Do I Need a Structural Engineer for My Utah ADU?

Yes. Every Utah ADU requires PE-stamped structural drawings for the building permit. That includes foundation design, lateral load analysis, and snow load calculations specific to your city's design table. Attached, basement, and detached ADUs all qualify as structural work in the permit office's eyes. Typical engineering cost: $1,500 to $3,500. Most projects close in 2 to 3 weeks from complete architectural plans.

What the ADU permit office actually requires

Utah's SB 174 (2021) and follow-on local ordinances opened up ADUs across the state, but every municipality still enforces the underlying building code. That means the IRC (International Residential Code) for most single-family ADUs, plus city-specific amendments.

For the structural portion of your permit submittal, plan checkers look for:

A foundation plan showing footing size, depth, reinforcement, and frost-line compliance
A framing plan showing wall studs, headers, beams, and floor/roof framing
A lateral plan showing how wind and seismic loads transfer from roof to foundation
Snow load calculations matched to the city's design ground snow load (varies from 30 psf in St. George to 100+ psf in Park City)
A wet PE stamp from a Utah-licensed structural engineer

Cities won't issue the permit without those. Realtors and ADU builders sometimes hand homeowners a stock plan and say "this should work for the permit." It rarely does. Every site has different soil conditions, different setbacks, different snow load zones. A stock plan is a starting point, not a permit package.

The three ADU configurations and what changes

Detached ADU. Standalone structure on the same lot. The most permit-heavy because it's effectively a small house: full foundation design, full framing package, full lateral system. Engineering scope is the largest of the three.

Attached ADU. Connected to the primary residence. The structural analysis usually includes the shared-wall connection, load transfer between the two structures, and any modifications to the existing framing. Typically lighter scope than detached, but discovery work on the existing house can add time.

Basement / interior ADU. A new dwelling unit inside the existing footprint. The structural work focuses on egress modifications (often a new window well), any wall removals or beam additions, and verifying the existing foundation supports any added loads. Smallest engineering scope, fastest turnaround.

Foundation design is where most ADUs get stuck

The footing detail on the permit drawings has to match what the soil under your lot can actually support. Most of the Wasatch Front is alluvial fan soil that performs fine for a standard footing. But pockets of expansive clay, fill, and high water table exist throughout Salt Lake, Davis, and Utah counties.

A structural engineer's foundation design either uses a presumptive soil bearing pressure (1,500 psf per IRC) or, if the site has known issues, calls for a geotechnical evaluation. The geotech report gets attached to the structural drawings and the city accepts both as a package.

The most common ADU mistakes we see in plans we review for permit:

1. Footing too shallow for the frost line. Utah jurisdictions require 30 inches minimum below grade. Some homeowners and ADU kit suppliers spec 24 inches and the plan kicks back. 2. No frost-protected slab design on detached slab-on-grade ADUs. Required in this climate. 3. Lateral system incomplete. A pretty framing plan that doesn't show how wind loads get from the gable to the slab.

Every one of those is fixable. None of them are fixable after the slab pours.

What an ADU engineering package costs

Real numbers by configuration in our 7-state footprint:

Detached ADU (400-800 sf): $2,500 to $3,500. Full foundation, framing, lateral, snow load.
Attached ADU (400-800 sf): $1,800 to $2,800. Includes shared-wall analysis.
Basement ADU: $1,200 to $2,000. Egress and partial structural mods.

These price ranges include the site visit, the engineering, the drawings, the PE stamp, and any plan-checker correspondence during permit review. They don't include the architectural drawings (separate scope) or geotechnical work (separate report if needed).

Express compresses the schedule from 2-3 weeks to 7 to 10 days for an additional 25-40%, useful when your contractor is booked and waiting on permit approval.

Snow load: the Utah-specific gotcha

Utah ADUs from Park City to Logan to Cedar City have wildly different design ground snow loads. The city's adopted table is the legal reference; ignore the federal map. Some specifics:

Park City: 100-115 psf design ground snow load (steepest in our footprint)
Salt Lake City proper: 30-43 psf depending on elevation
Provo: 30-40 psf
Ogden: 35-50 psf
St. George: 20-30 psf

Your roof framing, beam sizes, and footing widths are all driven by this number. A Park City ADU will have substantially heavier framing than the same architectural plan built in St. George. Stock plans don't account for this. Engineered plans do.

What to do next

If you're scoping an ADU project in Utah, the fastest path is to book a structural site visit. A licensed PE walks the property, confirms the configuration (attached, detached, basement), checks foundation and snow load implications, and produces the engineering scope and quote in writing.

Schedule a site visit

If you already have architectural plans and need PE-stamped drawings for permit, our Plan Review service handles that on a 2 to 3 week turnaround.