The number you build: estimated water use
Estimated total water use is the yearly water your design would draw, in gallons, computed zone by zone. Each zone follows one formula: the plants' water use rating, divided by irrigation efficiency, times the zone's square footage, times your city's annual reference evapotranspiration (ETo), times 0.62. Thirstier plants push the number up. A bigger area pushes it up. More efficient irrigation pulls it down.
You only control some of those inputs. The efficiency values are fixed by the state landscape ordinance, the Model Water Efficient Landscape Ordinance, not by your equipment brand: 0.75 for spray heads and rotors, 0.81 for drip. The plant rating comes from WUCOLS, the state's plant water classification. ETo comes from the ordinance's own city table in Appendix C, and the 0.62 simply converts inches of water over square feet into gallons.
Add every zone together and you have the total. On the official worksheet this figure is called ETWU, estimated total water use. It is the number the plan reviewer puts on the left side of the comparison.
The number you are given: the budget cap
The cap is the most water the whole project can use in a year. For a home it is ETo times 0.62 times 0.55 of the regular landscape area. Notice what is missing: plants. Nothing you plant changes the cap. It is built entirely from climate and square footage.
There are two adjustments. Non-residential projects use 0.45 instead of 0.55, so a business gets a tighter budget for the same area. And areas dedicated to edible plants, recreational areas, and areas irrigated with recycled water count at full value, 1.0, instead of 0.55. The ordinance calls these Special Landscape Areas. One catch for homeowners: recreational area means park and sports-field type space for active play or public assembly, and the definition states outright that private single-family residential landscapes are not recreational areas (§490.2(a)(68)). For a home, the practical route to the 1.0 counting is a dedicated edible garden or recycled water.
On the worksheet the cap is called MAWA, the maximum applied water allowance. Think of it as the budget line the design has to fit inside. You cannot negotiate it up; the only things that move it are the size of the landscape and how much of it qualifies as edible or recreation area.
Why plant choice is the biggest lever
The plant rating sits at the front of the formula, so it scales everything behind it. High water plants such as cool season lawn carry a rating around 0.8. Low water shrubs carry around 0.2. That is a fourfold gap before area, climate or irrigation enters the math, and nothing else in the formula comes close.
Irrigation helps too, just less. Drip divides the zone by 0.81 instead of 0.75, so the same plants on drip always compute lower than on spray. It is a real cut, but it cannot rescue a thirsty zone on its own. The order that works: pick lower water plants first, then put them on drip, and the two moves multiply.
That is why the fastest fix for a plan that runs over is almost never fancier hardware. It is changing what grows in the biggest, thirstiest zone.
One Sacramento yard, start to finish
Take a Sacramento home with 1,200 square feet of landscape and an annual ETo of 55.2 inches. The design has an 800 square foot lawn on spray, high water at 0.8, and 400 square feet of low water shrubs on drip at 0.2. The lawn computes to (0.8 ÷ 0.75) × 800 × 55.2 × 0.62 = 29,204 gallons a year. The shrubs compute to (0.2 ÷ 0.81) × 400 × 55.2 × 0.62 = 3,380 gallons. Estimated total water use: 32,584 gallons.
The cap for those 1,200 square feet is 55.2 × 0.62 × 0.55 × 1,200 = 22,588 gallons. The design is over by roughly 10,000 gallons. The lawn is the problem: that one zone uses more water than the entire budget for the whole yard.
Now replace the lawn zone with low water plants on drip. The same 800 square feet drops to 6,760 gallons a year. Together with the shrubs, the design now sits far below the 22,588 gallon cap, with room to spare for a small thirsty accent zone somewhere else.
What happens at plan check
Plan check runs the exact comparison you just did: estimated total water use at or under the cap. It arrives on a worksheet with your zones, ratings, areas and irrigation types filled in. If the total fits under the cap, the water budget portion of the review is satisfied and the reviewer moves on to the rest of the package.
If the total is over, the cap does not bend. The comparison is arithmetic, so the fix is always on the design side: swap plants, convert zones to drip, shrink the irrigated area, or some mix of the three, then resubmit the worksheet.
The good news is that none of this needs to happen at the counter. The formula has no judgment calls in it, so you can test a design in minutes before anyone draws a planting plan. Run your zones through the water budget calculator, see which side of the cap you land on, and adjust while changes are still free.