Quick answer
A building code is a set of minimum rules for how buildings must be designed and built so they are safe to live in, covering structure, fire safety, exits, plumbing, wiring, ventilation and energy use.
Most codes start as model codes written by the International Code Council. A state or city then adopts them into law, often with its own changes, and the local building department enforces the result.
Quick facts
- Who writes the model
- The International Code Council (ICC) publishes the model codes most US states start from 2
- Who makes it law
- A state or local government adopts the model, often with amendments 1
- California edition in force
- 2025 Title 24, effective 1 January 2026 1
- Which edition applies to you (California)
- The one in effect locally when your building permit application is submitted 3
Key takeaways
- A building code is the rulebook for safe construction. It sets the minimum, not the ideal.
- Most US codes begin as ICC model codes. They become law only when a state or city adopts them 2.
- California adopts its codes as Title 24. The 2025 edition took effect on 1 January 2026 1.
- In California, the code that applies is the one in force when you submit the permit application 3.
- Cities may add stricter rules, but California makes them show a local climate, geology or terrain reason 4.
- The building code is not the zoning code. Zoning says where and how big. The building code says how.
What a building code means, in plain English
A building code is a list of rules for building things safely. It tells a designer how strong a beam must be. It tells a builder how far apart the smoke alarms go. It says how big a bedroom window must be so you can climb out in a fire.
The rules are minimums. A code does not say how nice a home should be. It says how safe it must be at the very least. You can always build better than the code. You cannot legally build worse.
For an ADU (an accessory dwelling unit, a small second home on a lot with a house), the building code is the reason the plans need so much detail. Every wall, wire and pipe in the drawings must show it meets a rule.
Why it matters to you
The code shapes your cost. Each rule turns into material and labor. A deeper footing, a fire-rated wall or a bigger window all come from code lines.
The code shapes your timeline. Plan check (the city review of your drawings) is mostly a code check. Each missed rule comes back as a correction, and each round adds time.
The code shapes what you can sell or rent later. A unit built and inspected to code has a permit record. Lenders, buyers and insurers look for it.
How a building code goes from a book to your job site
- A model code is written. The ICC publishes new editions of its codes on a three-year cycle, such as the 2024 International Building Code 2.
- A state adopts it. The state picks a model edition and writes its own changes. California does this through the Building Standards Commission, which publishes Title 24 1.
- A city or county may amend it. Local changes must follow state rules. In California, the city must make a written finding and file it with the Commission 4.
- The building department enforces it. It reviews plans, issues the permit and inspects the work 5.
- The edition locks in. In California, the standards in force when you apply govern your plans and your build 3.
- Inspections confirm it. Work stays open for inspection until the building official approves it 2.
The family of codes an ADU touches
| Topic | ICC model code | California version (Title 24) |
|---|---|---|
| Larger or non-house buildings | International Building Code (IBC) | Part 2, California Building Code |
| Houses and small homes | International Residential Code (IRC) | Part 2.5, California Residential Code |
| Wiring | National Electrical Code (NFPA, not ICC) | Part 3, California Electrical Code |
| Heating, cooling, venting | Mechanical code | Part 4, California Mechanical Code |
| Pipes and fixtures | Plumbing code | Part 5, California Plumbing Code |
| Energy use | International Energy Conservation Code | Part 6, California Energy Code |
| Fire | International Fire Code | Part 9, California Fire Code |
| Work on existing buildings | International Existing Building Code | Part 10, California Existing Building Code |
| Green building | No single model | Part 11, CALGreen |
Which code your ADU falls under
The IBC covers almost every kind of building. It has one big exception. Detached one- and two-family homes and townhouses of up to three stories above grade, with their own exits, may follow the IBC or the IRC 2.
A typical ADU is a small house. So in most places it is designed under the residential code, not the building code. In California that is the California Residential Code.
The general building code still matters. It holds the rules the residential code points to. It also governs an ADU built inside a larger apartment building. The California Building Code page explains when that happens.
