Take a 740 ft² detached ADU going into a Sacramento backyard — climate zone 12, one bed, one bath, mini-split on the wall. The owner assumes that because the main house was built in 1978, the ADU gets graded on some kind of curve. It does not. In the eyes of Title 24, that little building is new construction, full stop, and it answers to the 2025 code on its own.

Detached means its own compliance model

A detached new-construction ADU gets its own energy model, its own CF1R, its own registration. The existing house is irrelevant to the analysis — you model the ADU as the standalone dwelling it is. That cuts both ways. You cannot borrow the main house's thermal mass or its recent window upgrade; you also are not dragged down by its 1978 envelope. Small buildings are unforgiving in the performance math, though: one bad west-facing slider is a much bigger fraction of a 740 ft² model than of a 2,800 ft² one.

Attached rides in the parent's file

An ADU created by an addition — building onto the house, converting attached garage volume with new conditioned area — is modeled as part of the parent building, in the same compliance file, under the rules for additions. That is usually a gentler standard than standalone new construction, and it is one reason attached conversions sometimes pencil where detached new builds strain. Pure conversions of existing conditioned space are gentler still, but that is a different article. The line between “addition” and “new detached building” is drawn by your building department, so confirm the classification before anyone runs a model — it changes everything downstream.

Yes, the PV requirement applies

A detached newly constructed ADU is a newly constructed dwelling, so the solar mandate of §150.1(c)14 applies. The array is sized by the standards' formula from floor area and climate zone — for a small ADU it typically lands around 1 to 2 kW, not a full-roof array. The exceptions do real work here:

  • No usable roof: if shading or geometry leaves less than the minimum solar-access area, the requirement can drop away. Backyard ADUs tucked under mature trees hit this exception constantly.
  • Existing PV: a sufficient system already on the property can satisfy the ADU's obligation — document it in the compliance run.
  • Battery trade-offs and community solar options exist in some territories, though availability varies by utility.

Do not assume the exception; claim it in the model with the shading study or documentation to back it up. Plan checkers ask.

The typical mechanical package

The 2025 standard design assumes heat-pump space and water heating, which happens to match what ADU builders were doing anyway. A single-zone mini-split heat pump handles a small open plan easily and models well. Water heating is usually a heat-pump water heater — a 40 or 50 gallon unit in a closet with the required volume or ducting — or, where gas is genuinely unavoidable, a penalty to overcome. Ducts, when they exist at all, are short; many ADUs skip them entirely, which conveniently also skips duct-leakage verification.

ADUs feel like small projects, and the construction is. The compliance workload is not proportionally smaller — it is most of the same checklist in miniature, plus a few state and local ADU-specific wrinkles. Jurisdictions also layer their own ADU ordinances on top of the state rules, so the energy code is one voice in a chorus. Get the classification and the PV question answered in week one and the rest is routine.