Is a garage conditioned space under Title 24?

Not unless it is heated or cooled above the thresholds in Section 100.1, or meets the indirect test. An ordinary attached garage is unconditioned, and the wall between it and the house is part of the envelope.

Does Title 24 apply to an unconditioned building?

In part. Part 6 covers unconditioned buildings of the occupancies it regulates, and its lighting requirements apply to unconditioned space.

Does a closet with no vent count as conditioned floor area?

Generally yes. Surrounded by conditioned rooms and with no openings to the outside, it is indirectly conditioned.

Does putting a mini-split in a garage make it an addition?

Yes, if it turns unconditioned space into conditioned space. Newly conditioned space must meet the requirements for an addition.

What Is Unconditioned Space Under Title 24?

Under Section 100.1, unconditioned space is enclosed space that is neither directly nor indirectly conditioned. A typical garage, vented attic or crawlspace is unconditioned, so the envelope rules apply to the walls, ceilings and floors between it and the house, while the lighting rules still apply inside it.

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The three definitions

Directly conditioned: enclosed space with wood heating, mechanical heating of more than 10 Btu/hr per square foot, or mechanical cooling of more than 5 Btu/hr per square foot.
Indirectly conditioned: not directly conditioned, but either its thermal transmittance-area product (UA) to directly conditioned space exceeds its UA to the outdoors or to unconditioned space, with no fixed vents or openings to them, or air from conditioned spaces passes through it at more than three air changes per hour.
Unconditioned: enclosed space that is neither.

Conditioned space is the first two together, and conditioned floor area, the figure much of the code scales with, counts only that space, measured at the exterior surfaces of the walls around it. The indirect test is why an interior closet or hallway with no register of its own still counts: it is surrounded by heated rooms and has no vents to the outside.

Unconditioned does not mean outside the code. Part 6 covers unconditioned buildings of the occupancies it regulates, and Section 100.0(c) applies the lighting requirements to unconditioned space in any story.

Garages

An attached garage with no heating or cooling is unconditioned. The wall, ceiling and doors between it and the house are each a demising partition, the code's term for an element separating conditioned space from enclosed unconditioned space, and they are part of the building envelope. Rules that follow from that:

Fire-rated swinging doors between the garage and the house are excepted from the prescriptive door U-factor (Exception to Section 150.1(c)5).
In new homes, the indoor air quality requirements the CEC's manual summarises include sealing or gasketing the walls and openings between the house and an attached garage, and ventilation air may not be drawn from a garage.
At least one garage luminaire is controlled by an occupancy or vacancy sensor that switches it off automatically (Section 150.0(k)2Ei).
Ducts through a garage are outside conditioned space, so they need at least R-6 insulation (Section 150.0(m)1B).

Attics

Section 100.1 defines an attic as an enclosed space directly below the roof deck and above the ceiling beams. A vented attic is unconditioned, and the insulation goes in the ceiling below it. Where one open attic runs over both the house and the garage, the manual says to insulate the whole ceiling, garage part included, unless a continuous air barrier separates the two. An unvented attic insulated at the roof deck is treated differently: ducts in it may have R-4.2 instead of R-6 where the attic has at least R-30 in contact with the roof deck, the gable ends meet the wall insulation requirements and the house tests at 3.0 ACH50 or less (Exception 2 to Section 150.0(m)1Bi).

Crawlspaces

A crawl space is the space immediately under the first floor, next to grade. A vented one is unconditioned, so the floor above it is the envelope: the prescriptive package in Table 150.1-A sets a raised floor at U-0.037, R-19 in cavity terms, in every climate zone. For unvented crawl spaces the manual calls for a Class I or Class II vapor retarder over the earth floor.

When unconditioned becomes conditioned

Adding heating or cooling changes the category. Section 100.1 defines newly conditioned space as space converted from unconditioned to directly or indirectly conditioned, and requires it to meet the rules for an addition; a garage conversion is the familiar case. The manual also covers the reverse: a sunspace with no heating or cooling, closed off from the house by weatherstripped doors and windows, stays unconditioned, though its lighting must still comply and the building official may require it to be conditioned if it appears habitable.