Complex sentence: AFSC first had its headquarters in the Friends Institute, but expanded into the meeting house, building offices in the balcony of the main meeting room in the 1930s, and occupying them until 1960.
Split into simpler sentences: AFSC first had its headquarters in the Friends Institute, but soon expanded into the meeting house. In the 1930s, the balcony of the main meeting room was divided into offices for AFSC, which it occupied until 1960.