Two builders quote the same house in two neighboring cities. One quote includes low-emission flooring, formaldehyde-free insulation and a construction-phase filter package; the other doesn't. Neither builder is cutting corners. They are reading the same code — one is reading a column the other city has not switched on.
That is the thing to understand about California's green building standards. The code sets mandatory measures that apply to covered projects statewide, and alongside them a menu of elective measures sorted into two stricter tiers. The elective menu is voluntary as printed. Its own checklist says so:
"Green building measures listed in this table may be mandatory if adopted by a city, county, or city and county" — checklist footnote
Key numbers
| Measure | The threshold | Status |
|---|---|---|
| Bathroom fan humidity control range | Adjustable from 50 percent or lower to a maximum of 80 percent relative humidity | Mandatory residential |
| Low-emission resilient flooring | 90 percent of flooring at the first tier, 100 percent at the second | Elective tiers |
| Filters on return air openings during construction | MERV 8 or higher when using the heating system mid-build | Elective |
| Recycling areas for commercial additions | Floor area increase of 30 percent or more in a 12-month period | Nonresidential mandatory |
| Building commissioning | New nonresidential buildings 10,000 square feet and over | Nonresidential mandatory |
What's mandatory in a home
The residential mandatory measures are less about solar panels than about keeping water, fumes and bad air out of the building. Four show up on nearly every project:
- Interior moisture control. A vapor retarder and capillary break at slab-on-grade foundations, and a check on the moisture content of materials used in wall and floor framing before enclosure — that is, before the drywall goes on.
- Bathroom exhaust. An ENERGY STAR fan in each bathroom, ducted to terminate outside the building, controlled by a humidity control or working as part of a whole-house ventilation system, with the control adjustable across the range above.
- Duct and equipment sizing. Duct systems sized and designed, and heating and cooling equipment selected, using the referenced industry methods: an established method for calculating heat loss and heat gain, another for sizing ducts, and a third for selecting equipment.
- Qualified people and paperwork. Heating and cooling installers trained and certified in proper installation; special inspectors employed by the owner able to demonstrate competence in what they are inspecting; and verification of compliance shown through construction documents, plans, specifications, builder or installer certification, inspection reports, or other methods the enforcing agency accepts.
The duct-sizing item is the one that quietly changes projects. It rules out sizing a system by rule of thumb — the calculation is the requirement, and oversized equipment that short-cycles is a comfort complaint the code is trying to prevent before it happens.
What's on the optional menu
The elective measures are where the two-tier structure lives, and they are worth recognizing because they read exactly like requirements. Among the residential electives: composite wood products made with no-added-formaldehyde or ultra-low-emitting-formaldehyde resins; low-emission resilient flooring at 90 percent coverage for the first tier and 100 percent for the second; thermal insulation meeting emission limits, with formaldehyde-free insulation at the higher tier; MERV 8 or better filters on return air openings when the heating system runs during construction; and direct-vent appliances where equipment sits in conditioned space, or the equipment isolated in a mechanical room (checklist).
None of that binds you because it is printed in the code. It binds you if your jurisdiction adopted it — which many have, in whole or in part. This is the single most useful question to ask a plan checker early: has this city adopted a green tier, and if so which one? The answer moves material selections, not just paperwork.
The commercial side
Nonresidential projects get their own chapter of mandatory measures, aimed at material conservation, resource efficiency and reducing greenhouse gas emissions — through moisture protection, construction waste diversion, recycling, lower-emission products and building commissioning (§ 5.401).
Two thresholds there matter to anyone doing tenant work. Recycling areas serving the whole building are required, and they reach additions that increase floor area by 30 percent or more within a 12-month period under single or multiple permits — with an exception for additions inside a tenant space that grow that space by less than 30 percent. And commissioning — a formal process of documenting design intent and then testing that the systems actually deliver it — applies to new buildings of 10,000 square feet and over, with exceptions for unconditioned warehouses of any size, tenant improvements under 10,000 square feet, and open parking garages (§ 5.410). A local recycling ordinance that is more restrictive controls instead.
The one thing to check before you spec anything
Find out which column your project is being reviewed against. The residential mandatory measures apply as printed; everything in the tiered menu depends on a local adoption. Cities may go beyond the state baseline, and on green measures many deliberately do. What no city may do is drop below it.
More plain-English guides to California's code parts are on our building codes hub.
Sources
- 2025 California Green Building Standards Code § A4.602 — residential application checklist, mandatory and tiered measures
- 2025 California Green Building Standards Code § 5.401 — scope of the nonresidential mandatory measures
- 2025 California Green Building Standards Code § 5.410 — recycling and building commissioning
- 2025 California Green Building Standards Code, Chapter 4 — residential mandatory measures
