Retaining Walls 101December 11th, 2022
If you’re considering building an accessory dwelling unit (ADU) and have a sloped or uneven property, you may need to consider incorporating a retaining wall into your design. When deciding if a retaining wall is necessary for your project, it’s important to consider the slope of your property, the type of soil present, and any potential erosion issues. You’ll also need to obtain a permit for your retaining wall, and it’s important to familiarize yourself with the regulations and requirements in your area. The cost of building a retaining wall for an ADU can vary, depending on the size and complexity of the project.
Read on to learn more about retaining walls and how they can be incorporated into your ADU build.
What is the purpose of a retaining wall?
Retaining walls serve a variety of purposes, including stabilizing slopes, creating usable space, and improving the appearance of a property. When getting to a buildable pad for an ADU, the purpose of a retaining wall is to retain soil to allow a large enough space to accommodate a foundation. In other words, a retaining wall will keep soil within a certain boundary so it can be used to create a more solid and resilient foundation for your granny flat. Such walls are used in lieu of a stem wall, which is a small wall built alongside the cement slab to help reinforce the foundation of the home. The two are similar in construction, but a retaining wall will most commonly stand outside of your unit while a stem wall is built-in.
When do you need a retaining wall for your ADU?
When deciding whether or not you need a retaining wall, it’s important to consider the slope of your property, the type of soil present, and any potential erosion issues. Retaining walls are used in situations where the ground foundation of the ADU may be compromised, due to a slope, additional structure, or pool. It serves to create more buildable space for your unit by maintaining a suitable amount of soil along the pad. This specifically accommodates a slab on grade foundation. A slab on grade foundation is a type of foundation that is built directly on top of the ground, rather than being raised above the ground to accommodate a crawl space or basement. It is typically made of concrete and is poured directly onto the ground, creating a flat, even surface on which the building can be constructed.
What types of retaining wall are there and which do I need?
Your retaining wall will sit outside of the unit as a standalone structure (which, if you remember, distinguishes it from a stem wall). There are two materials the wall is most commonly made out of: CMU (“concrete masonry unit,” more commonly known as cinderblock) and keystone block. The method of building depends entirely on the material you choose to use, as CMU will be poured in place with concrete and rebar, and keystone blocks will interlock with one another. The wall is then sealed to protect against the elements. The two methods do not affect the ADU design in any way, other than the location of the wall in proportion to the ADU. CMU tends to be larger than keystone, but neither can exceed six feet tall in most cases.
As previously discussed, the two types of retaining walls are determined by the material the wall is made of. When questioning which method is best for you, it is most important to consider the way your retaining wall needs to be structured. CMU retaining walls, also known as PIP walls or “poured in place,” stand upright and vertical. On the other hand, keystone blocks lay back slightly and retain soil at an angle. That being said, it’s important to consider the specific needs of your build and consult with a contractor for their expert advice.
Do I need a permit for a retaining wall in San Diego?
As with all other aspects of your build, your retaining wall will require a permit before it is built. If you are building within San Diego, the county offers standard designs for the retaining walls. This simplifies the building process and ensures that your wall will be built up to code and as effectively as possible.
There are only a few situations where a retaining wall build is not feasible. On especially steep hillsides, retaining walls are subject to a grade limit: these walls must have a layback of 2 to 1, meaning the wall must span over two feet for every one foot you build upwards. If your lot falls under a category with this kind of grade, your contractor will let you know ahead of time that a build in that particular location may not work given your site specifics.
How much does it cost to build a retaining wall for an ADU?
Including the cost of the dirt that will be retained by the retaining wall, the entire retaining wall structure will cost roughly $20-30K on top of the usual construction costs for your ADU (more on ADU costs). Fortunately, few ADUs end up needing a retaining wall. In our experience, only 10-15% of the projects we have started require such reinforcement. It will be evident early on whether or not your lot requires a retaining wall, so you can decide beforehand whether or not you would like to continue with the project.
Can you use a retaining wall as one wall of ADU structure?
Although you can use a retaining wall as one wall of your unit, it is better advised to leave the wall as its own standalone structure. These walls retain soil which also means they have a tendency to retain water. Though a drainage system is built in, a buildup of water close to your home’s foundation has the potential to cause problems down the line. While it may be tempting to incorporate the retaining wall into the overall ADU design for cost or aesthetic reasons, it’s important to prioritize the safety and stability of your unit. It’s always best to consult with a professional before making any decisions about the structure of your ADU.
What are disadvantages of a retaining wall?
While retaining walls can be an effective solution in some cases, they do have some potential drawbacks when it comes to building an ADU. Firstly, they can take up a significant amount of space, which may not be practical for homeowners with limited land. Additionally, they can be costly to build, adding to the overall expense of the ADU project. It’s important to weigh the pros and cons of using a retaining wall before making a decision. In some cases, alternative methods of soil retention may be a more suitable solution.