What is the difference between prescriptive and performance compliance?

Both are valid ways to prove your ADU meets Title 24, recorded in the same registered CF1R. The prescriptive path requires every regulated component to individually meet a fixed minimum, while the performance path uses energy-modeling software to show your whole-building design performs as well as or better than a code-compliant baseline — trading flexibility for a more complex analysis.

Prescriptive: meet every minimum, no trade-offs

On the prescriptive path, each component must individually meet its fixed requirement — wall and ceiling R-values, window U-factor and SHGC, HVAC efficiency, water-heater type, and lighting all have to clear their own minimum. It is the simplest, most predictable route, but it offers no flexibility: if one element falls short, the design doesn't comply, even if it is generous everywhere else.

Performance: model the whole building

On the performance path, approved energy software models the entire ADU and compares it against a code-compliant baseline building. As long as your design meets or beats that baseline overall, you can trade a weaker spot in one area for a stronger one elsewhere — for example, offsetting a larger window area with better insulation or a more efficient heat pump. This is useful when site or space constraints push you off the standard prescriptive solution.

Side-by-side

Which one fits your ADU

A practical rule of thumb: if your ADU uses conventional, all-electric systems — a ductless mini-split heat pump, a heat-pump water heater, LED lighting — the prescriptive path is usually the cleanest. If a design choice (an oversized window wall, a tankless or storage water heater, a space-constrained equipment location) won't meet a prescriptive minimum, the performance path lets the rest of the design carry it.

Either way, the path chosen in the CF1R sets the efficiency targets your mechanical, electrical, and plumbing sheets must match — and on the performance path, the equipment the model relied on must be exactly what the plans specify, or the plan checker issues a correction. The applicable code cycle and local amendments vary — confirm with your local building department. We select and document the appropriate path in the registered CF1R as a +$240 Title 24 add-on alongside your MEP plans; see Title 24 Reports.