ほんかん本館honkan
1.
The primary structure within a complex, often serving as the central hub that distinguishes it from auxiliary buildings like annexes or new constructions.
2.
The original and principal edifice in a setting, typically housing the main functions of an institution such as a library or museum.
3.
The main facility that hosts the essential activities and resources of an organization, contrasting with additional or newer facilities.
Example:
別館は本館の北側にある。