips come at a time of substantial cutbacks in HUD's budget for subsidized housing and community development programs. President Reagan has proposed to cut the agency's budget further in fiscal 1984 from $14.8 billion to $13.7 billion.

HUD Undersecretary Donald I. Hovde has encouraged senior officials to accept travel expenses from industry trade groups, even if they do business with the agency. Employes are prohibited from accepting expenses only from individual companies or organizations that have dealings with HUD. The general counsel's office generally approves any outside reimbursement for Pierce's trips.

In October, 1981, for example, the National Bankers Association paid for Pierce's stay at San Francisco's Hyatt Regency Hotel. A spokesman for the bankers' group said he had no record of expenses at the annual meeting, but that hotels generally provide a number of free rooms for large conventions.

On other occasions, the National Association of Market Developers paid for Pierce to visit Philadelphia, the Mortgage Bankers Association paid for his hotel and meals in New Orleans, and the National Association of Housing and Rehabilitation Officials paid for his hotel in Colorado Springs.

In similar fashion, Time Inc. paid for Pierce's hotel and flight to the Bahamas for a housing conference sponsored by the company. The U.S. Conference of Mayors paid for his hotel in Minneapolis, the Municipal Association of South Carolina paid for his stay in Hilton Head, S.C., and the Heritage Foundation paid for his travel and hotel in Chicago.

Pierce's trips to Europe and Asia are among a number of foreign trips by top officials at HUD, an agency concerned mainly with domestic problems. For example, Pierce's executive assistant, Lance Wilson, has gone to the Bahamas, where the National Association of Real Estate Brokers paid for his hotel.

Antonio Monroig, assistant secretary for fair housing, went to Nova Scotia, Canada, as the guest of the International Association of Human Rights Agencies. Hovde has gone to London, Stockholm and Paris at HUD's expense and to Milan, Italy, as a guest of the International Real Estate Federation.