Can My Existing Garage Slab Support an ADU Conversion?
Converting a detached garage into an Accessory Dwelling Unit (ADU) is one of the most efficient ways to add value to a property, but the foundation is often the biggest "unknown" in the project. Most residential garages built 30+ years ago were designed to support a vehicle and basic storage—not the concentrated loads of interior walls, kitchens, and second-story additions.
Before you pull permits or start framing, you need to determine if your concrete slab is a solid foundation or a structural liability.
Planning a garage conversion and need to know if your slab is up to code? Click here to contact our team and let’s take a look at your structural plans!
The "Slab-on-Grade" Reality Check
In many older homes, garage slabs were poured thin—sometimes as little as 3 inches—with minimal reinforcement. When you transition a space from "storage" to "habitable," the structural requirements change significantly.
3 Red Flags to Look For
Lack of Footings: A standard garage wall often sits on a "monolithic" slab without deep footings. To meet modern building codes for living spaces, you may need to add thickened edges or proper footings to carry new structural loads.
Minimal Reinforcement: Many older slabs utilized thin wire mesh rather than rebar. While this works for a parked car, it often fails under the weight of new plumbing trenches, heavy kitchen appliances, or tile flooring.
Moisture Issues: Garages rarely have a vapor barrier beneath the concrete. Without one, moisture can wick up through the slab, causing flooring to warp or mold to develop behind your new drywall.
Engineering Solutions for Conversions
If your slab doesn't meet the requirements, you don't always have to start from scratch. Common engineering workarounds include:
Saw-Cutting and Trenching: We often design plans that involve cutting specific sections of the slab to pour deeper footings exactly where the new load-bearing walls will sit.
Epoxy Doweling: This allows us to tie the existing concrete into new structural elements, ensuring the entire unit acts as a single, stable foundation.
Slab Leveling and Toppings: For slabs that are structurally sound but uneven, high-strength fiber-reinforced toppings can create a level surface for interior finishes.
The Bottom Line
Identifying these issues during the design phase saves thousands in "change orders" once construction has already begun. A structural assessment ensures your ADU is safe, legal, and built to last.
Don’t leave your foundation to chance and schedule a structural consultation for your project.