- If Detached ADU or one attached to a one- or two-family home, three stories or less
- Then Usually the residential code (in California, the California Residential Code)
- If ADU inside or added to an apartment building
- Then Usually the general building code (in California, the California Building Code)
- If A detail the residential code does not cover
- Then The residential code points to the building code for that item
Model-code scoping. Your state and city adopt their own versions; the building official decides. Source: [2]
Sources: [2]
Worked example: which edition applies to two applications
This is a hypothetical example. It uses the real California rule on edition timing 3 and the real 2025 effective date 1. The owners and dates are made up.
| Owner A | Owner B | |
|---|---|---|
| Permit application submitted | 15 December 2025 | 12 January 2026 |
| Title 24 edition in force that day | 2022 edition | 2025 edition |
| Edition that governs the plans | 2022 edition | 2025 edition |
| If the city adopts a local amendment in March 2026 | Does not apply to this permit | Does not apply to this permit |
Both owners keep the rules in force on the day they applied. A new local residential amendment applies only to applications submitted after it takes effect, with limited exceptions such as declared emergencies 3. If you plan to apply near a code change, ask your designer which edition the drawings follow.
Where you will see the building code
- On the cover sheet of your plans. It lists the code editions the design follows, such as “2025 CRC” or “2025 CBC.”
- In plan-check corrections. Each comment usually cites a code section. That is the rule the reviewer says you missed.
- On structural calculations. The engineer names the code and the loads used.
- On the energy forms. They show how the unit meets the energy code.
- On inspection cards. Each sign-off confirms a stage met the code.
- In a contractor’s bid. Lines such as “code upgrade” or “per code” point back to a rule.
California’s rule, then other states
California’s building code is Title 24 of the California Code of Regulations. The Building Standards Commission publishes it. The current edition is the 2025 Triennial Edition. It was published 1 July 2025 and took effect 1 January 2026 1.
State law makes the building department of every city or county enforce these standards for homes and apartments 5. A city may adopt stricter local rules.
Before it does, it must find the change is reasonably necessary because of local climate, geology or terrain. It must file that finding with the Commission. The change has no effect until it is filed 4.
Timing is fixed by Health and Safety Code 18938.5. Only standards in effect locally when the permit application is submitted apply to that permit 3.
California’s ADU law also limits what a city can demand of an ADU. Those limits are in the Government Code, not the building code. See California ADU law.
Other states choose their own editions and changes. Some adopt codes statewide. Others leave it to each city or county. A few rural areas have little or no code enforcement. Check with your local building department, and see ADU laws by state.
Code items that drive what an ADU costs
No dollar figures here. Costs vary by city and job. These are the code lines that tend to move a bid.
- Foundation design. Soil type and seismic zone set the footing size and steel.
- Lateral bracing. Shear walls and hold-downs resist wind and earthquakes.
- Fire separation. Walls near a property line or another building may need fire-rated assemblies.
- Egress windows. Every sleeping room needs a window or door you can escape through.
- Energy compliance. Insulation, windows, water heating and solar rules come from the energy code.
- Electrical service. A new unit can push the main panel past its capacity. See ADU electrical panel.
Building codes: benefits and limits
What it does well
- They set one clear safety floor that every builder must meet.
- Inspections catch hidden problems while walls are still open.
- A code-built, permitted ADU is easier to insure, finance and sell.
- Model codes are updated on a regular cycle, so new safety lessons get in.
Where it falls short
- They add cost. Some rules feel heavy for a very small unit.
- Local amendments mean the rules can differ from one city to the next.
- Edition changes can force a redesign if you apply after a new code starts.
- Codes set minimums. Passing inspection does not mean the work is high quality.
Building code compared with related rules
| Rule set | Answers | Who applies it | ADU example |
|---|---|---|---|
| Building code | How the unit is built | Building department | Footing depth, window size, smoke alarms |
| Zoning code | Where and how big | Planning department | Setbacks, height, lot coverage |
| State ADU law | What a city may and may not require | Courts and the state housing agency | Limits on parking and size rules |
| Fire code | Fire access and protection | Fire department | Fire access path, sprinklers in some cases |
| HOA rules | Private covenants | Homeowners association | Exterior color or style |
Common misreadings
- Myth “The building code and the zoning code are the same thing.”
- Reality They are separate laws. Zoning says what may go where. The building code says how it must be built. An ADU must pass both.
- Myth “A small ADU is exempt from the code.”
- Reality Size does not exempt a home. A dwelling of any size must meet the code for a dwelling.
- Myth “The code is the same everywhere in the US.”
