Complex sentence: It became the headquarters of East Suffolk County Council in 1906 and the headquarters the headquarters of Suffolk County Council in 1974before they decided to move out of the building to more modern premises in 2003.
Split into simpler sentences: It became the headquarters of East Suffolk County Council in 1906. In 1974 it became the headquarters of Suffolk County Council who decided to move out of the building to more modern premises in 2003.