Is a hotel residential or nonresidential under Title 24?

Nonresidential. A building with six or more guest rooms, or a lobby serving six or more, is a hotel/motel, and Part 6 puts new hotels and motels in the same sections as offices and shops, with NRCC certificates.

Is a duplex single-family or multifamily?

Single-family, where it is a Group R-3 building with two or fewer dwelling units. The same definition takes in townhouses, Group R-3.1 buildings and garages on a residential site.

Which forms does a four-storey apartment building use?

The nonresidential NRCC set. It still follows the multifamily sections of Part 6; only buildings of three or fewer habitable stories use the LMCC forms.

What if the use of a building changes?

Part 6 applies the requirements of the occupancy after the alteration, under Section 141.0, 150.2 or 180.0 depending on the new use.

Residential vs Nonresidential Title 24: Which Rules Apply?

Part 6 sorts a building by its occupancy group: Group R-3 houses and townhouses follow the single-family sections, R-2 and R-4 buildings the multifamily sections, and offices, shops, schools, warehouses and also hotels and motels the nonresidential sections. Each group has its own certificate of compliance, CF1R, LMCC or NRCC, and multifamily buildings of four or more habitable stories use the NRCC.

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The occupancy group decides, not the owner

Part 6 applies to buildings of Occupancy Group A, B, E, F, H, I, L, M, R, S or U, using the groups defined in the California Building Code. Section 100.1 then sorts them into three families, and the family decides which sections of the code you read.

Single-family: a Group R-3 building with two or fewer dwelling units, a townhouse, a Group R-3.1 building, and a Group U building such as a garage when it sits on a residential site.
Multifamily: Group R-2 buildings, except hotels, motels and timeshare properties; Group R-4; and certain nontransient congregate residences in Group R-3.
Nonresidential: Groups A, B, E, F, H, I, L, M and S. A hotel or motel, meaning six or more guest rooms or a lobby serving six or more, is placed with the nonresidential buildings too.

Two institutional groups are outside the code altogether: Section 100.0(a) exempts Occupancy Groups I-3 and I-4. Healthcare facilities in Groups I-1 and I-2 came into scope with the 2019 code, with many exceptions.

Where each family sits in the code

Sections 100.0 to 110.12 apply to everything. After that the code divides. New single-family buildings use Sections 150.0 and 150.1, and work on existing houses uses Section 150.2. New multifamily buildings use Sections 160.0 to 170.2, with Section 180.0 for additions and alterations. Nonresidential buildings and hotels use the 120 and 140 series, with Section 141.0 for work on existing buildings. A change of use is read forward: the section for the new occupancy is the one that governs the alteration.

The forms follow the family

The Energy Commission sets the document families out by building type:

Single-family: CF1R to show compliance, CF2R for installation, CF3R for field verification.
Multifamily of three or fewer habitable stories: LMCC, LMCI and LMCV. A performance LMCC is registered with an ECC-Provider when the scope includes field-verified measures.
Nonresidential, and multifamily of four or more habitable stories: NRCC, NRCI and NRCV, plus NRCA acceptance tests. For a multifamily building of four or more stories, the Multifamily Compliance Manual notes that the NRCC is not registered with an ECC-Provider even where ECC measures apply.

So a five-storey apartment building follows the multifamily sections but files nonresidential forms. The low-rise or high-rise test explains the storey count, and Title 24 forms covers each document.

Buildings that are two things at once

Plenty of buildings mix uses. Section 100.0(f) settles them space by space: a shop on the ground floor meets the nonresidential rules and the flats above it meet the multifamily ones. Two exceptions save work. Where one use takes up 80 per cent or more of the conditioned floor area, its rules may govern the envelope, HVAC and water heating for the whole building, but lighting and the mandatory measures are still checked use by use. Where one use reaches 90 per cent of conditioned and unconditioned area combined, its indoor lighting rules may cover everything. The Nonresidential Compliance Manual says live-work units lean residential, since they are conditioned around the clock, while the designated workspace still meets the nonresidential lighting rules of Section 140.6. See mixed-use buildings for the detail.

Why the line falls where it does

The split is as old as the statute. The Warren-Alquist Act defined a nonresidential building by reference to the 1973 Uniform Building Code: a heated or cooled building of any occupancy other than hotels and apartment houses, other residential occupancies and minor structures such as garages. National codes draw the line differently. The U.S. Department of Energy, consistent with the International Code Council and ASHRAE, treats multifamily buildings over three stories, and hotels of any height, as commercial. California keeps all multifamily housing in one set of sections, which is why a comparison with ASHRAE 90.1 or the IECC has to start by agreeing what counts as residential.