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Chapter 4 — SPECIAL DETAILED REQUIREMENTS BASED ON OCCUPANCY AND USE

Section 405 — UNDERGROUND BUILDINGS

2025 California Building Code (Title 24, Part 2) · 2025 edition · updated 2026-07-29 · California

Italicized text is a California amendment to the model code, as printed in the official publication.

405.1 General.

The provisions of Sections 405.2 through 405.9 apply to building spaces having a floor level used for human occupancy more than 30 feet (9144 mm) below the finished floor of the lowest level of exit discharge.

Exceptions: The provisions of Section 405 are not applicable to the following buildings or portions of buildings:

  1. One- and two-family dwellings, sprinklered in accordance with Section 903.3.1.3.

on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.

SPECIAL DETAILED REQUIREMENTS BASED ON OCCUPANCY AND USE

  1. Parking garages provided with automatic sprinkler systems in compliance with Section 405.3.
  2. Fixed guideway transit systems.
  3. Grandstands, bleachers, stadiums, arenas and similar facilities.
  4. Where the lowest story is the only story that would qualify the building as an underground building and has an area not greater than 1,500 square feet (139 m [2] ) and has an occupant load less than 10.
  5. Pumping stations and other similar mechanical spaces intended only for limited periodic use by service or maintenance personnel.

405.2 Construction requirements.

The underground portion of the building shall be of Type I construction.

[F] 405.3 Automatic sprinkler system. The highest level of exit discharge serving the underground portions of the building and all levels below shall be equipped with an automatic sprinkler system installed in accordance with Section 903.3.1.1. Water-flow switches and control valves shall be supervised in accordance with Section 903.4.

405.4 Compartmentation.

Compartmentation shall be in accordance with Sections 405.4.1 through 405.4.3.

405.4.1 Number of compartments. A building having a floor level more than 60 feet (18 288 mm) below the finished floor of the lowest level of exit discharge shall be divided into not fewer than two compartments of approximately equal size. Such compartmentation shall extend through the highest level of exit discharge serving the underground portions of the building and all levels below.

Exception: The lowest story need not be compartmented where the area is not greater than 1,500 square feet (139 m [2] ) and has an occupant load of less than 10.

405.4.2 Smoke barrier penetration. The compartments shall be separated from each other by a smoke barrier in accordance with Section 709. Penetrations between the two compartments shall be limited to plumbing and electrical piping and conduit that are firestopped in accordance with Section 714. Doorways shall be protected by fire door assemblies that comply with Section 716, automatic-closing by smoke detection in accordance with Section 716.2.6.6 and installed in accordance with NFPA 105 and Section 716.2.2.1. Where provided, each compartment shall have an air supply and an exhaust system independent of the other compartments.

405.4.3 Elevators. Where elevators are provided, each compartment shall have direct access to an elevator. Where an elevator serves more than one compartment, an enclosed elevator lobby shall be provided and shall be separated from each compartment by a smoke barrier in accordance with Section 709. Doorways in the smoke barrier shall be protected by fire door assemblies that comply with Section 716, shall comply with the smoke and draft control assembly requirements of Section 716.2.2.1 with the UL 1784 test conducted without an artificial bottom seal, and shall be automatic-closing by smoke detection in accordance with Section 716.2.6.6.

405.5 Smoke control system.

A smoke control system shall be provided in accordance with Sections 405.5.1 and 405.5.2.

405.5.1 Control system. A smoke control system is required to control the migration of products of combustion in accordance with Section 909 and the provisions of this section. Smoke control shall restrict movement of smoke to the general area of fire origin and maintain means of egress in a usable condition.

405.5.2 Compartment smoke control system. Where compartmentation is required, each compartment shall have an independent smoke control system. The system shall be automatically activated and capable of manual operation in accordance with Sections 907.2.18 and 907.2.19.

[F] 405.6 Fire alarm systems. A fire alarm system shall be provided where required by Sections 907.2.18 and 907.2.19.

[BE] 405.7 Means of egress. Means of egress shall be in accordance with Sections 405.7.1 and 405.7.2.

[BE] 405.7.1 Number of exits. Each floor level shall be provided with not fewer than two exits. Where compartmentation is required by Section 405.4, each compartment shall have not fewer than one exit and not fewer than one exit access doorway into the adjoining compartment.

[BE] 405.7.2 Smokeproof enclosure. Every required stairway serving floor levels more than 30 feet (9144 mm) below the finished floor of its level of exit discharge shall comply with the requirements for a smokeproof enclosure as provided in Section 1023.12.

