I think some of what you're looking for is CEBC Section 302, also check out CEBC 313. And CEBC does have a chapter 5, I think the bulk of what you're looking for is there. Specifically:
502.1 General
[BSC & HCD] Additions to any building or structure shall comply with the requirements of the California Building Code or California Residential Code, as applicable, for new construction. Alterations to the existing building or structure shall be made to ensure that the existing building or structure together with the addition are not less complying with the provisions of the California Building Code than the existing building or structure was prior to the addition. An existing building together with its additions shall comply with the height and area provisions of Chapter 5 of the California Building Code, or the height provisions of Chapter 3 of the California Residential Code, as applicable.
And:
503.1 General
Except as provided by Section 302.4, 302.5 or this section, alterations to any building or structure shall comply with the requirements of the California Building Code or California Residential Code, as applicable, for new construction. Alterations shall be such that the existing building or structure is not less complying with the provisions of the California Building Code or California Residential Code, as applicable, than the existing building or structure was prior to the alteration.
Exceptions:
An existing stairway shall not be required to comply with the requirements of Section 1011 of the California Building Code where the existing space and construction does not allow a reduction in pitch or slope.
Handrails otherwise required to comply with Section 1011.11 of the California Building Code shall not be required to comply with the requirements of Section 1014.6 of the California Building Code regarding full extension of the handrails where such extensions would be hazardous because of plan configuration.
Where provided in below-grade transportation stations, existing and new escalators shall have a clear width of less than 32 inches (815 mm).
[BSC] For state-owned buildings, including those owned by the University of California and the California State University and the judicial council, the requirements of Sections 503.3 through 503.4 are replaced by the requirements of Sections 317 through 322.
Also, for reference:
302.4 Existing Materials
Materials already in use in a building in compliance with requirements or approvals in effect at the time of their erection or installation shall be permitted to remain in use unless determined by the building official to be unsafe.
302.5 New and Replacement Materials
Except as otherwise required or permitted by this code, materials permitted by the applicable code for new construction shall be used. Like materials shall be permitted for repairs and alterations, provided that unsafe conditions are not created. Hazardous materials shall not be used where the code for new construction would not permit their use in buildings of similar occupancy, purpose and location. [HCD 1] Local ordinances or regulations shall permit the replacement, retention and extension of original materials, and the use of original methods of construction, for any building or accessory structure, provided such building or structure complied with the building code provisions in effect at the time of original construction and the building or accessory structure does not become or continue to be a substandard building. For additional information, see Health and Safety Code Sections 17912, 17920.3, 17922(d), 17922.3, 17958.8 and 17958.9.
Exception: No replacement residential garage door shall be installed to connect the replacement door to an existing residential automatic garage door opener that does not have a battery backup function designed to keep the garage door operational without interruption during an electrical outage. See Health and Safety Code Section 19892.
[BS] 302.5.1 New Structural Members and Connections
New structural members and connections shall comply with the detailing provisions of the California Building Code for new buildings of similar structure, purpose and location.
Exception: Where alternative design criteria are specifically permitted.
302.6 Occupancy and Use
Where determining the appropriate application of the referenced sections of this code, the occupancy and use of a building shall be determined in accordance with Chapter 3 of the California Building Code.
