Awaiting expert review. This page covers rules that vary by state and local jurisdiction. It is kept out of search results until a qualified reviewer has checked it; the sources below are dated so you can verify each point.
Quick answer
The California Building Code (CBC) is Part 2 of Title 24 of the California Code of Regulations: the state’s main construction code for buildings other than small houses, based on the International Building Code with California changes.
The 2025 edition took effect on 1 January 2026. Most ADUs are designed under a sister code, the California Residential Code. The CBC takes over when an ADU sits inside or beside an apartment building.
Quick facts
- Where it lives
- Title 24, Part 2 of the California Code of Regulations (two volumes) 1
- Current edition
- 2025 Triennial Edition, published 1 July 2025 1
- Effective
- 1 January 2026 1
- Model code it starts from
- International Building Code (ICC) 2
- Edition that applies to your permit
- The one in force locally when you submit the application 5
Key takeaways
- The CBC is Title 24, Part 2. It is California’s general building code 1.
- The 2025 edition took effect 1 January 2026 1.
- It starts from the International Building Code, then adds California changes 2.
- A detached or attached ADU on a house lot is usually designed under Part 2.5, the residential code.
- The CBC usually governs ADUs built inside an existing apartment building, which state law allows 4.
- Cities can tighten it only with a filed finding about local climate, geology or terrain 6.
What the CBC means, in plain English
California keeps all its building rules in one big set called Title 24. Title 24 has many parts. Each part covers one subject, such as wiring, plumbing or energy.
Part 2 is the California Building Code. People call it the CBC. It is the general rulebook for how buildings are built. It covers offices, stores, schools and apartment buildings.
Small houses get their own, simpler rulebook. That is Part 2.5, the California Residential Code (CRC). Most ADUs are small houses. So most ADU owners live with the CRC, not the CBC.
Important distinction
CBC (Part 2) and CRC (Part 2.5) are two different books in Title 24. A plan-check comment that starts “CBC” means the reviewer used the general building code, not the house code.
Why it matters to you
Searching “California building code” often lands you in the wrong book. If your ADU is a backyard cottage, the CBC may not be the code your designer uses. Reading the wrong code can lead to wrong answers about windows, stairs or fire walls.
It matters more if you own an apartment building. Turning a storage room or garage in a multifamily building into an ADU usually puts the work under the CBC. Those rules are stricter. They often bring in fire separation, exits and accessibility rules a house does not have.
Knowing the right code also helps you read plan-check comments. A comment that cites “CBC” versus “CRC” tells you which book the reviewer used.
The current edition at a glance
| Item | What it is |
|---|---|
| Name | 2025 California Building Code, Title 24, Part 2 1 |
| Published | 1 July 2025 1 |
| In effect | 1 January 2026 1 |
| Publisher of the state code | California Building Standards Commission 1 |
| Model code base | International Building Code, published by the International Code Council 2 |
| Updates between editions | The Commission issues supplements and errata to the 2025 edition 1 |
| Free to read? | The Commission says Title 24 is viewable at no cost through publisher sites and depository libraries 1 |
How to tell if the CBC covers your ADU
- Count the homes in the building the ADU is in or attached to. The IBC treats a building with no more than two dwelling units as Group R-3 3.
- If the building will hold more than two dwelling units, it is generally Group R-2, the apartment group 3.
- Check the scope rule. Detached one- and two-family homes and townhouses up to three stories with their own exits may follow the residential code 2.
- A backyard ADU on a single-family lot is a separate detached dwelling. It usually fits the residential code.
- An ADU carved out of an apartment building usually does not. It is designed under the CBC.
- Ask your designer to name the code on the plan cover sheet. Then ask the city to confirm.
- If The ADU is a separate, detached building on a house lot
- Then Usually the CRC (Part 2.5) 2
- If The ADU attaches to a house, so the building holds two homes
- Then Usually the CRC, Group R-3 3
- If The ADU is carved out of a building with more than two homes
- Then Usually the CBC (Part 2), Group R-2 3
- If The unit is factory-built or a manufactured home
- Then State or federal factory programs apply too; ask the building official
- If You are unsure which case you are
- Then Ask the building official before design starts; they decide
Typical outcomes only. Local building officials make the final call. Source: [2] [3]
This is a general guide. The building official decides which code applies to a given project. The guide below sorts the usual cases by one question: how many homes share the building.
