Duplex Title 24: One Report or Two?
A duplex is one single-family building under Part 6, not a pair of multifamily reports. Solar is sized once for the whole building with both units counted, while installation and verification records follow each dwelling unit. No section of the Energy Code sets how many CF1R files a permit set carries, so ask the plan checker.
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The classification that sets the forms
Part 6 never uses the word duplex. Section 100.1 defines a single-family building to include a Group R-3 building with two or fewer dwelling units, and the Multifamily Compliance Manual sends duplexes to the single-family manual. The CF1R family applies; the reasoning sits on the duplex classification page.
What is figured once for the building
Equation 150.1-C, the solar formula, includes a term for the number of dwelling units, and the single-family manual states that it is assessed on the building rather than on each unit. With two units the formula alone gives at least 2.12 kWdc in any climate zone, above the 1.8 kWdc exemption, so only the roof-area cap or another listed exception removes the requirement. Worked sizes are on the new-home solar size page. Battery-ready wiring under Section 150.0(s) also covers buildings of one or two dwelling units whose service is above 125 amps.
What follows each unit
- Installation and verification: the ECC-Rater confirms that the CF1R and CF2R are registered for every unit to be tested, and a registered CF3R must be available for each unit at final inspection.
- Ventilation: Section 160.2 does not apply to two-unit dwellings, so each unit follows Section 150.0(o). The Aext exterior-wall ratio is defined for attached units that share no floor or ceiling with another unit, which a side-by-side duplex satisfies and a stacked one does not.
- Sampling: the manual describes group sampling for several similar units in one subdivision, so plan a lone duplex as two tested units.
The mistake to avoid
Borrowing multifamily rules because there are two kitchens. That brings in the LMCC set, the Equation 160.2-B ventilation rates and a 4 kWdc exemption threshold, none of which a duplex uses. The line moves at a third unit, covered in triplex requirements. Two detached houses are two buildings with their own documents; see SB 9 projects, the residential CF1R and the multi-family service.
Common Title 24 Questions
- Do apartments need Title 24?
- Does a duplex need Title 24?
- Do triplexes and fourplexes need Title 24?
- Which Title 24 forms does a multi-family project use?
- What is an LMCC certificate?
- How much does a multi-family Title 24 report cost?
Frequently Asked Questions
No. Section 100.1 counts a Group R-3 building with two or fewer dwelling units as single-family, so the single-family standards and CF1R forms apply.
Yes, under Section 150.1(c)14, unless an exception applies, such as a solar access roof area under 80 contiguous square feet or a required size under 1.8 kWdc.
Explore Multi-Family
- Does a Duplex Need Title 24?
- What Is Dwelling Unit Compartmentalization?
- Fourplex Title 24 Requirements
- What Is an LMCC Certificate?
- Low-Rise or High-Rise Multifamily: Which Applies?
View all Multi-Family guides → · Permits and Compliance · Title 24 guide