Can I convert my basement into an ADU in California?

Yes. A basement is existing space within your house, so state law allows it to become an ADU, or a JADU of up to 500 square feet, and HCD's handbook also lists an ADU in a newly built basement as an attached ADU. How the 2025 Energy Code treats it depends on whether the basement is already conditioned: the CEC calls converting a conditioned basement an alteration, while turning an unconditioned basement into living space creates newly conditioned space, which must meet the requirements for an addition.

Building code items unique to basements

• Emergency escape. The California Residential Code requires emergency escape and rescue openings for basements and for every sleeping room, which often means a new egress window with a window well.
• Ceiling height. Beams, ducts, and pipes often hang below basement joists. The code's minimum ceiling heights for habitable space apply, with limited allowances for beams and similar obstructions.
• Moisture. Below-grade walls and floors need a plan for water and vapor before finishes go in.
• Separation and access. A full ADU is a separate dwelling unit inside your house, so it needs fire-resistive separation from your living space and its own exterior access.

Energy Code: alteration or addition?

• Already conditioned basement: an alteration. Requirements apply to what you change, such as a new water heater, a new mini-split, lighting, or envelope work.
• Unconditioned basement: newly conditioned space that follows Section 150.2(a), including insulation of below-grade walls in the Title 24 model, whole-dwelling ventilation for the new unit, and ECC-rater verification where required.
• New basement: excavating a new basement for the ADU adds conditioned floor area, so it is an addition.

Plumbing: gravity decides a lot

Basement fixtures may sit below your sewer lateral. Drains that cannot reach the sewer by gravity must go to a sealed sump with a sewage ejector pump, and fixtures located below the next upstream sewer manhole cover generally need a backwater valve to protect against backups. The plumbing designer needs the elevations of your building drain and lateral to decide which applies.

Mechanical and electrical

• The ADU cannot share a forced-air system with your house; the CEC notes the California Mechanical Code bars return air from one dwelling unit from being discharged into another. If house ducts pass through the basement, they must stay sealed with no supply or return openings in the ADU and be coordinated with the fire separation.
• Continuous ventilation and a plan for humidity help control below-grade moisture.
• Panels cannot be placed in bathrooms, and each household needs ready access to the breakers that serve its unit, which matters if your home's main panel is already in the basement.

What it means for your MEP plans

Start with a survey of what the basement already holds: the home's panel, furnace, water heater, ducts, and drain line. The MEP plans then show a separate heating and cooling system for the ADU, any ejector pump and backwater valve, the electrical feed and access arrangement, and the ventilation the Title 24 report requires. Useful reading: the California ADU requirements guide, ADU sewer connections, and the ADU Title 24 guide.

Sources: hcd.ca.gov, energy.ca.gov, energy.ca.gov, energy.ca.gov.