Is my ADU new construction, an addition, or an alteration under Title 24?

It depends on how the ADU is built, not what it is called. Under the 2025 California Energy Code, a detached ADU built from the ground up is a newly constructed building, an attached ADU or a garage conversion is an addition, and turning space that is already conditioned into an ADU is an alteration. That one label decides which rules apply to your project, including whether solar PV is required.

The three categories in plain English

• Newly constructed building: the Energy Code defines this as a building that has never been used or occupied for any purpose. A detached ADU built from scratch fits here. So does a new ADU linked to the house only by a breezeway or covered walkway, because it shares no wall, floor or ceiling with the home.
• Addition: any change that increases conditioned floor area and conditioned volume. Building an ADU that shares a wall with your house is an addition. So is converting an unconditioned garage, attached or detached, because the code says newly conditioned space must meet the requirements for an addition. The CEC also treats an ADU built on top of a detached garage as an addition.
• Alteration: a change to a regulated system or component that is not an addition. Converting an already conditioned basement, bonus room or part of the house into an ADU or junior ADU is an alteration, and only the components you change (HVAC, water heater, lighting, ventilation, envelope) pick up requirements.

Why the label matters

• Solar PV: required for newly constructed ADUs unless an exception applies. The code states that PV is not required for additions, and alterations don't trigger it either.
• Battery-ready and electric-ready: additions must meet a narrower list of mandatory measures (Sections 150.0(a) through (n), (p) and (q)). The battery-ready rules in 150.0(s) and the electric-ready rules for gas furnaces, cooktops and dryers in 150.0(t) through (v) are written for new construction.
• Envelope and HVAC sizing: additions follow Section 150.2(a), which has its own size-based options, such as relaxed rules for additions of 700 square feet or less. A new detached ADU follows the same Sections 150.0 and 150.1 as any new single-family home.
• Field verification: both new ADUs and ADU additions need ECC-Rater verification of items like ventilation airflow.

Gray areas

A garage that was already heated, a partly finished space, or a structure built without permits may not fit neatly. For unpermitted work, the CEC says the building department decides whether the ADU is treated as new construction or an addition. Raise any doubt with plan check early instead of after the report is done.

What this means for your MEP plans and Title 24 report

The project type is the first input in the compliance software. It sets the standard design, the PV calculation and the mandatory items your electrical and mechanical sheets must show. If the drawings describe one type and the CF1R models another, expect a plan check correction. We confirm the classification before modeling. See the ADU Title 24 guide, our Title 24 reports page, and prescriptive vs. performance compliance.

Sources: energy.ca.gov, energy.ca.gov, energy.ca.gov.