Title 24 for multifamily: which parts govern your project

Every California building permit runs through the California Building Standards Code — Title 24 of the California Code of Regulations, republished on a three-year cycle. The 2025 edition took effect statewide on January 1, 2026, and the edition that governs your project is fixed by the date you file a permit application, not the date you break ground. File under the old books and you generally keep them; file after the changeover and the new edition's energy and green-building changes are yours.

Title 24 is not one code but a family of parts, and a multifamily deal touches nearly all of them. The threshold question is which primary code the building falls under: the California Building Code for apartment buildings, or the California Residential Code for houses, townhouses and many ADUs. That single classification drives construction type, egress, fire protection and accessibility scope more than any other early design decision.

Title 24 republishes every three years with intervening supplements, and each part has its own job. The parts a multifamily project touches:

• Part 1 — the California Administrative Code: how the Building Standards Commission and state agencies adopt, amend and enforce the standards.
• Part 2 — the California Building Code (Volumes 1–2): the primary code for apartment buildings, which are Group R-2.
• Part 2.5 — the California Residential Code: detached one- and two-family dwellings, townhouses and many ADUs.
• Parts 3–5 — the Electrical, Mechanical and Plumbing codes governing the trades.
• Part 6 — the Energy Code — and Part 11 — CALGreen: efficiency, electrification, EV and green mandatory measures.
• Parts 8–10 — the Historical Building Code, Fire Code and Existing Building Code: historic stock, fire and life safety, and renovation work.
• Part 12 — the Referenced Standards Code: the controlling editions of NFPA, ASTM and other standards the rest of Title 24 cites.

The application date is the pivot. A project that filed a complete permit application under the prior edition generally rides it through issuance; applications on or after January 1, 2026 are on the 2025 books, heat-pump baselines and all. Anything entitling through 2026 and beyond should be designed and priced on the 2025 edition.

The threshold classification: three or more attached dwelling units make a Group R-2 building under the CBC. The CRC covers detached one- and two-family dwellings and townhouses — units with independent egress and no dwelling stacked above another, up to three stories — plus most ADUs riding on single-family lots. A duplex with a detached ADU stays CRC; a triplex of stacked flats is a CBC building with sprinklers, rated separations and Chapter 11A adaptable-unit standards.

Everything cascades from that call: sprinkler standard, fire-resistance ratings, egress design, energy-compliance path, CALGreen scope and accessibility. What the R-2 side entails is covered in CBC basics for apartments.

• Underwriting watch-outs:
• Missing-middle plays live on the boundary — the third attached, stacked unit drags a project from CRC into CBC territory, and sprinklers, separations and adaptability standards land all at once. Price the jump before locking the unit count.
• Townhouse configurations can stay under the CRC at higher unit counts if every unit meets the townhouse definition (independent egress, nothing stacked) — often the cheaper structure for the same density.
• ADU code paths follow configuration, not label — HCD's ADU Handbook plus local practice decide CRC vs. CBC when units attach to or sit inside multifamily buildings.

Cities and counties adopt Title 24 by reference and may amend it — but only with express findings that each change is reasonably necessary because of local climatic, geological or topographical conditions, filed with the Building Standards Commission (Health & Safety Code §§ 17958.5–17958.7). The filter is generous in practice: hillside jurisdictions amend for fire, coastal cities for wind and corrosion, and hundreds of jurisdictions ratchet sprinkler, WUI, seismic and CALGreen provisions above the state floor every cycle.

• Underwriting watch-outs:
• Always pull the jurisdiction's current adopting ordinance next to the state text — the state code alone is never the complete answer.
• Local amendments lapse and re-adopt with each triennial edition, so a 2022-cycle amendment compilation may not reflect the ordinances adopted for the 2025 books. Verify against the current ordinance.
• Common cost-movers buried in amendments: stricter sprinkler thresholds, seismic-retrofit mandates layered onto the Existing Building Code, CALGreen tier adoption, and fire-code access standards beyond the state minimums.