James Biddle said yesterday that he intends to resign as president of the National Trust for Historic Preservation. He has held the post since Jan. 1, 1968. At the same time, it was learned, some 20 to 30 lower level staff employees have been laid off.

Biddle said, in a letter to the Board of Trustees that he found "my life is centering more and more in Philadelphia, with my many personal commitments there." The Biddle family home, an early example of 19th century Greek Revival architecture, is in Bucks County near Philadelphia. No date was set for his resignation.

Before coming to the Trust, Biddle was curator of the Metropolitan Museum of Art's American Wing. Biddle is 50 years old.

The staff cuts at the Trust are said to reflect a shift in emphasis from the national to regional Trust offices. The Trust just recently has bought and renovated the historic Mellon apartment building just off Dupont Circle for its offices.

During Biddle's presidency, the Trust has grown from 13,000 members to more than 155,000 served by a staff of some 200 here and in the regional offices.

The National Trust owns and cares for historic houses and sites, gives advice to others interested in local preservation projects, and publishes a recently enlarged magazine on preservation. Its money comes from government grants, private contributions, entry fees to the various houses and membership fees.