Tub-to-shower conversion: what California code asks of the new shower
Swapping a bathtub for a walk-in shower looks simple from the doorway. Under the tile it is a new waterproof floor, a new valve and a permit, each with numbers in the code.
A tub-to-shower conversion usually fits the old space: California's plumbing code sets a minimum shower of 1,024 square inches that can hold a 30-inch circle, but that minimum does not apply to a shower receptor at least 30 by 60 inches. Measure your tub alcove against that. What changes is underneath: a waterproofed floor sloped to the drain, a curb or a curbless entry, a scald-protected valve, safety glass and, in the City of San Diego, a permit.
Below, each part of the conversion in the order you would plan it, with the code section that sets the number. Everything quoted is from the 2025 editions that took effect in California on January 1, 2026. Local building departments can add their own amendments, so treat this as the baseline your city starts from.
Will a shower fit where the tub was?
Section 408.7 of the California Plumbing Code sets the minimum shower: "(1) Not less than 1024 square inches (0.6606 m2). (2) Be capable of encompassing a 30 inch (762 mm) circle." It also gives the exception that matters most for a conversion: "The minimum required area and dimension shall not apply for a shower receptor having overall dimensions of not less than 30 inches (762 mm) in width and 60 inches (1524 mm) in length."
So measure the alcove wall to wall. At 30 by 60 inches or more, the shower meets the size rule as a full-length receptor. Smaller than that, the finished shower has to reach 1,024 square inches and hold the 30-inch circle on its own.
The opening matters too: "Shower doors shall open so as to maintain not less than a 22 inch (559 mm) unobstructed opening for egress."
Curb or curbless
A curbed shower has two limits. The curb must sit lower than the shower's sides: "not less than 1 inch (25.4 mm) lower than the sides and back of such receptor." And its height above the drain is bounded: "In no case, shall a dam or threshold be less than 2 inches (51 mm) or exceeding 9 inches (229 mm) in depth where measured from the top of the dam or threshold to the top of the drain."
A curbless shower is allowed, with a consequence for the rest of the room: "Where there is a shower without a threshold, the floor space within the same room shall be considered a wet location". In practice that means planning the bathroom floor, not only the shower, for water.
Either way, the finished shower floor must "slope uniformly from the sides towards the drain not less than 1/8 inch per foot (10.4 mm/m), nor more than 1/2 inch per foot (41.6 mm/m)." In a curbless conversion, check early that the floor has the depth to make that slope.
The shower floor: pan, lining and the flood test
A tub came with its own waterproof shell. A tiled shower has to be made watertight on site, and the code is specific about a lined shower floor:
- The lining goes up the walls: the subfloor and rough walls are lined "to a height of not less than 3 inches (76 mm) above the top of the finished dam or threshold".
- The lining slopes to the drain: "Lining materials shall be pitched 1/4 inch per foot (20.8 mm/m) to weep holes in the subdrain of a smooth and solidly formed subbase."
- It is tested before tile: "Shower receptors shall be tested for watertightness by filling with water to a depth of not less than 2 inches (51 mm) for not less than 15 minutes."
If a liquid or sheet waterproofing system is used in place of a traditional liner, ask which product it is and confirm that your building department accepts it before the tile goes on.
Walls: what goes behind and on the tile
- Backer board from the code's list. CRC R702.4.2: backers for wall tile in tub and shower areas "shall be of materials listed in Table R702.4.2", which names glass mat gypsum backing panel, fiber-reinforced gypsum panels, fiber-cement backer board and fiber mat-reinforced cementitious backer units.
- Not ordinary water-resistant drywall where water hits it. CRC R702.3.7.1: "Water-resistant gypsum backing board shall not be used where there will be direct exposure to water, or in areas subject to continuous high humidity."
- A nonabsorbent finish to 6 feet. CRC R327.2: shower walls "shall be finished with a nonabsorbent surface. Such wall surfaces shall extend to a height of not less than 6 feet (1829 mm) above the floor."
The valve and the water temperature
A conversion is the moment to replace the old tub valve, because the code is specific: "Showers and tub-shower combinations shall be provided with individual control valves of the pressure balance, thermostatic, or combination pressure balance/thermostatic mixing valve type that provide scald and thermal shock protection for the rated flow rate of the installed showerhead."
