General Definition

Accessory Use refers to a secondary space within a primary occupancy that supports the main function of the building but does not dominate the overall use.

• Both the International Building Code (IBC) and Japan’s Building Standard Law (BSL) regulate fire safety, egress, and structural requirements for accessory spaces.
  • However, BSL enforces stricter fireproofing (耐火建築物), compartmentalization, and seismic design, particularly in high-rise and mixed-use developments.

IBC Classification & Requirements for Accessory Use

• The IBC defines Accessory Use Areas (IBC 508.2) as spaces that:
  • Are ancillary to the primary occupancy and do not exceed 10% of the total building area.
  • Do not require fire-rated separation from the main occupancy.

Common Accessory Use Spaces in IBC:

1. Office Space in a Factory or Warehouse – A small administrative area inside a Group F or S building.
2. Storage Rooms in an Office or School – Small rooms storing equipment or supplies within a Group B or E building.
3. Retail Space in a Hotel or Office Building – A gift shop inside a Group R-1 or B building.
4. Kitchen or Cafeteria in a Hospital or Office Building – Food service areas supporting a Group I-2 or B occupancy.
5. Conference Room in a Warehouse or Factory – Meeting space inside a Group S or F occupancy.

IBC Fire Safety & Exiting Requirements for Accessory Use:

• No fire-rated separation required if the area is ≤10% of the total building area.
• Fire sprinklers required if the main occupancy requires sprinklers.
• Exiting requirements are based on the main occupancy.

BSL Classification & Requirements for Accessory Use (附属建築物 – Fuzoku Kenchikubutsu)

• The Building Standard Law of Japan (BSL) classifies accessory use areas as 附属建築物 (Fuzoku Kenchikubutsu, Accessory Buildings) when detached or 付属室 (Fuzoku Shitsu, Accessory Rooms) when within the primary building.
  • Common Accessory Use Spaces in BSL:

1. 事務室 (Jimushitsu) – Office Space in a Factory or Warehouse – An administrative area in an industrial building.
2. 倉庫 (Sōko) – Storage Rooms in Offices & Schools – A storage space within a Group B or E occupancy.
3. 売店 (Baiten) – Small Retail Areas – Shops inside hotels, hospitals, or large offices.
4. 厨房 (Chūbō) – Kitchens & Cafeterias – Food service within a hospital or office.
5. 会議室 (Kaigishitsu) – Conference Rooms – Meeting areas inside industrial or office buildings.

BSL Fire Safety & Exiting Requirements for Accessory Use:

• Fireproofing (耐火建築物) required for attached accessory uses in high-rise buildings.
• Fire-rated separation may be required if the accessory use involves hazardous activities (e.g., kitchens).
• Egress must comply with the primary occupancy but may require additional exits for large spaces.

Key Differences:

• BSL regulates accessory spaces more strictly, often requiring fireproofing (耐火建築物) for larger mixed-use buildings.
• IBC allows accessory spaces to be integrated without fire separation if they are ≤10% of the building area, while BSL may require compartmentalization.
• Seismic design is a greater priority in BSL for accessory spaces in high-rise and mixed-use buildings.

IBC vs. BSL – Summary of Accessory Use Requirements

Summary

• IBC allows accessory uses to be integrated without fire-rated separation if they occupy ≤10% of the total building area.
• BSL regulates accessory spaces more strictly, often requiring fireproofing (耐火建築物) for high-rises and mixed-use developments.
• Seismic design is a greater priority in BSL for accessory spaces, particularly in high-rise and transit-integrated developments.
• Both codes allow flexibility for small accessory uses but prioritize fire safety in high-risk spaces like kitchens and hazardous storage.