Title 24 for an ADU on a Multifamily Property

What state law allows

HCD's ADU Handbook (March 2026) describes two kinds of ADU that a local agency must allow on a lot with a multifamily dwelling under Government Code section 66323:

• Converted ADUs in parts of an existing multifamily structure not used as livable space, such as storage rooms, boiler rooms, passageways, attics, basements or garages: at least one, and up to 25 per cent of the existing units.
• Detached ADUs: up to two on a lot with a proposed multifamily dwelling, or up to eight on a lot with an existing one, not exceeding the number of existing units.

HCD adds that a leasing office can be converted, and that a JADU cannot be built on a multifamily site. Setbacks, heights and the single-family rules are on California ADU rules. None of that is Part 6, which governs how the unit is built.

The Energy Code has no ADU category

The CEC's Single-Family Compliance Manual says so directly: the Energy Code does not define an accessory dwelling unit. An ADU is documented as whatever its construction makes it, under the part of the code that governs the building it is in. An ADU inside an apartment building and one beside it are therefore different reports.

Inside an existing multifamily building

Section 100.0(e)3 sends additions and alterations in existing multifamily buildings to Section 180.0. Storage rooms, garages and attics are usually unconditioned, so converting one creates newly conditioned space, which Section 100.1 treats as an addition under Section 180.1:

• The envelope follows Section 170.2(a) with the Section 180.1 changes. Over 700 square feet, a conversion may skip the air-sealing part of QII where the existing air barrier stays. At 700 or less, QII does not apply, and windows meet Table 180.2-B.
• A new dwelling unit created by an addition needs whole-dwelling ventilation airflow based on its own conditioned floor area, under Section 180.1(a)2Aiii. No dwelling-unit air leakage test is required for additions.
• New water heating for the unit meets Section 170.2(d) or an equivalent, and new heating serving the addition may be a heat pump or gas (Exception 7).
• Where heating or cooling is extended from existing systems, the existing equipment need not comply (Exception 4), and PV and batteries are not required (Exception 6).

Converting space that is already conditioned, such as a conditioned leasing office, adds no conditioned floor area. It is an alteration under Section 180.2, regulated where components change. Where ventilation must be field verified, buildings of up to three habitable stories use the Residential Appendices and taller ones Nonresidential Appendices NA1 and NA2.

Detached ADUs

A new detached ADU has never been used or occupied, so it is a newly constructed building in its own right. Section 100.1 makes a Group R-3 building with two or fewer dwelling units a single-family building, documented on the single-family forms with the single-family solar requirement; HCD confirms that newly constructed detached ADUs carry it, subject to exceptions. Several ADUs in one detached building classified Group R-2 would form a multifamily building instead.

Duplexes

State ADU law treats any structure with two or more attached dwellings as multifamily. Part 6 does not: a Group R-3 building with two or fewer dwelling units is single-family, as explained on does a duplex need Title 24. Which part governs an ADU added to a duplex follows the occupancy the building department assigns, so settle it before the design.

Common Title 24 Questions

1. Do apartments need Title 24?
2. Does a duplex need Title 24?
3. Do triplexes and fourplexes need Title 24?
4. Which Title 24 forms does a multi-family project use?
5. What is an LMCC certificate?
6. How much does a multi-family Title 24 report cost?

Frequently Asked Questions

Does an ADU converted from storage in an apartment building need solar?

Can a JADU be built in an apartment building?

Does the rest of the apartment building need upgrading?

Which forms does a converted ADU use?