Building on a steep slope starts under the ground and at the road. The soil decides the foundation. The road decides how the trucks, the crane and the concrete get to your lot.
Vision In Design Inc is a design-build company, and hillside construction is the first thing on my list of services. I design the house and I build it. Here is what changes when your lot is not flat, in the order it comes up on the job.
Nothing gets drawn for real until a soils engineer and a geologist have looked at your lot. They drill or dig, log what they find, and write a report. It tells the structural engineer what the ground can carry and where firm material starts.
In Calabasas, the Building and Safety submittal checklist lists a geology and soils report with the plans and structural calculations. The California Building Code also has its own section for buildings on or next to slopes steeper than 1 to 3. That is about 33 percent. It sets how far the building and its footings must stay back from the face of the slope. A building official can approve other setbacks, and can ask for a geotechnical investigation to back them up.
A licensed third party writes that report. I don’t. I design around it. On a hillside the report sets the shape of the house before I draw a single room.
Flat-lot houses sit on footings a few feet down. On a steep slope the top layer of soil is often loose, and it can creep downhill. You have to go past it.
Caissons. A caisson is a deep hole drilled into the slope, filled with a steel cage and concrete. It reaches down through the loose soil into firm ground or rock. The house sits on top of a row of them.
Friction piles. A friction pile holds its load through the grip of the soil along its sides, not only at the bottom. Engineers use them where hard rock is too deep to reach.
Grade beams. A grade beam is a heavy concrete beam that ties the tops of the caissons or piles together. The walls of the house sit on the grade beams. Caissons tied with grade beams is a common hillside system.
Retaining walls. A retaining wall holds back the earth where the hill is cut for a driveway, a garage or a flat yard. On a slope they carry real load. They need their own engineering and their own drainage behind the wall.
The structural engineer designs all of this from the soils report. My job is to build it the way it was drawn, and to catch problems at the hole before the concrete goes in.
Owners tend to think about this last. I think about it first.
Many canyon roads in the Santa Monica Mountains are narrow, steep and tight on the curves. Many lots have no flat ground at all. There is nowhere to park a lumber delivery, no room to stack rebar, no spot for a dumpster, so every delivery has to be planned around the road and the lot.
What I plan before we start:
If a crane, a pump or a truck has to stand in a public road, that usually needs a permit from whoever owns the road. In unincorporated LA County, any temporary construction in the county right-of-way needs an encroachment permit from Public Works. Cities run their own. Street closures can also mean a traffic control plan and flaggers. I find out early which agency owns the road, because it changes the schedule.
On a flat lot a concrete truck backs up and pours. On a steep lot the truck often can’t get near the forms. A concrete pump sits on the road and reaches the concrete down to the caissons and grade beams through a boom or a line.
Steel beams, roof framing and big loads come in by crane the same way. The crane needs a level, firm spot to set up, and the road has to hold its weight. Some days the whole job is built around one pour or one lift. If the pump or the crane can’t reach, the design changes.
On a hillside, water is the enemy. It softens soil, pushes on retaining walls and washes out slopes.
The building code sets some of this. Ground next to the house has to fall away from the foundation, at least 5 percent for the first 10 feet, or water has to be carried away another approved way. On graded sites the top of the foundation has to sit above the point where the water leaves the site.
On a slope I also plan drains behind every retaining wall, and where roof water and yard water go. It has to go somewhere safe. Not over the edge onto the neighbor below.
All of Calabasas is in a Very High Fire Hazard Severity Zone, per the State Fire Marshal map.
That brings in Chapter 7A of the California Building Code. It covers how the house resists embers and flame: roof rating, vents, siding, decks, windows. In a Very High zone the roof has to be Class A. The city says paint or coatings do not count as protection.
LA County Fire also requires a Fuel Modification Plan for new houses and remodels of 50 percent or more in its fire hazard zones. It has to be approved before the building permit. State law asks owners to keep 100 feet of defensible space around buildings in these areas.
All of this goes into the design. Materials, deck framing, plant layout and access for fire trucks get decided at the start.
On a steep lot, every design choice shows up again at the build. Move the house 10 feet and the caissons get deeper. Add a lower level and the retaining walls get taller. Pick a beam the crane can’t reach and you redraw.
When the designer and the builder are separate, those problems show up after the plans are approved. Then the plans go back to be redrawn and resubmitted.
That is why I work design-build. I am involved in every project from the beginning to the end. The person reading your soils report is the one drawing your plans and supervising the build. Access, cranes, drainage and fire rules get solved on paper first.
If you call me for an estimate on a hillside lot, I can’t give you an honest answer, because I don’t know yet what you want to build, so I do a preliminary design first. Once you like the design, I give you the estimate. If you agree, we move forward. If not, we walk away.
If you go ahead, I design the project, prepare all the required plans, and file and pull the building permit myself. Then we build what was drawn and permitted.
You can read more on my hillside construction page, or see what goes into a new custom home from the ground up.
If you own a hillside lot, or are thinking of buying one, call me at (818) 645-1129. Have the address ready, and the soils report if you already have one.
Almost always. In Calabasas a geology and soils report is on the Building and Safety submittal checklist. The California Building Code has special rules for buildings on or next to slopes steeper than 1 to 3, and the building official can require a geotechnical investigation.
Most hillside houses sit on deep foundations: caissons or piles drilled down into firm ground, tied together at the top with concrete grade beams. Retaining walls hold back the soil where the lot is cut. The structural engineer picks the system based on the soils report.
They usually set up on the road above or below the lot and reach over. If they stand in a public road, you normally need a permit from the city or county that owns it, and sometimes a traffic control plan.
Yes. All of Calabasas is a Very High Fire Hazard Severity Zone. New houses there follow Chapter 7A of the building code, need a Class A roof, and in LA County Fire’s area need an approved Fuel Modification Plan before the building permit.
Monday – Saturday: 9:00 AM – 5:00 PM
Sunday: Closed