Building an accessory dwelling unit in California is one of the smartest investments a homeowner can make — but the decisions made at the design stage, before a single permit is pulled, can cost or save you thousands of dollars.
California’s Title 24 energy code applies to ADUs, and how your unit is designed determines which rules apply and how expensive compliance will be.
Here are three code-based facts worth knowing before you start.
Attached or Detached Makes a Bigger Difference Than You Think
An ADU that shares a wall with your existing home is treated as an addition under California’s building energy code. That means it follows addition compliance rules rather than new construction rules — a meaningful distinction. New construction triggers the full suite of Title 24 requirements: mandatory solar, mechanical ventilation systems, HERS verification of HVAC and insulation, and more.
An attached ADU keeps many of those requirements tied to the existing building’s scope, which is typically narrower and less expensive to satisfy. If your lot and design allow for it, attached is almost always the more cost-effective path from a compliance standpoint.
Size Matters — Stay at or Below 700 Square Feet
California’s Title 24 energy code (§150.2(a)1) draws a clear compliance line at 700 square feet. At or below 700 square feet, Quality Insulation Installation — known as QII — is not required. Above 700 square feet, QII becomes the prescriptive default.
QII is a detailed insulation standard that requires a certified HERS Rater to inspect and verify the installation before walls are closed. Alternatives to QII exist, but they typically involve performance modeling and equipment upgrades that cost more than simply meeting QII. Either way, once you cross 700 square feet, your compliance options become more expensive. It is worth knowing that 701 square feet and 699 square feet are not the same project.