Basement Conversion Title 24 Requirements in California
Finishing a basement is an alteration, an addition, or a new attached ADU if you dig one, and the route sets what the rest of the house must do. Beyond the walls, the items that surprise owners are whole-dwelling ventilation above 1,000 square feet, extended ducts and a second water heater.
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Three starting points
- Already conditioned. Finishing it, or turning it into an ADU or junior ADU, is an alteration under Section 150.2(b), and only the components you alter must comply.
- Unconditioned today. Heating or cooling it is newly conditioned space, an addition under Section 150.2(a).
- Newly excavated. It adds conditioned floor area and volume, so it meets the definition of an addition. HCD lists an ADU built in a newly constructed basement among attached ADUs.
The wall and floor values are on the basement conversion page. This page covers what the rest of the house must do.
The systems around the basement
- Ventilation. An addition that raises a dwelling's conditioned floor area by 1,000 square feet or less skips the whole-dwelling airflow. Above that, the airflow is sized on the existing floor area plus the addition, so a 1,200 square foot basement under a 1,800 square foot house is sized for 3,000.
- Whole-house fan. Not required up to 1,000 square feet. Above that it is a prescriptive item in zones 8 to 14.
- Heating. Extending ducts from the existing furnace leaves the furnace itself alone, but its heating capacity must satisfy CRC 303.10 and the combined duct system is tested at 10 percent leakage or 7 percent to outside. A basement ADU cannot share a central-return furnace, because the Mechanical Code bars moving return air between dwelling units.
- Hot water. A new basement bath or laundry served by an additional water heater needs a heat pump water heater or another listed option; an electric unit with point-of-use piping is allowed when the addition is 500 square feet or less.
A junior ADU built as an addition is excused from the whole-dwelling airflow, though the Mechanical Code may still ask for ventilation. A new bathroom or kitchen needs local exhaust either way.
The ADU wording is on alteration versus addition, ventilation sizing is on whole-house ventilation, and the attic equivalent is attic conversion to living space. Our residential reports start from your plans.
Common Title 24 Questions
- When is Title 24 required for a home?
- Does a remodel need Title 24?
- Does a room addition need Title 24?
- What plans are needed for a residential Title 24 report?
- How much does a residential Title 24 report cost?
- How long does a residential Title 24 report take?
Frequently Asked Questions
No. Conditioned space that is repurposed is an alteration, and only altered components such as lighting, windows, water heaters or mini-splits must comply.
Not if it is an addition of 1,000 square feet or less to an existing dwelling and not a new dwelling unit. Local exhaust for a bathroom or kitchen still applies.
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