Ask someone in Del Mar about bluffs and they’ll likely point west, toward the rail line and the ocean. The City’s hazard planning points in two directions. Its 2023 annex to the county hazard mitigation plan lists locations on the North Bluff and South Bluff as vulnerable to coastal bluff erosion, and, as a separate item, locations in the North Hills and South Hills as vulnerable to inland bluff erosion.
That second line is easy to skip. For owners of hillside homes, it’s the one that matters.
What an “inland bluff” is
An inland bluff has no surf at its foot. What shapes it is rain, runoff and gravity working on steep ground. In Del Mar, the City’s Local Coastal Program Land Use Plan describes much of the North Hills District as steep hillside above the San Dieguito Lagoon and its watershed. It designates that part of the district for larger-lot single-family homes, notes that much of it lies within the Bluff, Slope and Canyon Overlay Zone, and asks that development there minimize disruption of the topography and keep the open-space character of these visible slopes.
The ground has a local pedigree, too. The California Geological Survey’s San Diego quadrangle pamphlet describes the Eocene Delmar Formation, a sandy claystone interbedded with sandstone, as named for sea-cliff exposures here, with the Torrey Sandstone also part of the local sequence. Which unit is under a particular house, and how weathered it is, takes a site investigation to answer. General maps only set the scene.
How the City measures “steep”
Del Mar’s slope rules turn on one number: 25 percent. The Land Use Plan defines “steep slope areas” as slopes with a gradient of 25 percent or greater, identified through a slope analysis. For projects in the overlay zone, that analysis is prepared by a licensed civil engineer, drawn on two-foot contours, with the gradient calculated between individual contour lines rather than averaged across several. The same submittal list calls for a geological reconnaissance and a preliminary engineering geology report, including subsurface work where development is proposed. The plan exempts projects that clearly involve no new grading, construction or footprint expansion unless the Planning Director asks otherwise.
The plan also uses a stricter term, “substantial slopes”: 25 percent or steeper where the slope area itself drops 20 feet or more, or where it joins similar slopes next door. Construction, grading or other encroachment on those slopes, or within 20 feet of the top or 10 feet of the bottom, is prohibited except where the plan’s policies specifically allow it.
The encroachment table
Where a site is mostly steep, the plan allows only a limited share of the steep area to be disturbed. A simplified reading of its table:
| Share of site in steep slopes | Maximum encroachment into steep areas |
|---|---|
| 96–100% | 20% |
| 91–95% | 18% |
| 86–90% | 16% |
| 81–85% | 14% |
| 76–80% | 12% |
| 75% or less | 10% |
Two details are easy to miss. First, “encroachment” is broad. It includes grading, paving, structures and even removal of native vegetation, including clear-cutting for brush management. Second, any allowed grading is checked by the Planning Director, with the City Engineer, to confirm it’s the least needed for the project.
Water, by the rules
The plan’s overlay policies send runoff from impervious surfaces into existing or new public drainage systems with enough capacity. Where none is nearby, an alternative may be allowed if the Planning Director, consulting the City Engineer, finds it carries the water to a defined legal natural drainage course without adverse impact. Drainage designs are prepared by a licensed civil engineer and reviewed by the City Engineer.
For an existing home, that’s a useful benchmark even when no project is planned. Ask where your roof, patio and driveway water actually ends up. On a hillside lot, the honest answer is sometimes “down the slope.”
ADUs on a hill
Many hillside owners are weighing an accessory dwelling unit. Del Mar’s ADU chapter (June 2025 version) carries its own slope language. It defines a “substantially steep slope” as 25 percent or greater with at least 20 feet of elevation difference between top and bottom, bars ADU development from encroaching into one, and requires the ADU to sit at least 20 feet from the top edge and 10 feet from the bottom, without overhanging the slope or its setback. A new ADU’s development pad must sit on ground no steeper than 4 to 1, and grading is to be minimized.
That doesn’t mean an ADU is off the table on a sloped lot. It means the site plan starts with a slope map, not a floor plan.
What we look for on a hillside house
Homes in the hills often step down their lots. Depending on era and design, that can mean stepped footings, a tall downhill crawl space, or a garage tucked into the slope with retaining walls on two or three sides. We don’t assume any of it until we’ve seen the house. When we do, these are the usual questions:
- Downhill side first. Is there cracking, separation or rotation on the low side of the house or deck?
- Retaining walls. Are they leaning, bulging, or missing working drains? Is there more than one tier?
- Cripple walls and posts. On raised sections, are tall downhill walls braced and is the house bolted down? See seismic retrofit.
- Water from above. Does runoff from the street or uphill neighbors cross the lot? Where does it concentrate?
- Water from you. Irrigation, pool or spa lines, and downspouts on the slope side.
- Records. Any geotechnical report, slope analysis, or as-built drawings from prior work.
Our bluff and hillside homes and retaining wall repair pages cover the repair side.
Review and permits
Work on these lots is rarely just a building permit. The Bluff, Slope and Canyon Overlay Zone, design review and the City’s coastal development permit path can all apply. Our note on design review for foundation projects explains how those processes intersect. We help owners prepare a clear scope and coordinate with licensed engineers as needed for slope analysis, geology and structural design. We can’t forecast what reviewers will decide or how long review will take, and we don’t try.
Scope drivers
On an inland-bluff lot, scope generally turns on slope gradient and height, access for equipment, whether work reaches into a steep-slope area, the condition and number of retaining walls, foundation type, drainage routes to an approved outlet, geotechnical findings, and the review path.
A note to close
The Del Mar hills are some of the most pleasant streets in the county to live on. The inland bluff hazard doesn’t change that. It simply asks owners to treat water and slope as part of the house. Our Crest Canyon note covers the homes along that canyon rim, and our earlier entry on Del Mar’s coastal bluffs in the news covers the ocean side.
For a measured look at a hillside property, request a Free Quote.