The same section caps the water: "The maximum water temperature discharging from an individual showerhead shall be limited to 120 degrees F (49 degrees C)". Moving the valve, the drain or the shower head is plumbing work, and in the City of San Diego the building-permit exemptions "are not exemptions from the electrical, plumbing, and mechanical permit requirements."
Glass and air
Shower glass is in what the code calls a hazardous location: glazing next to a shower "where the bottom exposed edge of the glazing is less than 60 inches (1524 mm) measured vertically above any standing or walking surface shall be considered to be a hazardous location." Glass in those locations must carry a visible mark: each pane "shall be provided with a manufacturer's designation specifying who applied the designation, the type of glass and the safety glazing standard with which it complies, and that is visible in the final installation." Look for it in a corner of the panel.
And the room needs a fan. CRC R325.2.1: "Each bathroom containing a bathtub, shower or tub/shower combination shall be mechanically ventilated for purposes of humidity control", and "Window operation is not a permissible method of providing bathroom exhaust for humidity control." California's green building code adds fans ducted outside and controlled by a humidity control; how much of that applies to a remodel is the building department's call.
The permit, in the City of San Diego
The City's no-plan permit bulletin lists, among work that needs no building permit, "Tub-to-tub and shower-to-shower replacements that do not change the size or location of the tub or shower." A tub-to-shower conversion is not on that list, so plan on a permit and confirm the type with the building department. The same bulletin's no-plan route covers a bathroom remodel "with no changes to structural elements, wall framing or exterior walls and no added plumbing fixtures."
A permit brings one more rule with it: "Where alterations, repairs or additions requiring a permit occur, the individual dwelling unit shall be equipped with smoke alarms located as required for new dwellings." Outside the City, the County or your own city's building department sets the permit path.
Before you book it
A conversion takes a bathroom out of use for a while. If it is the only one, read our notes on living in your house during a remodel before you schedule it. For the whole room, see bathroom remodeling; when the bathroom is one of several rooms in the job, see whole-house remodels.
Quick answers
Can any bathtub space become a walk-in shower?
Most can. The California Plumbing Code's minimum shower (1,024 square inches, able to hold a 30-inch circle) does not apply to a shower receptor at least 30 by 60 inches. Measure the alcove; if it is smaller, the finished shower must meet the minimum on its own.
How high can a shower curb be?
Measured from the top of the curb to the top of the drain, the code allows 2 to 9 inches, and the curb must sit at least 1 inch lower than the sides and back of the shower.
Is a curbless shower allowed in California?
Yes. When a shower has no threshold, the code treats the floor space of the whole room as a wet location, and the shower floor still has to slope 1/8 to 1/2 inch per foot toward the drain.
Do I need a permit to convert a tub to a shower in San Diego?
Plan on one. The City of San Diego exempts only tub-to-tub and shower-to-shower replacements that keep the same size and location, and its exemptions never cover the plumbing permit. Confirm the permit type with the building department for your address.
What kind of shower valve does the code require?
An individual control valve of the pressure balance, thermostatic or combination type that protects against scalding and thermal shock, with the water at the shower head limited to 120 degrees F.
Does shower glass have to be special glass?
Glass beside a shower with its bottom edge less than 60 inches above the floor is in a hazardous location under the Residential Code, and each pane there must carry a visible manufacturer's mark naming the safety glazing standard it meets.
Sources
- 2025 California Plumbing Code, Section 408 Showers (IAPMO (publisher of the California Plumbing Code, Title 24 Part 5))
- 2025 California Residential Code, Chapter 3 Building Planning (with January 2026 errata) (International Code Council (publisher of Title 24 Part 2.5))
- 2025 California Residential Code, Chapter 7 Wall Covering (International Code Council)
- 2025 California Green Building Standards Code 4.506.1 Bathroom exhaust fans (International Code Council)
- Information Bulletin 203: No-Plan Building Permit (City of San Diego Development Services)
- San Diego Municipal Code 129.0203, Exemptions from a Building Permit (Ch. 12, Art. 9, Div. 2) (City of San Diego)
- Building Services Division (County of San Diego, Planning & Development Services)
- California Building Standards Commission home page (California Building Standards Commission)
Read on 2026-10-16.