WASHINGTON (AP) _ President Reagan plans to nominate Philip D. Winn to be ambassador to Switzerland, succeeding Faith Ryan Whittlesey, the White House announced Tuesday.

Since 1987, Winn has served as chairman of the board of the Philips Development Corp. in Englewood, Colo. He served as chairman of the board for Philip D. Winn & Associates Inc. from 1976-87. From 1981-82, he was assistant secretary at the Department of Housing and Urban Development and was at one time the federal housing commissioner.

Winn, 63, is a graduate of the University of Michigan and native of New Britain, Conn.

Whittlesey is resigning after four years as the ambassador to Bern.

The White House also announced Reagan's intention to nominate Richard Llewellan Williams, a career foreign service officer, to be ambassador to Mongolia. Diplomatic relations were established at the ambassadorial level with the Mongolian People's Republic in January 1987.

Williams, 58, currently is director of the Office of Chinese and Mongolian Affairs at the State Department. Previously he served as deputy counsul general in Hong Kong from 1981-85 and as consul general in Canton.