Whether a detached, new-construction accessory dwelling unit is built with fire sprinklers is the same decision as what its exterior wall has to be. Two documents answer the sprinkler question, keyed to different facts about one house: California's ADU statute limits what a local agency may require where sprinklers "are not required for the primary residence," while the residential code's exception for an accessory dwelling unit sets its own condition, "The existing primary residence does not have automatic fire sprinklers." The statute's four feet is a ceiling on what an agency may demand at the side and rear lot lines, not a dimension the drawing has to hit — and that distance is what the code's two exterior-wall tables read.
Key numbers
| Figure | What it governs | Section |
|---|---|---|
| Four feet | Ceiling on a required side or rear setback | Gov. Code § 66314(d)(7) |
| 1,200 square feet | Floor-area condition in the exception | CRC R309.2 |
| ≥ 2 feet to < 5 feet | The printed projection band | Table R302.1(1) |
| ≥ 5 feet | Unrated walls and projections begin | Table R302.1(1) |
| 3 feet, then 5 feet | Openings read 25% maximum of wall area, then unlimited | Table R302.1(1) |
| 3 feet | Unrated walls and projections, unlimited openings | Table R302.1(2) |
The two sprinkler tests are keyed to different facts
Government Code § 66314(d)(12) states the statutory half in one sentence:
Accessory dwelling units shall not be required to provide fire sprinklers if they are not required for the primary residence.
That limits what a local agency may require; it does not say the unit has none.
R309.2 answers from the other side, opening with a default, "An automatic sprinkler system shall be installed in one- and two-family dwellings," then printing exceptions, the second headed "Accessory Dwelling Unit, provided that all of the following are met." The difference sits in the verb: the statute is keyed to a requirement, the exception to an installation — "The existing primary residence does not have automatic fire sprinklers."
A rebuild in the same place is not treated as a new unit
Government Code § 66314(d)'s seventh item requires no setback for a conversion, and for everything else reads "a setback of no more than four feet from the side and rear lot lines shall be required for an accessory dwelling unit that is not converted from an existing structure or a new structure constructed in the same location and to the same dimensions as an existing structure."
What it caps is what "shall be required" — a ceiling on the local agency, not a dimension the drawing must hit. Nothing keeps a project from setting back five feet.
The exception's other condition is floor area
There are four conditions in all; another reads "The accessory detached dwelling unit does not exceed 1,200 square feet in size." Whether a particular unit meets all four is a determination for the design professional and the jurisdiction.
The setback is the distance the tables read
R202 opens "FIRE SEPARATION DISTANCE. The distance measured from the building face to one of the following:" and prints three targets, the first "To the closest interior lot line." It closes "The distance shall be measured at a right angle from the face of the wall." At a side or rear lot line that first target is what a setback dimension describes, so the two are one number.
One sentence picks the table, and the table prices the wall
Government Code § 66314(d)'s eighth item reads "Local building code requirements that apply to detached dwellings" — the statute's own handoff, not what makes the code apply.
R302.1 prints the switch: exterior walls of dwellings and accessory buildings "shall comply with Table R302.1(1) based on fire separation distance; or dwellings and accessory buildings equipped throughout with an automatic sprinkler system installed in accordance with Section R309 shall comply with Table R302.1(2)".
In Table R302.1(1) the wall rows are keyed to 0 feet and to ≥ 5 feet, only the second "Not fire-resistance rated"; projections carry a printed band, "≥ 2 feet to < 5 feet"; openings are barred below 3 feet, read "25% maximum of wall area" at 3 feet, and become unlimited at 5 feet. Table R302.1(2) reads at 3 feet: unrated walls, unrated projections, unlimited openings. No row in either table is keyed to four feet.
Where the cheaper lane is, and what it costs
The cheaper lane is what both documents describe — § 66314(d)(12) as a limit on the agency, R309.2 as a release, "Accessory Dwelling Unit, provided that all of the following are met" — and its price is Table R302.1(1), for a dwelling without that system: its unrated wall row begins at five feet, leaving the one other wall row, "1 hour—tested in accordance with ASTM E119, UL 263 or Section 703.2.2 of the California Building Code with exposure from both sides"; the projection band containing four feet reads "1 hour on the underside, or heavy timber, or fire-retardant-treated wood"; and the openings row keyed to three feet reads "25% maximum of wall area". The other lane costs a sprinkler system and buys Table R302.1(2): unrated and unlimited at three feet.
What this looks like on the drawing
Three drawn objects come out of this chain, all at the side and rear lot lines: the wall assembly, rated or unrated by table and by distance to the closest interior lot line; the eave, which carries rows and a band of its own; and the window schedule, measured against a wall area no cited section computes.
Currency
Government Code § 66314 was amended by Stats. 2025, Ch. 67, Sec. 109 (AB 1170), effective January 1, 2026 — inside the last twenty-four months, so its effective-date status is worth confirming. Its page records §§ 66310–66342 as a 2026 edition, updated 2026-09-22.
R202, R302.1 and R309.2 carry an edition and no amendment history: 2025 California Residential Code (Title 24, Part 2.5), 2025 edition, updated 2026-10-24. The research established no amendment line or effective date for those three pages, and none is invented here; whether that edition is in force for a given jurisdiction is worth confirming first.
Frequently asked questions
Is a window allowed at four feet?
Openings are barred below 3 feet, so at four feet they are permitted: the next row reads "25% maximum of wall area" at 3 feet, and openings become unlimited at 5 feet. Whether that budget binds depends on a wall area no cited section computes.
What do the projection row's markers mean?
Footnote (a) permits the rating to be "reduced to 0 hours on the underside of the eave overhang if fireblocking is provided"; footnote (b) the same on the rake overhang "where vent openings that communicate with the attic are not installed". Whether either is available on a given overhang is a design and plan-review question.
What if the rear lot line is an alley?
R202 answers with a list: its second item reads "To the centerline of a street, an alley or public way," measured to the centerline rather than the lot line. R202 assigns no target to a given wall, so which item applies is a determination for the design professional and the jurisdiction.
Sources
- Government Code § 66314 — accessory dwelling unit standards: https://gocodebook.com/library/us/ca/ca-gov-66310-adu-jadu-law/title-7.-planning-and-land-use-65000-66499.58/division-1.-planning-and-zoning-65000-66345.4/chapter-13.-accessory-dwelling-units-66310-66342/article-2.-accessory-dwelling-unit-approvals-66314-66331/66314#66314
- California Residential Code R202 — fire separation distance: https://gocodebook.com/library/us/ca/ca-residential-code/chapter-2-definitions/section-r202-definitions#R202
- California Residential Code R302.1 — exterior walls, both tables: https://gocodebook.com/library/us/ca/ca-residential-code/chapter-3-building-planning/section-r302-fire-resistant-construction#R302.1
- California Residential Code R309.2 — sprinkler systems: https://gocodebook.com/library/us/ca/ca-residential-code/chapter-3-building-planning/section-r309-automatic-sprinkler-systems#R309.2
Related reading: How Close to the Fence Can You Build in California?: https://gocodebook.com/blog/california-fire-separation-distance-rules
Code text quoted as published in GoCodebook's library on 2026-09-13 and re-checked against the library on 2026-11-02. This article does not replace permit review or professional judgment.