Converting an attached garage into a livable ADU is the single most popular ADU strategy in Davis County right now, and it's almost always more involved structurally than homeowners expect. The garage was never designed to be a house. Making it one is a real engineering project, even if the floor plan looks simple.
Five things that have to be engineered
1. The slab
Garage slabs are typically 4 inches thick, unreinforced, sloped to a drain, and uninsulated. To become a livable space they usually need:
- An insulating layer (often a new slab on top, or rigid foam plus a new slab)
- A vapor barrier
- Either filling the slope or accepting a slightly sloped floor
Some garage slabs aren't structurally adequate for the new loads from interior partition walls and finishes. Sometimes they need reinforcement or replacement. A site visit answers this.
2. Where the garage door used to be
The garage door opening becomes a wall — usually with windows, a door, and significant exterior framing. The existing header above the garage door is sized to span the wide opening with no bearing in the middle. When you fill in the opening with new wall framing, you can sometimes keep the existing header and just add new studs underneath; sometimes you need a different header strategy. The right answer depends on what's above the garage (if anything) and the existing framing details.
For garages with living space above, this is a more involved problem because the new wall has to maintain the load path from the floor above. We see this a lot in 1990s and 2000s Davis County subdivisions.
3. The exterior wall framing
Garage walls are often built with less stringent rules than livable walls. Stud spacing, sheathing nailing, and the specific connection details all need to be evaluated for habitable use. In some cases the existing walls are fine. In some cases additional sheathing or new shear wall configurations are needed.
4. Egress windows
Any sleeping room in the new ADU needs an egress window. If the garage doesn't have windows that meet egress requirements, you have to cut new ones. Cutting windows into a garage exterior wall is similar to cutting an egress window in a basement — header design, plus structural evaluation of the wall above.
5. Fire separation between units
Code requires a 1-hour fire-rated assembly between the existing house and the new ADU. That assembly has to wrap the wall and the ceiling and any common framing. The engineering scope doesn't usually include the fire protection design specifically, but the structural design has to accommodate it.
The lateral system question
Here's the catch most homeowners don't see coming. The existing garage door opening was a "soft story" condition — a wide gap in the wall on one side of the structure. Modern seismic design tries to avoid soft stories because they're a weak point in lateral systems. When you fill in the garage door opening with new wall framing, you're actually improving the lateral system on that side of the house. But you have to design the new wall to be a shear wall, not just an infill panel. That means specific sheathing, nailing schedules, and hold-downs.
Done right, a garage conversion can leave the house with a better lateral system than it had before. Done wrong, it creates a partial fix that doesn't actually function as a shear wall.
The fee picture
Engineering scope for a typical garage conversion is $1,500-$3,000 fixed fee. That includes site visit, slab evaluation, header and lateral design, egress window design, and the stamped plan set. Plan check is included.
See our ADU services page for the broader scope, or our HB 82 explainer for the legal background.