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Building Code Reference

Got a CRC R401.3 correction?

Site drainage slope

For builders in California, Greater Seattle, the Las Vegas area, the Denver area and Kansas City · CrossBeam Permits

What this code requires, how to fix it, and how cities and counties review it

What this code requires

CRC R401.3 requires that the ground surface around a residential foundation slope at least 6 inches downward within the first 10 feet from the building. Impervious surfaces in that same zone — driveways, patios, and concrete walks — must slope at least 2% away from the structure. When lot lines or physical barriers make that grade impossible, the code allows drains or swales instead, as long as water still moves away from the foundation. Most single-family residential and ADU permits get flagged on R401.3 when the grading plan is missing, shows flat grade, or drains toward the building.

How to fix it

  1. Add grading arrows to the site plan

    On your plot plan or grading plan, draw drainage arrows showing water flowing away from the building. Place arrows at the four building corners and along each elevation. Annotate the slope: '6 inches minimum fall within 10 feet of foundation' for natural grade, or '2% minimum slope' for concrete driveways, walkways, and patios. This callout is what plan-checkers are looking for — without it, R401.3 is an automatic flag even if the physical grading is correct.

  2. Add spot elevations or percent-grade callouts

    Drainage arrows alone sometimes aren't enough for a detailed plan check. Add spot elevations at the foundation edge and at 5 feet and 10 feet out, so the checker can confirm you're achieving the required drop. Alternatively, call out a percent grade: 5% grade (6 inches over 10 feet) or better for unpaved areas, 2% minimum for paved surfaces. Either format works; pick whichever your sheet scale supports more clearly.

  3. Verify all impervious surfaces within 10 feet

    Go around the building footprint and identify every impervious surface within 10 feet: concrete flatwork, pavers, driveway apron, covered patio slab. Each one needs a 2% slope callout directed away from the house. If a patio slab currently shows no slope or drains toward the building, either revise the plan to show correct slope or add a trench drain to intercept runoff before it reaches the foundation. Note the drain size and connection point on the plan.

  4. Show a swale or drain when grading is constrained

    When a property line, wall, or retaining structure prevents you from achieving 6 inches of fall within 10 feet, the code allows an alternative: show a swale or drain that routes water away from the building. Locate the swale parallel to the foundation, size it for the contributing drainage area, and show where it outlets. If you're using a trench drain or catch basin, note the pipe size (typically 4-inch minimum), slope (1% minimum to maintain self-cleaning velocity), and outlet location.

  5. Address drainage conflicts between ADU and main house

    On ADU projects and additions, verify that drainage from the new structure doesn't conflict with the existing building. The plan-checker will look at the relationship between the new and existing foundation grades. If the new ADU slab is downhill from the main house, you may need to show how runoff from the main house travels around (not toward) the ADU foundation. A simple drainage notes section on the site plan addressing this relationship clears the comment.

  6. Show compliant slope for room additions and slab extensions

    Room additions, patio covers, and garage conversions that add new concrete must show compliant drainage slope. It's not enough to note that the existing concrete drains correctly. Pull spot elevations at the new slab edge and the existing foundation wall. If the new slab is at or above the existing finished floor, you'll need to show how water exits the new concrete without ponding against the wall. A 2% slope away from the building, shown on the plan with a spot elevation, resolves this.

  7. Add a drainage note to the general notes sheet

    Many plan-checkers want to see a blanket drainage note even when the site plan already shows arrows and elevations. Add to your general notes: 'All grading shall slope a minimum of 6 inches within the first 10 feet from the foundation. Impervious surfaces within 10 feet of the building shall slope a minimum of 2% away from the building. Where constraints exist, drains or swales shall be provided per CRC R401.3.' This note doesn't replace the graphical callouts but gives the checker a clear written confirmation.

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Why you got flagged

Plan-checkers flag R401.3 more often than most contractors expect, and the corrections almost always come down to one of four situations on the grading and drainage sheets.