[F] 405.8 Standby and emergency power. A standby power system complying with Section 2702 shall be provided for the standby power loads specified in Section 405.8.1. An emergency power system complying with Section 2702 shall be provided for the emergency power loads specified in Section 405.8.2.

[F] 405.8.1 Standby power loads. The following are classified as standby power loads:

  1. Smoke control system.
  2. Ventilation and automatic fire detection equipment for smokeproof enclosures.
  3. Elevators, as required in Section 3003.

[F] 405.8.2 Emergency power loads. The following are classified as emergency power loads:

  1. Emergency voice/alarm communications systems.
  2. Fire alarm systems.
  3. Automatic fire detection systems.
  4. Elevator car lighting.

4-14 2025 CALIFORNIA BUILDING CODE

on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.

SPECIAL DETAILED REQUIREMENTS BASED ON OCCUPANCY AND USE

  1. Means of egress and exit sign illumination as required by Chapter 10.

  2. Fire pumps.

[F] 405.9 Standpipe system. The underground building shall be equipped throughout with a standpipe system in accordance with Section 905.

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▸ Contents — 2025 California Building Code (Title 24, Part 2)
2025 California Building Code (Title 24, Part 2)
  1. Chapter 1 — ADMINISTRATION
  2. Chapter 2 — DEFINITIONS
  3. Chapter 3 — OCCUPANCY CLASSIFICATION AND USE
  4. ▸ Chapter 4 — SPECIAL DETAILED REQUIREMENTS BASED ON OCCUPANCY A…
    Overview
    1. Section 401 — SCOPE
    2. Section 402 — COVERED MALL AND OPEN MALL BUILDINGS
    3. Section 403 — HIGH-RISE BUILDINGS AND GROUP I-2 OCCUPANCIES HA…
    4. Section 404 — ATRIUMS
    5. Section 405 — UNDERGROUND BUILDINGS
    6. Section 406 — MOTOR-VEHICLE-RELATED OCCUPANCIES
    7. Section 407 — GROUP I-2
    8. Section 408 — GROUP I-3
    9. Section 409 — MOTION PICTURE PROJECTION ROOMS
    10. Section 410 — STAGES, PLATFORMS AND TECHNICAL PRODUCTION AREAS
    11. Section 411 — SPECIAL AMUSEMENT AREAS
    12. Section 412 — AIRCRAFT-RELATED OCCUPANCIES
    13. Section 413 — COMBUSTIBLE STORAGE
    14. Section 414 — HAZARDOUS MATERIALS
    15. Section 415 — GROUPS H-1, H-2, H-3, H-4 AND H-5
    16. Section 416 — SPRAY APPLICATION OF FLAMMABLE FINISHES
    17. Section 417 — DRYING ROOMS
    18. Section 418 — ORGANIC COATINGS
    19. Section 419 — ARTIFICIAL DECORATIVE VEGETATION
    20. Section 420 — GROUPS R-1, R-2, R-2.1, R-2.2, R-3, R-3.1 AND R-4
    21. Section 421 — HYDROGEN FUEL GAS ROOMS
    22. Section 422 — AMBULATORY CARE FACILITIES
    23. Section 423 — STORM SHELTERS
    24. Section 424 — PLAY STRUCTURES
    25. Section 425 — HYPERBARIC FACILITIES
    26. Section 427 — MEDICAL GAS SYSTEMS
    27. Section 435 — SPECIAL PROVISIONS FOR LICENSED 24-HOUR CARE FAC…
    28. Section 436 — GROUP I-4 [SFM]
    29. Section 437 — RESERVED
    30. Section 438 — RESERVED
    31. Section 439 — ROAD TUNNELS, BRIDGES AND OTHER LIMITED-ACCESS H…
    32. Section 440 — HORSE RACING STABLES [SFM]
    33. Section 441 — PET KENNELS AND PET BOARDING FACILITIES [SFM]
    34. Section 442 — COMBUSTION ENGINES AND GAS TURBINES [SFM]
