Fourplex Title 24 Requirements
A fourplex in one building of up to three habitable stories is low-rise multifamily. Four units push the solar formula above 4 kWdc in every climate zone, leave a central water heater free of the recirculation valve rule, and set a 25 pascal duct leakage test for a shared continuous ventilation shaft. Four attached townhouses are single-family instead.
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Four townhouses are a different building
Four attached units that each run from foundation to roof, with open space on two sides, meet the Section 100.1 townhouse definition and follow the single-family rules; see townhome Title 24. A fourplex of stacked or corridor-served units is multifamily, and the unit count then trips rules that a smaller building misses.
Solar cannot drop under 4 kWdc by formula
The unit term of Equation 170.2-C is four times the zone factor, at least 4.24 kWdc even in zone 5, before any floor area is added. The under 4 kWdc exemption therefore needs the solar access roof area cap: 250 square feet of usable low-sloped roof times 14 watts is 3.5 kWdc. A battery of 7.5 kWh or more allows a 25 percent cut in the computed size, though the Standards do not say whether the 4 kWdc test comes before or after that cut.
Hot water for four units
- Individual heaters: Section 170.2(d)1 allows a single 240 volt heat pump water heater, or one meeting NEEA Tier 3, with extra measures in zones 1 and 16. The gas instantaneous option is for buildings of four or more habitable stories, so it is out here.
- One central heater: it is a central system under Section 170.2(d)2, but the recirculation loop with a master mixing valve is waived for buildings of eight or fewer dwelling units.
A shared ventilation shaft
Where one central fan serves several units with continuous or balanced airflow, Section 160.2(b)2C caps duct leakage at 6 percent of fan design airflow. Ducts serving six or fewer units are tested at 25 pascals, so every fourplex gets the lower pressure. It is an acceptance test under NA7.18.3, certified before the occupancy permit, and three-story buildings in zone 6 are excepted.
Unit-level ventilation and air tightness work the same as in a triplex, set out on triplex requirements. More detail sits on multifamily water heating, multifamily ventilation 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
Not by unit count. The line is habitable stories: up to three is low-rise, four or more is high-rise, so most fourplexes are low-rise.
Not in new construction of three habitable stories or fewer. Exception 1 to Section 170.2(d)1 reserves that option for buildings of four or more habitable stories.
Explore Multi-Family
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- Multifamily Common Area HVAC Title 24 Requirements
View all Multi-Family guides → · Permits and Compliance · Title 24 guide