Does a tiny house on wheels need a Title 24 report?

Not if it is an RV or park trailer built to RV standards, because those are not built under the Building Standards Code. Where you may live in it is a separate question for the local agency.

Is a tiny home on a foundation an ADU?

It can be, if it meets the Government Code definition, and HCD says such units are subject to ADU law. Either way it is built to the Building Standards Code, Energy Code included.

Does a tiny home need solar panels?

A new site-built tiny home is a newly constructed building, so PV applies unless an exception does. Because the minimum system is calculated from floor area, a small home may come in under 1.8 kWdc, and then none is required.

Do Tiny Homes Need Title 24?

It depends on what the tiny home legally is: built on site on a foundation, as a house or an ADU, it is a dwelling under the California Building Standards Code and needs Title 24 energy compliance like any new home. On a chassis with wheels it is usually a recreational vehicle or park trailer built to RV standards, and the Energy Code does not apply.

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"Tiny home" is not a legal category

HCD's Information Bulletin 2016-01 explains that no state or local agency has a specific definition or approval process for tiny homes as a product; the bulletin describes them as anything from 80 to 400 square feet. To be lawfully occupied, a tiny home has to qualify as one of a recognised set: a recreational vehicle, a park trailer, a HUD-code manufactured home, factory-built housing, a camping cabin, or a dwelling built to the California Building Standards Code. Which of those it is decides whether Title 24 applies.

On wheels: built to RV standards

The bulletin describes a recreational vehicle as built on a single chassis, with less than 320 square feet of internal living area and 400 square feet or less of gross area. Park trailers, often sold as tiny homes, are a type of RV: built on a single chassis, 400 square feet or less when set up (excluding a qualifying loft), and meant for recreational or seasonal use rather than as a permanent home. Both are built to RV construction standards, ANSI A119.5 or NFPA 1192, not to the Building Standards Code, so there is no Title 24 energy report. The bulletin adds that unless a local ordinance allows otherwise, RVs and park trailers generally may be occupied only in mobilehome parks or special occupancy parks.

On a foundation: a house like any other

A tiny home that is not on a chassis, sits on a foundation and is built on site is a dwelling unit under the Building Standards Code, and the Energy Code is Part 6 of that code. If it meets the ADU definition in Government Code Section 66313, HCD's ADU Handbook says it is subject to ADU law; see California ADU rules. For Title 24, a new detached unit is a newly constructed building, with a CF1R, the mandatory measures, ECC field verification where required and, unless an exception applies, solar PV. Our page on detached ADUs walks through it.

Factory-built housing generally has to meet the same standards as conventional housing. In the 2016 bulletin, HCD said it had not approved any factory-built units as tiny homes. For prefabricated units, see prefab ADUs.

Where small size helps

The 2025 Energy Code has several allowances for new dwelling units of 500 square feet or less, which takes in most tiny homes:

an electric water heater with point-of-use distribution in place of the prescriptive heat pump water heater (Exception 1 to Section 150.1(c)8); a unit with one bedroom or less may instead use a 120-volt heat pump water heater (Exception 2)
no whole-house fan requirement (Exception to Section 150.1(c)12)
in climate zone 5, a maximum window U-factor of 0.30 rather than 0.27 (Table 150.1-A, note 12)
for heat pumps with backup heat in buildings under 500 square feet, simpler control options in place of the 35°F lockout (Exception 1 to Section 150.0(h)7)
no 90-minute defrost delay-timer setting in climate zones 3, 5 to 10 and 15 (Exception 2 to Section 150.0(h)6)

Solar is calculated rather than waived for size. Equation 150.1-C sets the minimum system from conditioned floor area and the number of dwelling units, with factors that vary by climate zone, and no PV is required when that minimum is under 1.8 kWdc (Exception 2 to Section 150.1(c)14). More in solar requirements for new homes.