Common ADU cases and the code that usually governs
| ADU case | Usual code | Why |
|---|---|---|
| Detached backyard ADU on a single-family lot | CRC (Part 2.5) | A detached dwelling within the residential scope 2 |
| Attached ADU added to a house | CRC (Part 2.5) | The building holds two dwelling units, Group R-3 3 |
| Garage conversion on a house lot | CRC, plus the existing building code for the old structure | Still a small residential building |
| ADU inside an apartment building (storage room, basement, garage) | CBC (Part 2) | The building already has more than two dwelling units, Group R-2 3 |
| Detached ADU on a lot with an apartment building | Often CRC for the new detached unit | The ADU itself is a separate small building; confirm locally |
| Factory-built or manufactured ADU | State or federal programs, not only the CBC | See prefab ADU and manufactured ADU |
Worked example: two owners, two codes
A hypothetical example. The code rules are real; the properties are made up.
| Owner A | Owner B | |
|---|---|---|
| Property | Single-family house | Existing eight-unit apartment building |
| Plan | Detached ADU in the backyard | Convert a ground-floor storage room into an ADU |
| State law basis | A local agency must approve a qualifying detached ADU 4 | State law allows ADUs in non-livable space of existing multifamily buildings, if each meets state building standards for dwellings 4 |
| Occupancy group of the building | R-3 (two or fewer dwellings) 3 | R-2 (more than two dwellings) 3 |
| Code on the plans | 2025 CRC | 2025 CBC |
| Extra design items to expect | Few beyond the house rules | Fire separation between units, exit paths, possibly accessibility |
Owner B also has a numbers rule. The law requires a city to allow at least one ADU in an existing multifamily building, and up to 25 percent of the existing units 4. For eight units, 25 percent is two.
Where you will see the CBC
- On the plan cover sheet: “2025 California Building Code” in the code list.
- In plan-check comments that start with “CBC” and a section number.
- On a multifamily conversion, in the fire and exit plan sheets.
- In engineering calculations, where the CBC sets loads even for some house projects.
- In a city’s local amendment ordinance, which lists changes to CBC sections.
California’s rules on adopting and changing the CBC
The Building Standards Commission publishes Title 24, including the CBC, every three years. The 2025 edition is in force now 1.
Every city and county building department must enforce the state building standards for homes and apartments 8.
A city may change those standards for its area. First, it must find the change is reasonably necessary because of local climate, geology or terrain. It must file the finding and the change with the Commission. Until then, the change has no effect 6.
Timing is set by law. Only standards in effect locally when you submit the permit application apply to your plans and your build.
A later local residential amendment applies only to later applications, with narrow exceptions such as declared emergencies 5.
The Commission’s Bulletin 25-01 applies that rule to the 2025 edition: applications submitted on or after 1 January 2026 follow it 7.
State ADU law adds one rule that points straight at building standards. ADUs inside existing multifamily buildings must each comply with state building standards for dwellings 4.
Source and scope
Scope: these adoption rules are California’s. The 2025 CBC governs permit applications submitted on or after 1 January 2026; applications submitted by 31 December 2025 generally stay on the 2022 edition 7. Other states pick their own editions.
Rules outside California
Other states have their own building codes. Many start from the same ICC model, the International Building Code. Each picks its own edition and writes its own changes 2.
Some states adopt one code statewide. Others let each city choose. The split between a general building code and a residential code is common, but the names differ. See ADU laws by state for state ADU rules, and ask your local building department which code edition it enforces.
What drives cost when the CBC applies
No dollar figures. These are the CBC-type items that tend to add design and build cost compared with a house-code ADU.
- Fire-rated walls and floors between the new unit and existing apartments.
- Exit paths and doors that must meet commercial-style rules.
- Accessibility features in some multifamily cases.
- Fire alarm or sprinkler tie-ins with the existing building system.
- Engineering for openings cut into existing walls.
- More review disciplines, which can mean more plan-check rounds.
Working under the CBC: benefits and limits
What it does well
- One clear code for complex buildings with several homes.
- Strong fire and exit protection between neighbors in a shared building.
- Lets owners add homes inside apartment buildings that state law opens up 4.
Where it falls short
- Usually more detail and cost than a house-code ADU.
- More technical; you will likely need an architect or engineer.
- Local amendments can change sections from city to city 6.
- Searching the CBC for a backyard cottage often gives the wrong answer.
The CBC compared with related codes
| Code | Title 24 part | Covers | ADU role |
|---|---|---|---|
| California Building Code | Part 2 | Most buildings, including apartments | ADUs inside multifamily buildings |
| California Residential Code | Part 2.5 | One- and two-family homes, townhouses | Most backyard and attached ADUs |
| California Existing Building Code | Part 10 | Work on existing buildings | Conversions of existing space |
| California Energy Code | Part 6 | Energy use | Every new ADU |
| California Fire Code | Part 9 | Fire safety and access | Fire access, some sprinkler cases |
| Building code (general term) | Any | The idea of a construction code | The concept behind all of these |
Common misreadings
- Myth “The California Building Code is the code for my backyard ADU.”
- Reality Usually not. A small detached or attached ADU is normally designed under the California Residential Code, Part 2.5.
- Myth “Title 24 means the energy code.”
- Reality Title 24 is the whole set of building codes. The energy code is only Part 6 of it 1.
- Myth “The CBC is the same as the IBC.”