- Reality Most states start from ICC models, but editions and local changes differ.
- Myth “The newest code always applies.”
- Reality In California, the edition in force on your application date applies to that permit 3.
- Myth “Passing the code means the house is well built.”
- Reality The code is a minimum. Finish quality and durability above the minimum are up to your contract.
What to ask your designer and the city
- Which code editions do these plans follow?
- Will our application go in before or after the next code change?
- Does the city have local amendments that affect an ADU? Where are they posted?
- Is the unit designed under the residential code or the general building code? Why?
- Which items need an engineer’s stamp?
- Which inspections will the city do, and in what order?
- Are there wildfire, flood or seismic rules for this lot?
Checklist: building code basics for your ADU
- Find your local building department and its ADU page.
- Note the code editions the city is using today.
- Ask whether local amendments apply, and get a copy.
- Confirm with your designer which code the plans follow.
- Check for hazard zones on your lot: fire, flood, slope.
- Get structural and energy work done before you submit.
- Submit a complete set to lock in the edition.
- Answer each plan-check comment by code section.
- Keep the approved set on site for every inspection.
- Keep the final sign-off for your records.
Where the building code sits in an ADU project
Related guides, tools and research
- ADU permits guideThe full permit path, start to finish
- ADU fire code requirementsFire rules that often surprise owners
- Prefab ADU building standardsWhich codes cover factory-built units
- ADU permit times by cityHow long review takes in practice
- ADU feasibility checkerTest a lot before you pay for plans
- ADU engineerWho stamps the structural code work
Questions about building code
What is the purpose of a building code?
A building code protects the people who live in and near a building. It sets minimum rules for structure, fire safety, exits, plumbing, wiring, air and energy. It does not set style or quality above the minimum. Your contract and your design handle that.
Who creates building codes in the US?
Most start as model codes from the International Code Council, a private nonprofit. They have no legal force until a state or local government adopts them. That government may change them before adoption. The electrical code usually comes from a different group, the NFPA.
What building code does California use?
California uses Title 24 of the California Code of Regulations, called the California Building Standards Code. The 2025 edition took effect on 1 January 2026. It includes separate parts for buildings, homes, electrical, plumbing, energy, fire and more.
Do ADUs have to meet the building code?
Yes. An ADU is a home, so it must meet the code for a home. In most places that means the residential code. State ADU laws limit zoning rules, but they do not waive safety codes.
What is the difference between the IBC and the IRC?
The IBC covers most buildings. The IRC covers detached one- and two-family homes and townhouses up to three stories with their own exits. Those homes may use either code. Most small ADUs use the residential code.
How often do building codes change?
The ICC publishes new model editions every three years. States adopt them on their own schedule. California adopts a new Title 24 every three years, and it can issue supplements in between.
Can a city make the building code stricter?
Often, yes. In California, a city or county may change state building standards only after finding the change is reasonably necessary because of local climate, geology or terrain. It must file that finding with the Building Standards Commission first.
Which code edition applies if the code changes during my project?
In California, the edition in force locally when you submit the building permit application applies to that permit. A later edition does not apply to it. Ask your designer to confirm the edition on the plans.
Where can I read the California building code for free?
The Building Standards Commission says Title 24 can be viewed at no cost. Its Codes page links to the publishers’ free online versions and lists the parts. Some paid editions add notes and search tools.
Is the building code the same as zoning?
No. Zoning says where a building can go and how big it can be. The building code says how it must be built. An ADU has to pass both reviews, usually by different offices.
Sources
- California Building Standards Commission, Codes (2025 Triennial Edition of Title 24, effective 1 January 2026), retrieved .
- International Code Council, 2024 International Building Code, Chapter 1 (sections 101.2, 103.1, 105.1, 110.1), retrieved .
- California Legislature, Health and Safety Code §18938.5 (which standards apply to a permit), retrieved .
- California Legislature, Health and Safety Code §17958.7 (local amendment findings), retrieved .
- California Legislature, Health and Safety Code §17960 (local 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: Model-code scope read in the ICC 2024 IBC Chapter 1; Title 24 edition and dates read on the Building Standards Commission Codes page; California statutes read on leginfo.legislature.ca.gov on 2026-10-08.
Suggest a correction. We fix errors and say what changed.