Attached ADU Title 24 Report
An attached ADU is documented as an addition under Section 150.2(a), so solar PV is not required and a unit of 700 square feet or less gets a lighter prescriptive package. Because it is also a second home, it needs heating and cooling with its own thermostat, its own mechanical ventilation, and no return air shared with the main house.
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Addition rules for the shell, dwelling-unit rules for the systems
The Energy Commission’s ADU FAQs call a unit attached when it shares a common wall, floor or ceiling with another dwelling unit. Building it adds conditioned floor area and volume to an existing building, which is the Energy Code’s definition of an addition, so it complies under Section 150.2(a) rather than as new construction. HCD’s handbook gives examples that fit: a new second story on the house, a newly built basement, a unit on top of an attached garage.
That settles the envelope, not everything. The result is a separate household, and the mechanical and ventilation rules treat it as one; documenting it as a bedroom wing with a kitchen misses them.
The shell: the 700 square foot line
Above 700 square feet the unit meets the new-home package as modified by Section 150.2(a)1A. At 700 square feet or less, Section 150.2(a)1B relaxes it further:
- a ventilated attic takes R-38 (U-0.025) in climate zones 1, 2, 4 and 8 to 16, or R-30 (U-0.031) in zones 3 and 5 to 7, plus a radiant barrier in zones 2 to 15;
- walls that extend the house’s wood-framed walls may keep their depth, with R-15 cavity insulation in 2x4 framing or R-21 in 2x6;
- glass area follows the addition allowances, with west-facing glass held to 60 square feet in zones 2, 4 and 6 to 15;
- Quality Insulation Installation does not apply.
The glazing allowances per size band are worked through on the room addition page.
What an addition does not carry
Exception 6 to Section 150.2(a) says photovoltaic systems are not required for additions, and the ADU FAQs apply that to attached ADUs directly. The manual limits the electric-ready rules for gas appliances to newly constructed buildings, and Section 150.2(a) does not list the battery-ready measures for additions. At 300 square feet or less the cool-roof rule drops away; at 1,000 or less, the whole-house fan.
Heating and cooling of its own
Section 150.2(a) has two exceptions for stretching an existing system into an addition. Under Exception 3 the existing equipment need not meet Part 6 if its heating capacity meets CRC 303.10. Under Exception 4, once any duct is extended, old and new ducts meet the altered-system rules: R-6 or R-8 on new ducts in unconditioned space, by zone, and a leakage test.
For an ADU, the Mechanical Code gets in the way. CMC 311.4 does not allow return air from one dwelling unit to be discharged into another through a shared system, and the manual and the FAQs both conclude that a furnace with a central return cannot be extended to serve an ADU. The manual names ductless systems as an option. Either way the unit needs its own thermostat, heat able to hold 68°F in its habitable rooms, and load calculations under Section 150.2(a)1E. A new system serving it meets new-home rules, including refrigerant charge verification on a heat pump in every zone and fan airflow and watt draw testing.
Ventilation and hot water
A room addition of up to 1,000 square feet is excused from whole-dwelling ventilation airflow. An attached ADU is not: Section 150.2(a)1C gives a new dwelling unit added to a building mechanical ventilation sized on its own floor area. Only a JADU is exempt. A unit stacked above or below the house cannot lower that rate with the infiltration credit, which Section 150.0(o)1C bars for vertically attached units. An ECC-Rater verifies a vented range hood.
An added water heater must prescriptively be a heat pump model, not outdoors, on a rigid R-10 base, with a demand-response interface or CTA-2045-B port, or a NEEA Tier 3 model. At 500 square feet or less, one electric water heater with point-of-use distribution may be used instead.
Forms and the performance route
A prescriptive attached ADU under 1,000 square feet goes on CF1R-ADD-01-E, or CF1R-ADD-02-E when nothing needs field verification. The performance path models the addition alone, or the house, its alterations and the addition together. If an ECC-Rater (a HERS rater until 1 January 2026) is needed, the CF1R is registered with an ECC-Provider first. Units over a garage are covered on ADU above a garage, converted garages on garage conversion ADUs.
Other ADU Types
Frequently Asked Questions
No. Exception 6 to Section 150.2(a) removes the PV requirement for additions, and the Energy Commission’s ADU FAQs confirm it for attached ADUs.
Not through a ducted system with a central return. The Mechanical Code does not allow return air from one dwelling unit to be discharged into another, so the ADU normally gets its own system, and ductless equipment is one option.
Not on the prescriptive path, because Section 150.2(a)1B removes it at 700 square feet or less. If a performance model takes credit for it, an ECC-Rater has to verify the insulation and air barrier.
Not at 1,000 square feet or less, under Exception 5 to Section 150.2(a). It still needs whole-dwelling mechanical ventilation, which is a separate requirement.