William H. Hussmann, Montgomery County's chief administrative officer, said yesterday he will resign in December to work for Kettler Brothers, Inc., the building firm that developed Montgomery Village in Gaithersburg.

Hussmann will work as an assistant to a vice president, helping manage construction, land development, sales and purchasing.

Hussmann has worked for the county government for 11 years and the City of Rockville for seven years. His career in government started in 1960, when he became a draftsman for the City of Rockville. He became Rockville's planning director in 1963.

He bacame a program coordinator for the county in 1967, acting as a liason between the county and the Maryland Park and Planning Commission.

In 1970, when County Executive James P. Gleason assumed office, Gleason appointed Hussmann deputy chief administrative officer, in charge of the county's 30 departments and 5,000 employes.

He said he decided to resign because "it was time for a change" and because his job will end in December, when Gleason leaves office.

He said he did not see his new job as a conflict of interest. "I haven't dealt with any issues that created or denied opportunities for Kettler Brothers," he said.