Jay Janis, under secretary for Housing and Urban Development, was named the Carter administration's choice of the Federal Home Loan Bank Board yesterday. According to sources, Carter made the decision Monday.

A former Florida home builder, Janis, 46, has a reputation for understanding the home building and mortgage finance industries and for strong management and leadership capabilities, according to Vondal Gravlee, president of the National Association of Home Builders.

"This is a very important position in terms of the great challenges in working with the savings and loan associations, the home building industry and providing shelter for Americans," said Janis.

However, he hesitated to discuss the direction he plans to take as chairman of the three-member board until the Senate reconvenes for confirmation hearings in September.

Joseph T. Benedict, president of the U.S. League of Savings Associations, said President Carter "is honoring a widely recognized public service" with the nomination.

"We are most impressed with his knowledge, particularly in urban problems," said Benedict, who also lauded Janis for his support fo new kinds of mortgages and his efforts at HUD to revitalized federal housing programs.

Janis would join the board at a critical time for the housing industry. Not only is mortgage money tight as interest rates approach 11 percent, but a regulation allowing savings and loans to pay one-quarter percent more interest on savings than commercial banks reportedly is being phased out.

Janis would succeed Robert McKinney, who resigned June 1 for personal reasons to return to his home in Indiana.

Acting board Chairman Anita Miller said Janis' "dedication to excellencec will assist the board's efforts to help savers, home buyers, and housing and thrift industries.

After coming to Washington in 1965, he served as special assistant and director of equal opportunity in the Commerce Department. He was executive assistant to the secretary of HUD from 1966 to 1969.

During the next six years, Janis was a co-principal for a Florida building and community development company before being named vice president of the University of Massachusetts.

Janis, who is from Los Angeles, served on the board of the National Association of Home Builders.

The bank board governs 2,000 federally chartered savings and loans institutions, and its actions also affect about 2,000 federally insured thrift institutions.

Directed to help supply money for home mortgages, the board's Federal Savings and Loans Insurance Corp. insures savings accounts of more than 75 million Americans for$40,000. There are 12 regional home loan banks.