California’s 2025 Title 24 Energy Code: What Changed for Architects
California's 2025 Building Energy Efficiency Standards (Title 24, Part 6) took effect January 1, 2026. If you're an architect with projects in California, this affects your design process. The changes are significant enough that designing first and handing off to an engineer for compliance later no longer works.
This post covers the highest-impact changes for architects and design teams. For a complete list, the Energy Commission has published condensed two-page memos that are worth reading:
Key Changes for Architects
New Compliance Metric: Long-term System Cost (LSC)
The biggest shift is the switch from Time Dependent Valuation (TDV) to Long-term System Cost (LSC). TDV weighted energy based on when it was used during the day. LSC looks at lifecycle costs and greenhouse gas emissions instead.
In practice, this changes how systems score under the compliance math. Electric heat pump systems and renewable-ready designs perform better under LSC than they did under TDV. If your team has been modeling under the old framework, your baseline assumptions need updating.
Expanded Electrification Requirements
Multi-zone HVAC systems in offices and schools under 150,000 square feet or five stories must now be heat pump based when using the prescriptive compliance path. This includes systems like:
DOAS with heat recovery and VRF or fan coils
Air-to-water heat pump VAV systems
Dual duct dual fan systems
The code doesn't just require electrification. It sets detailed expectations for fan power, controls, heat recovery, and part-load operation. The result is a push toward integrated system design rather than simple fuel switching.
Heat pump water heaters are now prescriptively required in small school buildings (less than 25,000 square feet, fewer than four stories, Climate Zones 2-15). This signals growing confidence in HPWH performance and cost-effectiveness in nonresidential settings.
Tighter Envelope Performance
The prescriptive envelope requirements (Table 140.3-B) have been tightened across most climate zones. Maximum U-factors for many building assemblies have been reduced. Fenestration requirements have been updated with more stringent criteria.
Glazing selection and window-to-wall ratios need to be checked against the new thresholds early in design development, not at permit stage. If wall assemblies and roof systems that passed the 2022 code are now being reconsidered, this is why.
Mandatory Vestibules
Vestibules are now required for most nonresidential buildings (with exceptions). While this is a simple architectural move, it reinforces envelope performance. For high-traffic buildings, vestibule infiltration losses can quietly undermine efficiency goals if not designed in from the start.
Embodied Carbon Requirements
For the first time, embodied carbon is formally required in California's building code. New construction and major alterations over 50,000 square feet must now demonstrate at least a 10% reduction in embodied carbon compared to a baseline design. This focuses on primary and secondary structural elements and building enclosure components.
Compliance pathways include building reuse, whole-building lifecycle assessment, or prescriptive material limits. The shift is clear: operational efficiency alone is no longer sufficient. Carbon impacts are now evaluated across a building's full lifecycle.
Balanced Ventilation in Multifamily
Exhaust-only ventilation is no longer allowed in multifamily dwelling units. Units must now use balanced or supply ventilation, paired with compartmentalization testing. This improves indoor air quality, filtration capability, and control over infiltration and energy losses.
Designers now have more flexibility in system configuration. Central ERVs, in-unit systems, or hybrid approaches all work. But relying solely on exhaust fans is no longer an option.
Simultaneous Mechanical Heat Recovery
One of the most complex additions to Title 24 2025 is the requirement for simultaneous mechanical heat recovery in certain large buildings. This requirement is capacity-based, not square-footage based. Buildings with overlapping cooling and heating loads (especially those with significant domestic hot water or process cooling) may be required to recover heat between systems using water-to-water heat pumps or similar technologies.
If a building is rejecting heat while simultaneously needing it elsewhere, the code increasingly expects that energy to be reused.
Space Heating Hot Water Maximum Setpoint
Hot water systems used for space heating must be designed for a maximum supply temperature of 130 degrees Fahrenheit. This applies broadly and supports higher heat pump efficiency, better condensing boiler performance, and future-ready low-temperature hydronic systems. Rather than mandating specific equipment, the code shapes systems toward operating conditions that align with decarbonization.
Code Format Reorganization
One meaningful change that does not introduce new requirements is how the code is organized. A new, optional restructured format reorganizes Title 24 by building system (envelope, HVAC, lighting, plumbing, electrical). Mandatory, prescriptive, and performance requirements are now placed together instead of scattered across chapters.
While this version is not the legally adopted code, it dramatically improves usability. Navigation is easier for non-energy-code experts. System-level requirements are clearer. Interpretation during design and review is faster.
Prescriptive vs. Performance Compliance
The 2025 code offers two compliance pathways. Prescriptive compliance means following specific requirements for each building system. Envelope U-factors, equipment efficiency minimums, lighting power densities all must meet individual thresholds.
Performance compliance compares your proposed building to a code-baseline building model. As long as your building achieves equivalent or better overall energy performance, you comply. Even if individual systems don't meet prescriptive requirements, performance compliance can work.
For complex or architecturally ambitious projects, performance compliance often provides the latitude needed to pursue design goals while still meeting code. For more straightforward projects, prescriptive is typically faster.
The important thing to recognize is that performance modeling under LSC requires recalibration from prior-code workflows. Running the same modeling approach you used under the 2022 code will produce incorrect compliance results.
What This Means for Your Design Process
The 2025 code reinforces a shift that has been building for several code cycles. Energy compliance is now a design discipline, not a post-design exercise.
Key energy decisions need to be made at schematic design and early design development, not at permit. Specifically:
Glazing and envelope assemblies should be modeled against 2025 prescriptive thresholds before design development.
HVAC system type (heat pump vs. gas, VAV configuration) needs to be determined early enough to affect floor plans and mechanical room sizing.
PV and battery storage should be integrated into roof planning and electrical load calculations from the start.
Performance modeling should begin no later than design development to allow time for iteration.
Bringing an energy consultant in during schematic design rather than at permit typically reduces redesign costs and compliance risk.
Common Pitfalls to Avoid
Using old TDV-based modeling workflows. The switch to LSC changes how compliance is calculated. Verify that your energy modeling tools and consultants are updated for LSC.
Assuming 2022-compliant glazing still works. Reduced U-factor maximums mean previously compliant assemblies may now fail prescriptive requirements. Check early.
Under-sizing electrical infrastructure. Expanded electrification requirements mean higher electrical loads. Undersizing at permit leads to expensive changes during construction.
Late MEP coordination. Require mechanical specifications to reflect ASHRAE Guideline 36 controls and fault detection and diagnostics from the start, not added at the last minute.
Missing vestibule requirements. The new vestibule threshold can affect entry design in commercial, retail, and institutional projects. Check your occupancy type early.
Early-Design Energy Analysis
If you're working through your first 2025 project, early-design energy analysis is the best investment you can make. A performance assessment during schematic design catches conflicts before they become late-stage constraints. You'll understand:
Whether prescriptive or performance compliance is your fastest path
Which system choices (HVAC type, glazing, envelope assembly) have the biggest impact on energy performance and cost
What equipment and system flexibility you'll need to lock in early vs. optimize later
Whether embodied carbon requirements will drive material or structural decisions
The window for design flexibility is the schematic and early design development phase. After that, compliance becomes a constraint rather than an opportunity.
What A2 Efficiency Can Help With
A2 Efficiency helps architecture and design teams navigate California's energy codes from first sketch through permit. Our services include:
Early-design energy analysis under the 2025 LSC framework
Building energy modeling for prescriptive and performance compliance paths
Code compliance consulting for Title 24, Part 6
MEP engineering for HVAC design and heat pump system integration
Integrated design support for high-performance building projects
If you're working on a California project with a permit date on or after January 1, 2026, early-stage analysis will save time and cost later.