    35. Section 443 — FIXED GUIDEWAY TRANSIT AND PASSENGER RAIL SYSTEM…
    36. Section 444 — EXPLOSIVES [SFM]
    37. Section 445 — RESERVED
    38. Section 446 — WINERY CAVES [SFM]
    39. Section 447 — RESERVED
    40. Section 448 — RESERVED
    41. Section 449 — PUBLIC LIBRARIES [SL AND SFM]
    42. Section 450 — GROUP C [SFM]
    43. Section 451 — RESERVED
    44. Section 452 — SCHOOL FACILITIES FOR KINDERGARTEN THROUGH 12TH …
    45. Section 453 — GROUP L [SFM]
    46. Section 454 — RESERVED
    47. Section 455 — LARGE FAMILY DAY-CARE HOMES [SFM]
  5. Chapter 5 — GENERAL BUILDING HEIGHTS AND AREAS
  6. Chapter 6 — TYPES OF CONSTRUCTION
  7. Chapter 7 — FIRE AND SMOKE PROTECTION FEATURES
  8. Chapter 7A — MATERIALS AND CONSTRUCTION METHODS FOR EXTERIOR W…
  9. Chapter 8 — INTERIOR FINISHES
  10. Chapter 9 — FIRE PROTECTION AND LIFE SAFETY SYSTEMS
  11. Chapter 10 — MEANS OF EGRESS
  12. Chapter 11 — RESERVED
  13. Chapter 11A — HOUSING ACCESSIBILITY
  14. Chapter 11B — ACCESSIBILITY TO PUBLIC BUILDINGS, PUBLIC ACCOMM…
  15. Chapter 12 — INTERIOR ENVIRONMENT
  16. Chapter 13 — ENERGY EFFICIENCY
  17. Chapter 14 — EXTERIOR WALLS
  18. Chapter 15 — ROOF ASSEMBLIES AND ROOFTOP STRUCTURES
  19. Chapter 16 — STRUCTURAL DESIGN
  20. Chapter 16A — STRUCTURAL DESIGN
  21. Chapter 17 — SPECIAL INSPECTIONS AND TESTS
  22. Chapter 17A — SPECIAL INSPECTIONS AND TESTS
  23. Chapter 18 — SOILS AND FOUNDATIONS
  24. Chapter 18A — SOILS AND FOUNDATIONS
  25. Chapter 19 — CONCRETE
  26. Chapter 19A — CONCRETE
  27. Chapter 20 — ALUMINUM
  28. Chapter 21 — MASONRY
  29. Chapter 21A — MASONRY
  30. Chapter 22 — STEEL
  31. Chapter 22A — STEEL
  32. Chapter 23 — WOOD
  33. Chapter 24 — GLASS AND GLAZING
  34. Chapter 25 — GYPSUM PANEL PRODUCTS AND PLASTER
  35. Chapter 26 — PLASTIC
  36. Chapter 27 — ELECTRICAL
  37. Chapter 28 — MECHANICAL SYSTEMS
  38. Chapter 29 — PLUMBING SYSTEMS
  39. Chapter 30 — ELEVATORS AND CONVEYING SYSTEMS
  40. Chapter 31 — SPECIAL CONSTRUCTION
  41. Chapter 31A — SYSTEMS FOR WINDOW CLEANING OR EXTERIOR BUILDING…
  42. Chapter 31B — PUBLIC POOLS
  43. Chapter 31C — RADIATION
  44. Chapter 31D — FOOD ESTABLISHMENTS
  45. Chapter 31F — MARINE OIL TERMINALS
  46. Chapter 32 — ENCROACHMENTS INTO THE PUBLIC RIGHT-OF-WAY
  47. Chapter 33 — SAFEGUARDS DURING CONSTRUCTION
  48. Chapter 34 — RESERVED
  49. Chapter 35 — REFERENCED STANDARDS
  50. Appendix A — EMPLOYEE QUALIFICATIONS
  51. Appendix B — BOARD OF APPEALS
  52. Appendix C — GROUP U—AGRICULTURAL BUILDINGS
  53. Appendix D — FIRE DISTRICTS
  54. Appendix E — RESERVED
  55. Appendix F — RODENTPROOFING
  56. Appendix G — FLOOD-RESISTANT CONSTRUCTION
  57. Appendix H — SIGNS
  58. Appendix I — PATIO COVERS
  59. Appendix J — GRADING
  60. Appendix K — GROUP R-3 AND GROUP R-3.1 OCCUPANCIES PROTECTED B…
  61. Appendix L — EARTHQUAKE RECORDING INSTRUMENTATION
  62. Appendix M — TSUNAMI-GENERATED FLOOD HAZARDS
  63. Appendix N — REPLICABLE BUILDINGS
  64. Appendix O — PERFORMANCE-BASED APPLICATION
  65. Appendix P — SLEEPING LOFTS
  66. Appendix Q — EMERGENCY HOUSING

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