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B-02 2020-Current: Application of Building Code Provisions to Partially Developed Areas in Residential Construction

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The following guidelines are provided as an aid in determining the code requirements for partially developed areas in residential construction such as attic spaces and storage lofts. Attic spaces and similar areas should be clearly identified on the drawings. The intended use of such areas shall be specified on the plans.

Partially developed areas are different from Non-Habitable Storage Spaces, Habitable Spaces, and Living Spaces which meet Minimum Room Area requirements per R304, Ceiling Height Requirements per R305, Means of Egress Requirements per R311, and all other applicable code requirements based on their use. Refer to Habitable Space and Living Space definitions in CRC and Section B below.

Also included in this bulletin are occupancy determinations and whether the area of the space contributes to the overall floor area of the structure. The floor area determinations in this bulletin are to administer permit valuations and Sonoma County Code requirements but are not used for Allowable Building Areas per CBC Chapter 5.

Version: 04/01/2020
Revised: 12/08/2008, 11/01/2013, 12/28/2016, 06/01/2017
Effective date: 12/31/2001

Technical Details

  1. Uninhabitable Attics are partially developed spaces that are separated by walls, floors, or ceilings from designated Living Spaces. Uninhabitable attics should be noted on the plans if they are used for limited storage, non-storage, or for mechanical equipment. Uninhabitable attics do not contribute to the total floor area of a structure.
    1. Uninhabitable Attics without Storage. (Table R301.5)
      1. All walls are unfinished.
      2. No sheathing allowed on top of ceiling joists or bottom chord except for walkways to mechanical equipment, or as required for structural purposes. The head height is not limited directly above the walkway.
      3. Electric lights and receptacles outlets as required for mechanical service or attic access only.
      4. Attic Access shall be only per R807 or a door if it is on the same level as Living Space. Stairways are not permitted.
      5. No plumbing fixtures.
      6. Maximum clear height between the top of ceiling joists and roof rafters or rafter ties is 42 inches. Spacing of rafters/ties shall be at maximum 4 feet o.c. Where attic space is formed by trusses, there are not two or more adjacent trusses capable of containing a rectangle 42 inches high by 2 feet wide or greater, located within the plane of the truss.
      7. Ceiling joists or bottom chord shall be designed for a minimum 10 psf live load (need not act concurrently with other live load requirements). 
    2. Uninhabitable Attics with Limited Storage (Table R301.5) shall meet all the requirements of Uninhabitable Attics without Storage except for the following:
      1. Floor sheathing is permitted
      2. The attic floor is designed for a minimum 20 psf live load
      3. Only one ceiling light and one outlet is permitted
  2. Storage Lofts and Mezzanines:
    1. Storage Loft: Elevated floor areas where the height from the elevated floor to the ceiling above is less than 5 feet shall be considered a storage loft and a partially developed area.
      1. Storage loft floor areas do not contribute to the total floor area.
      2. Only one light and receptacle is permitted
      3. The floor area shall not be greater than one-third of the floor area of the room or space in which they are located and shall be open to the room or space.
      4. Code compliant stairs are not required.
    2. Mezzanine: Elevated floor areas where the height from the elevated floor to the ceiling/roof above is over 5 feet shall be considered a mezzanine and is not a partially developed area.
      1. Code compliant stairs are required.
      2. Shall have a guard
      3. Mezzanines used for storage contribute to the floor area as an U-occupancy. However, Mezzanines open to R-3 occupancy floor areas shall also be designated as R-3 occupancies. Mezzanines for storage shall meet all requirements of R325 and R505 except the ceiling height may be less than 7 feet.
      4. Mezzanines not used for storage shall meet all the requirements of R325 and CBC Section 505 as applicable. The floor area will contribute to the total floor area as a U or R-3 occupancy based on the specified use with the exception of equipment platforms.
  3. Knick-knack and plant shelves and very low ceiling height storage areas.
    1. Areas in buildings where decorations or minor storage may exist need not be developed with egress provisions or with rigidly applied live loads. These areas require judgment in application of code provisions consistent with the proposed use of the area.
    2. Small or narrow areas which would be accessible may be reached from portable ladders and which are unlikely to be used as walking surfaces fall in this category. Also areas with very low ceiling heights could fall in this category.
    3. These areas do not contribute to floor area totals and are considered partially developed areas.
  4. Space intended for occupancy or use. These spaces do not qualify as a Partially Developed Area. The intended use shall be specified on the plans.
    1. Non-Habitable Storage space:
      1. Code complying egress (R311).
      2. Minimum 6 feet to 8 feet ceiling height.
      3. Only one ceiling light and one outlet is permitted.
      4. Designed for 40 psf live load. Space shall be un-conditioned.
      5. Contributes to the floor area as an U-occupancy. However, storage spaces accessed on the same level via an R-3 Occupancy shall also be designated as an R-3.
    2. Habitable space and Living Space
      1. Code complying egress (R311). Minimum ceiling heights and room areas required (R305.1).
      2. Are required to be conditioned and meet California Energy Code standards.
      3. Designed for 30 psf live load for habitable attics with limited storage and sleeping areas, and 40 psf live load for all other residential areas.
      4. Lighting and receptacle outlets required and shall meet minimum California Electrical Code standards.
      5. Contributes to the floor area as a R-3 Occupancy


2019 California Residential Code, Sections R305, R311, R304, R305, R807, Table R301.5, footnotes (b&g)

California Residential Code Definitions: ‘Habitable Space’, ‘Living Space’, ‘Attic, Habitable’, ‘Mezzanine, Loft’