The most common trigger is a site plan that shows no drainage arrows at all. Plan-checkers want to see explicit slope direction callouts on the plot plan or grading plan. A note that says "drain away from building" doesn't satisfy it. They need arrows showing grade direction, plus a spot-elevation or percent-slope callout confirming the 6-in-10-ft minimum for pervious areas and 2% for any concrete or pavers near the house.

The second scenario involves tight urban lots where the setback from the property line is under 10 feet. On those sites, you physically can't achieve 6 inches of fall before hitting the neighbor's fence. The code allows swales or drains as an alternative, but you have to show it. If the plan is silent on the drainage solution, the checker marks it non-compliant even if you planned to install a trench drain.

A third common flag comes from ADU additions. When a detached ADU is placed in a rear yard, the grading around the new structure often conflicts with the existing house drainage. Checkers will ask: does water from the ADU pad flow toward the main house foundation? If the plan doesn't address that relationship explicitly, R401.3 gets cited.

Fourth, remodel plans that add a slab extension — like a room addition or a covered patio — can trigger R401.3 if the new impervious surface doesn't show a 2% slope away from the building. The existing house may already pass, but the new concrete changes the drainage picture and the checker will call it out.

The official code

California Residential Code R401.3 — Site Drainage

Read the source →

Common questions

Does CRC R401.3 apply to ADUs?
Yes. Any new structure with a foundation is subject to R401.3, including detached ADUs. The drainage requirements apply to the new structure independently of the main house. When the ADU is in a rear yard, plan-checkers also check that grading from the main house doesn't flow toward the ADU foundation. Show drainage arrows and slope callouts for both structures on the grading plan. If you're building a junior ADU inside the existing footprint, site grading around the existing house still needs to comply.
What if my lot is too small to slope 6 inches in 10 feet?
R401.3 has an exception for exactly this situation. When lot lines, walls, or physical barriers prevent the standard 6-in-10-ft slope, the code allows drains or swales as an alternative, provided they route water away from the foundation. You still have to show this on the plan. Draw the swale location, note the outlet point, and add a drainage note calling out the R401.3 exception. Without that plan callout, the exception doesn't protect you in plan check — the checker won't assume you know about it.
What slope does concrete need around a house?
Impervious surfaces — concrete slabs, pavers, asphalt — within 10 feet of the foundation need a minimum 2% slope away from the building. That works out to roughly 1/4 inch per foot. For a 4-foot-wide concrete walk, you need at least 1 inch of fall from the house side to the outer edge. Show this on the grading plan with a percent-slope callout or spot elevations at each edge.
Can a trench drain substitute for grading slope?
Yes, if you can't achieve the required slope due to site constraints, a trench drain is a code-compliant alternative under R401.3's exception. The plan needs to show the drain location (parallel to and near the foundation), the pipe size and slope (1% minimum is a reasonable baseline), and where it outlets. Connect to an approved collection point — not to a neighbor's yard or public walk. Include this information in a drainage note or on the utility plan.
Does R401.3 apply to a garage slab conversion?
R401.3 applies whenever you're modifying grading or adding impervious surface near the foundation. A garage conversion that adds a new slab, alters the existing slab, or changes how water flows around the garage wall triggers a review of R401.3 compliance. The plan-checker will look at whether the new finished floor or slab elevation creates a drainage problem. Show the existing and proposed grades on the site plan and confirm 2% slope away from all walls.
How do I show drainage on a plot plan?
Three things satisfy a typical plan-checker for R401.3: (1) drainage arrows at each building corner showing the direction water travels away from the house, (2) a percent-grade callout or spot elevations confirming the required slope, and (3) a drainage note on the cover sheet or general notes referencing R401.3. For tight lots or ADU projects, also address where runoff outlets so the checker knows water isn't headed toward a neighboring foundation or a public right-of-way.
Will the city re-check drainage during the field inspection?
Yes. The grading and drainage correction isn't just a plan-check issue — the field inspector can also flag it. Inspectors typically check that the finished grade around the foundation actually slopes away from the building and that no ponding is visible at the perimeter. If the physical grade doesn't match the approved plan, you'll get a correction at the field inspection stage. Build to the approved plan dimensions, and if site conditions changed during construction, revise the plan and get a re-approval before the final inspection.

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