What Title 24 Requires for Every New ADU in California
Title 24 Part 6 applies to every ADU and garage conversion built in California.
This is the section of California's Building Standards Code that governs energy efficiency for residential construction. For ADUs, it sets mandatory performance minimums for insulation, windows, HVAC systems, water heaters, and mechanical ventilation. Every requirement must be calculated, documented, and approved before a permit is issued. There are no exemptions based on size, and none based on how the unit is used.
Why Climate Zone 3 Changes the Numbers for San Mateo County
California's energy code divides the state into 16 climate zones, each with its own performance requirements based on local temperature, humidity, and solar exposure. Climate Zone 3 covers most of San Mateo County — mild temperatures, consistent marine influence from the Bay, and relatively low cooling loads compared to inland California.
In practice, Zone 3 doesn't carry the extreme summer heat that drives aggressive HVAC sizing in zones like 14 or 16 — but it does have specific insulation R-values, window performance thresholds, and ventilation requirements for every new ADU and conversion in the county. Those thresholds aren't identical to Sacramento or the Central Valley, and a compliance report prepared for the wrong climate zone will fail plan check. We apply Climate Zone 3 requirements to every project, with the calculations run against the correct zone from the first draft.
What Title 24 Covers for ADUs — Five Categories
Each category carries its own threshold and documentation requirement. The figures below are the Climate Zone 3 prescriptive values.
Insulation R-values
Walls: R-15 in a 2×4 cavity or R-21 in 2×6. Ceiling/attic: R-38. Raised floor: R-19. Verified at the insulation inspection — before drywall. Once drywall is up, the inspector can't confirm what's inside the wall, and a failed rough-in means the drywall isn't approved.
Window U-Factor & SHGC
U-Factor measures heat transfer through the assembly (lower is better); SHGC measures solar energy through the glass (lower cuts cooling load). Zone 3 requires U-Factor ≤ 0.30 and SHGC ≤ 0.23, documented with the manufacturer's rated values in the plan set.
HVAC efficiency
Split-system AC must meet a minimum SEER2; heat pumps must meet minimum HSPF2 and SEER2. Minimums depend on capacity and type. Equipment must be confirmed compliant before it's specified — substituting a unit mid-construction can force a compliance recalculation.
Mechanical ventilation
A tightly sealed envelope requires a ventilation system — an HRV, ERV, or controlled exhaust fan. It's triggered by how tight the envelope becomes, not by whether the unit is new or converted: you can't meet the insulation standard without triggering the ventilation standard.
Water heater efficiency
Heat-pump water heaters are commonly specified for Zone 3 — they meet the efficiency floor and qualify for incentive programs. Standard resistance electric units typically don't meet the current threshold alone. This is a design-phase decision, not an afterthought.
Common Scenarios Where Title 24 Catches Homeowners Off Guard
The garage conversion that already exists
When a structure shifts from non-habitable to habitable use, the code treats it as new residential construction for energy purposes — existing shell or not. Insulation, window, HVAC, and ASHRAE 62.2 requirements all apply. Submitting without Title 24 docs returns a correction, and one cycle can push a start date two months or more.
The new ADU with the wrong window spec
Windows chosen for aesthetics go into the set, the permit is submitted, and plan check flags a U-Factor rated 0.35 instead of the required 0.30. The fix means redesigning the schedule, updating the report, and resubmitting — avoidable by confirming U-Factor and SHGC against the calc before finalizing.
The floor plan without a Title 24 report
A pre-drawn plan or design-only package often lacks the CF1R — the California Energy Commission compliance certificate that's a required part of the permit set. An incomplete submission gets rejected at intake or returned as a correction, and nothing moves until the documentation is complete.
Why We Build Compliance In Before the First Submission
Our in-house design team runs Title 24 calculations during design — specifying window performance, insulation assemblies, HVAC equipment, and ventilation while the floor plan is still being finalized. That sequencing matters: when the compliance report and the drawings are developed together, the specs align. The window in the plan set matches the U-Factor in the CF1R; the wall-section R-value matches what the calc requires; the HVAC is sized to the standard before it's drawn.
Projects that arrive after a plan check correction often had compliance prepared separately — sometimes by a different party, sometimes after the plans were finished. The plans and the report don't agree, plan check finds the discrepancy, and the resubmission takes weeks. Building compliance into the first submission is the baseline we hold every permit package to.
Title 24 documentation in every permit set — in-house, CA License #1119304.
When You Need a Builder Who Handles Title 24 Directly
If your project needs a permit set, Title 24 documentation should come with it. The CF1R and CF2R are the California Energy Commission compliance documents submitted with an ADU permit: the CF1R is the certificate of compliance documenting what the design specifies; the CF2R confirms field installation at the inspection stage. Both must be present and accurate — a builder who understands the process prepares both as standard deliverables.
Serving San Mateo County ADU Projects
We handle Title 24 compliance documentation for ADU builds, garage conversions, and JADU projects as part of every permit package, throughout San Mateo County.
Title 24 is one part of the package — see how it fits the full ADU permitting process.
Start Your ADU Project With Compliance Built In
Every ADU permit set we submit includes Title 24 documentation, complete on the first submission. If you're planning a new ADU or garage conversion in San Mateo County and want to understand how Climate Zone 3 requirements apply to your specific project, we're ready to walk through it.
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