- Reality It starts from the IBC but adds California changes. Read the California version.
- Myth “A city can change the CBC however it wants.”
- Reality Changes need a filed finding about local climate, geology or terrain 6.
- Myth “If the code changes mid-design, I must redesign.”
- Reality Your permit follows the edition in force when you applied 5. Applying later can change that.
What to ask before you design
- Will this ADU be designed under the CBC or the CRC? Why?
- What occupancy group is the building now, and after the ADU?
- Does the city have local amendments to the CBC? Where are they?
- Will we submit before or after the next code edition or supplement?
- Which fire separation and exit rules apply to this conversion?
- Does any accessibility rule apply here?
- Who on the team is licensed to stamp the CBC work?
Checklist: getting the code right on your project
- Decide the ADU type: detached, attached, conversion or inside an apartment building.
- Count the dwelling units in the building before and after.
- Ask your designer which Title 24 part governs, and get it on the cover sheet.
- Confirm with the building department.
- Collect any local amendments.
- If the CBC governs, hire an architect or engineer early.
- Submit a complete application to lock in the edition.
- Answer plan-check comments section by section.
Related guides, tools and research
- ADU permits guideHow the permit path works
- ADU fire code requirementsFire rules by ADU type
- Prefab ADU building standardsWhich codes cover factory-built units
- California ADU ordinance trackerLocal ADU ordinances across the state
- ADU type selectorPick the ADU type that fits your lot
Questions about california building code
What is the California Building Code?
It is Part 2 of Title 24 of the California Code of Regulations. It sets construction rules for most buildings in the state. It starts from the International Building Code and adds California changes. The 2025 edition took effect 1 January 2026.
What is the current California Building Code?
The 2025 California Building Code is current. The Building Standards Commission published it on 1 July 2025, and it took effect on 1 January 2026. Supplements and errata can update it between editions.
Is an ADU built under the California Building Code?
Usually not. Most detached and attached ADUs are designed under the California Residential Code, Part 2.5. The CBC usually applies when an ADU is built inside an existing apartment building. Your building official decides.
What is the difference between the CBC and the CRC?
The CBC, Part 2, covers most buildings, including apartments. The CRC, Part 2.5, covers one- and two-family homes and townhouses. The CRC is shorter and written for houses. Most ADUs use the CRC.
Is Title 24 the same as the California Building Code?
No. Title 24 is the whole collection of state building codes. The CBC is one part of it, Part 2. Other parts cover homes, wiring, plumbing, energy, fire and green building.
Where can I read the California Building Code for free?
The Building Standards Commission says Title 24 is viewable at no cost. Its Codes page links to publisher sites with free read-only versions. Paid versions add search and notes.
Can a California city change the building code?
Yes, within limits. A city must find the change is reasonably necessary because of local climate, geology or terrain. It must file that finding with the Building Standards Commission. The change has no effect until it is filed.
Which edition applies if I applied in 2025 but build in 2026?
The edition in force locally when you submitted the permit application applies. If you applied before 1 January 2026, your permit generally follows the 2022 edition. Confirm with your building department.
Can I add an ADU inside my apartment building in California?
State law requires cities to allow ADUs in non-livable parts of existing multifamily buildings, such as storage rooms or garages. Each unit must meet state building standards for dwellings. Cities must allow at least one, and up to 25 percent of existing units.
Do I need an architect if the CBC applies?
Often, yes. California’s license exemption covers simple wood-frame houses and small dwellings. Work in a larger apartment building usually falls outside it, so a licensed architect or engineer prepares those plans. Ask the building official early.
Sources
- California Building Standards Commission, Codes (2025 Triennial Edition of Title 24, published 1 July 2025, effective 1 January 2026), retrieved .
- International Code Council, 2024 International Building Code, Chapter 1, section 101.2 Scope, retrieved .
- International Code Council, 2024 International Building Code, Chapter 3 (sections 310.3 Group R-2, 310.4 Group R-3), retrieved .
- California Legislature, Government Code §66323(a)(3), ADUs within existing multifamily structures (as amended by SB 543, Stats. 2025, ch. 520), retrieved .
- California Legislature, Health and Safety Code §18938.5 (standards in effect at application), retrieved .
- California Legislature, Health and Safety Code §17958.7 (local modification findings filed with the Commission), retrieved .
- California Building Standards Commission, Information Bulletin 25-01 (1 July 2025): 2025 Title 24 publication and effective date, retrieved .
- California Legislature, Health and Safety Code §17960 (building department enforcement), retrieved .
Expert review
Written by the HyreADU Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Edition, dates and parts read on the Building Standards Commission Codes page; scope and occupancy groups read in the ICC 2024 IBC Chapters 1 and 3; statutes read on leginfo.legislature.ca.gov on 2026-10-08. Held for review (G-REV).
Suggest a correction. We fix errors and say what changed.