Stormwater drainage
Illustration: water from the roof and driveway is collected and carried under the lawn to a dry well, with a pop-up emitter for overflow.
Drainage, grading and erosion control for Marin homes. Hillside lots and winter storms send a lot of water toward Marin houses. We find where it goes and carry it away safely, to the County's rules, working with our partner Joe Elliott of Elliott Land Management.
Typical: downspouts into catch basins and buried pipe, channel drains across driveways, dry wells, regrading and erosion control on slopes.
Why now
A strong El Niño is likely this winter.
NOAA's Climate Prediction Center, in its October 8, 2026 update, expects a strong to very strong El Niño through early 2027, which raises the odds of a wet season.
The rainy season has started.
From October 15 to April 15, grading in unincorporated Marin needs the County's approval. Now is the time to find the problems and design the fix, so any regrading is ready to build when the dry season comes.
Small problems grow.
Water against a foundation or under a deck does its damage slowly. A wet winter is when you find out where it is going.
The rules that shape a drainage job in Marin
Grading permits
In unincorporated Marin, the County needs a grading permit before work that moves more than 250 cubic yards of earth, cuts or fills past set heights, or works in or near a creek. Grading that is part of a building permit is reviewed under that permit instead. Cities and towns have their own rules.
The rainy season, October 15 to April 15
Marin County's code does not allow grading in the rainy season without the County's approval of an erosion control plan and schedule. Bigger earthwork is best planned for the dry months.
Only rain in the storm drain
In Marin it is illegal to let anything but rain water reach a storm drain: no soil, concrete wash, paint or soapy water. Building sites use erosion and sediment controls to keep soil out of the drains.
Runoff from new paving and roofs
Projects that add or replace 2,500 square feet or more of roof or paving, including single-family homes, must use at least one stormwater design measure, such as sending runoff to planted areas. Larger projects of other kinds need engineered treatment. The County decides how this applies to your project.
Your neighbors downhill
California courts expect every property owner to use reasonable care so that changes to drainage do not harm a neighbor, and the neighbor must take reasonable care too. What is reasonable depends on the facts, so we design to keep water on your own property where we can.
Dates to know
- October 15 to April 15. The rainy season in Marin County's code. Grading needs the County's approval of an erosion control plan and schedule in these months.
- All year. Erosion and sediment controls on projects that need them, and nothing but rain water into the storm drain.
- Coming: a new state stormwater permit. The State Water Board put a draft of its new permit for smaller cities and counties out for comment in July 2026. It was not adopted when we checked on October 21, 2026, so the 2013 permit still sets the rules in Marin.
Where the water goes
- Gutter and downspout. Catch the rain off the roof and bring it down to the ground.
- Catch basin. A box under a grate that collects water from the downspout and the lawn around it.
- Solid pipe. Carries the water underground, sloped downhill and away from the foundation.
- Channel drain. A long grate across the driveway that catches water running off the paving.
- Drywell. A buried pit of gravel that lets the water soak slowly into the ground, where the soil allows it.
- Pop-up emitter. Or the pipe ends in a cap that pops open and lets the water out gently onto a planted area.
- To the street. Where your town allows it, the rest leaves through the curb into the street gutter. Only rain water, never soil or soapy water.
What we do
- A walk-around in the rain, or after it, to see where the water actually goes
- Downspouts piped away from the foundation to a safe outlet or planted area
- French drains and other buried drains behind walls and along foundations
- Grading and swales that move surface water away from the house
- Dry creek beds, rain gardens and permeable paving that let water soak in
- Retaining walls built with drainage behind them, and erosion control on slopes: matting, fiber rolls and planting
- Patios, paths and other hardscape sloped to carry water away from the house
- Decks: water steered away from posts and footings, and rot repaired where it already got in
- Clearing clogged gutters, drains and culverts before the storms
We plan and run the job with Joe Elliott of Elliott Land Management, our partner for hardscape, landscape, drainage, retaining walls, decks and Firewise work.
Retaining walls and patios too: see our hardscape work. Getting ready for fire season as well? See wildfire protection.
Questions
Do I need a permit for drainage work?
It depends on the job and the address. In unincorporated Marin, moving more than 250 cubic yards of earth or working in or near a creek needs a grading permit, and cities and towns have their own rules. We check with the County or your town before we start.
Can you do drainage work in winter?
It depends on the work. Grading in unincorporated Marin needs County approval between October 15 and April 15, so we look at what your job involves and plan around the season.
Does an El Niño winter mean flooding?
Not necessarily. NOAA expects a strong El Niño this winter, which raises the odds of a wetter season, but nobody can say how much rain will fall on your street. Good drainage is worth having either way.
Sources
- NOAA Climate Prediction Center: El Niño diagnostic discussion
- Marin County Public Works: Grading permits
- Marin County Code 24.04 (grading, drainage and the rainy season)
- Marin County Public Works: Stormwater pollution prevention
- MCSTOPPP: New and redevelopment projects
- Keys v. Romley (1966), California Supreme Court
- State Water Board: Phase II small stormwater permit and the draft new permit
General information, checked in October 2026, not legal advice. We are builders, not lawyers; the County or your town has the final word on your project.
What clients say
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