Does a new shed need a Title 24 report?

An unconditioned shed is still covered by Part 6, but only its lighting rules apply: JA8 light sources, wall switches and controls for outdoor fixtures. Whether the building department asks for a certificate is its decision.

Does a detached garage need solar panels?

Not if it is unconditioned, because it has no conditioned floor area for Equation 150.1-C. A newly constructed conditioned one is assessed for PV, and no PV is required when the calculated size is under 1.8 kWdc.

Is adding a mini-split to my workshop an addition?

Yes. Installing heating or cooling converts unconditioned space into newly conditioned space, and the compliance manual says newly conditioned space must comply with the requirements for an addition.

When does a studio or pool house become an ADU?

When it has complete, independent living facilities, including permanent provisions for living, sleeping, eating, cooking and sanitation, as Government Code section 66313 defines an ADU. The planning department decides the classification.

Sheds, Detached Garages and Studios: When Title 24 Applies

A shed, detached garage or workshop on a house lot is a single-family building in Energy Code terms, so its hardwired lighting is regulated even while it is unconditioned. Once it is heated or cooled, the envelope and equipment rules apply too, with solar above a size threshold; once it has permanent provisions for living, sleeping, eating, cooking and sanitation, it is an ADU.

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The usual assumption is that an outbuilding sits outside the Energy Code. For insulation and heating that is often right, for its light fixtures it is not, and once the building is heated, cooled or lived in the picture changes completely. This guide covers the buildings on a house lot that are not the house: sheds, detached garages, workshops, studios, home offices and pool houses.

How the code classifies them

Section 100.0(a) brings buildings of Occupancy Group U, the building code's group for buildings of an accessory character, within Part 6 whether they are unconditioned or conditioned. Section 100.1 then defines a single-family building to include "a building of Occupancy Group U when located on a residential site", and the compliance manual gives private garages as the example. So a detached garage behind a house is, in energy terms, a single-family building. Which rules apply depends on whether it is conditioned, which turns on the heating and cooling thresholds explained in conditioned and unconditioned space.

Unconditioned: the lighting still counts

For an unconditioned outbuilding the envelope and HVAC requirements do not apply, because Table 100.0-A attaches them to conditioned space. Lighting is different: the table applies the residential lighting rules of Section 150.0(k) to conditioned and unconditioned space alike. Section 100.0(c) says the same of unconditioned space in any story. In practice:

Installed luminaires and light sources must be JA8-certified and marked, with exceptions such as lighting built into a garage door opener, or LED sources installed outdoors.
Lighting needs a readily accessible wall switch, and in a garage at least one luminaire must switch off automatically through an occupancy or vacancy sensor.
Outdoor lights mounted on the building need a manual switch plus a photocell with a motion sensor, a photocell with a time switch, or an astronomical time clock. Section 150.0(k)3 expressly covers other buildings on the same lot as the house.
A residential garage for eight or more vehicles follows the nonresidential garage lighting rules instead.

Changing fixtures in an existing outbuilding is a lighting alteration; see replacing light fixtures and JA8 lighting. Whether the department wants a certificate for an unconditioned outbuilding is its decision, so ask; the lighting rules apply either way.

Heated or cooled: an addition, or a new building

A mini-split or heater added to an existing workshop turns unconditioned space into newly conditioned space, which the compliance manual says must meet the requirements for an addition, much as a room addition does. A new building designed from the start as a conditioned studio, office or pool house is a newly constructed single-family building, held to the mandatory measures of Section 150.0 and to either the prescriptive or the performance approach of Section 150.1. If a pool house holds the pool heater or pump, those carry their own rules; see pool requirements.

Solar on a conditioned outbuilding

This is the surprise for most owners. The Energy Commission's manual, in Example 7-2, says a conditioned, newly constructed Group U building on a residential lot, such as a pool house, rec room or art studio, must meet the PV requirement of Section 150.1(c)14, with zero entered for dwelling units in Equation 150.1-C. Its Example 9-8a works one through: a new 1,200 square foot conditioned detached garage in climate zone 15 calls for 1.87 kWdc. Exception 2 then does much of the work for small buildings, since no PV is required when the calculated size is under 1.8 kWdc. With no dwelling unit in the equation, that line falls at about 1,150 square feet of conditioned area in zone 15, which has the highest factor in Table 150.1-C, and higher in every other zone, up to about 3,150 square feet in zone 7. An unconditioned garage has no conditioned floor area, so the equation gives zero.

When it becomes an ADU

The dividing line is the planning definition. Government Code section 66313, as HCD's ADU Handbook sets it out, defines an ADU as a unit with complete, independent living facilities, including permanent provisions for living, sleeping, eating, cooking and sanitation. A studio with a bathroom but no kitchen does not meet that definition. State law lets detached garages, sheds and other existing accessory structures be converted into ADUs, with up to 150 square feet of expansion for access. For the Energy Code the route matters:

Converting an existing unconditioned building, such as a detached garage, into an ADU is an addition; converting space that is already conditioned is an alteration. Per the Energy Commission, as HCD reports it, neither is subject to the solar requirement.
A new detached ADU is newly constructed, and the dwelling-unit term in Equation 150.1-C adds between 1.06 and 1.51 kWdc depending on the zone, so it reaches the 1.8 kWdc line far sooner than a studio of the same size.
Building a unit on top of an existing detached garage is an addition, according to the compliance manual.

See ADU Title 24 reports and garage conversion ADUs for the detail.

Questions to settle before it is drawn

Will it ever be heated or cooled? If so, design the envelope for that now rather than retrofitting.
Will it have a kitchen and a bathroom? Then it is an ADU question.
Is the lot in a fire hazard severity zone? Then the Wildland-Urban Interface Code may shape its construction as well.
Will a home battery go in it? The compliance manual's summary of the Fire Code lists detached garages and detached accessory structures among the allowed locations.