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Hillside & Elevated Decks
On a hillside lot the deck is the only flat ground you are going to get. It's also a structure with its foundation on moving earth, which is a different job from building a deck in a level backyard.
Three different hillside maps, and none of them agree
This is where most people get lost, so let's be clear about it.
Los Angeles has a zoning Hillside Area, defined by a Department of City Planning map. There is no slope percentage in that definition. It is purely a map designation, which is why "my lot is under 20% so I'm fine" gets you nowhere. If your lot is in it and zoned R1, RS, RE or RA, the Baseline Hillside Ordinance applies.
There is a separate Hillside Grading Area for building and grading purposes, defined off a Bureau of Engineering grid map. And there is a third overlay, the Hillside Construction Regulation District, with stricter rules in certain areas.
Three different maps, and they don't coincide. ZIMAS will tell you which ones your parcel falls in. We check before we quote.
Footings, slopes and the setback nobody reads
Once a slope gets steeper than 1 in 3, a 33.3% slope, the residential code's slope provisions kick in. Below that they don't. That threshold is worth knowing, because it's the line between a routine deck and an engineered one.
Above it, footings need a setback from the descending slope face, dimensioned off the height of the slope. The 40-foot figure people quote is the cap, not the requirement. The code also lets the building official approve an alternate setback, which is how a deck ends up sitting closer to a slope than the standard dimension allows. Usually on the strength of a geotechnical investigation.
Whether you need caissons or spread footings is a soils question, not a code question. No code section says "use drilled piers above X% slope." The geotechnical engineer sets the embedment based on reaching competent material below the theoretical failure surface. Anyone quoting you a standard caisson depth before a soils report exists is guessing.
One useful thing in the Los Angeles code: excavations for caissons or piles under a structure with a valid building permit are exempt from the grading permit requirement. Drilling piers for a deck doesn't, on its own, turn your project into a grading job.
When you need an engineer
There is no height, span, or attached-versus-freestanding threshold in the code that triggers a structural engineer. That's a myth, and a stubborn one.
What actually triggers it: leaving the prescriptive tables in the deck section of the residential code, deviating from conventional framing in a way that brings in the Business and Professions Code, a geotechnical report calling for a designed foundation, and in Los Angeles specifically, LADBS policy requiring structural plans signed by a licensed California engineer or architect. On a hillside deck of any size you're usually outside the prescriptive tables anyway.
The lot coverage trap
An elevated deck more than six feet above natural ground level counts as structure toward your lot coverage cap under the Baseline Hillside Ordinance. People find this out late, after they've already spent the coverage on the house.
A few other limits catch hillside decks. Unenclosed balconies may project only a limited distance beyond the maximum envelope height. Rooftop decks have to be set back from the required side yard. And on a Substandard Hillside Limited Street, elevated stairways, porches and platforms can't project into the front yard at all. The ordinary six-foot front-yard projection allowance doesn't apply to you.
There is no number in the municipal code for how far a deck can cantilever over a slope. The limits are indirect: yard projections, coverage, envelope height, and what the structure will actually carry.
What we do
We have been building on these hillsides since 1995. Bel Air, Brentwood, the Palisades, La Cañada, Calabasas, the Palos Verdes peninsula. Engineered post-and-beam framing, drilled piers where the soils report calls for them, enclosure of the underside where the fire code requires it, and railings detailed for a view rather than against one. More on cable and custom railings and on deck stairs.
If you're looking at a slope and want a straight answer about what can go there, call (818) 345-2866.