Foundation, framing, and load calculations sealed by a Colorado-licensed engineer and ready for your city permit.
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If you are planning an accessory dwelling unit in Colorado, almost every building department will ask for engineered, stamped structural plans before it issues a permit. That is where an ADU structural engineer comes in. At ISTA Engineers, we prepare complete, permit-ready structural plans for ADUs foundation design, framing, and load calculations sealed by a Colorado-licensed Professional Engineer and built around the code your specific jurisdiction enforces.
We work with homeowners, general contractors, and architects across the Denver metro area and Colorado’s mountain communities, turning a design concept into a structurally sound, code-compliant plan set that clears review the first time. Whether you are converting a garage, finishing a basement unit, or building a detached ADU from the ground up, the engineering is what protects your timeline, your budget, and the people who will live there.
Why this matters in Colorado. Most Front Range jurisdictions require foundation and structural plans stamped by a Colorado-licensed engineer, including snow load, wind speed, and frost-depth design. Skipping this step is the most common reason ADU permits stall.
Do You Need a Structural Engineer for an ADU?
In most cases, yes. An ADU is not a shed or a simple bump-out it is a complete, independent dwelling with its own structural system, its own loads, and its own life-safety requirements. Building departments treat it that way, and so should your plan set.
When a stamp is legally required
A licensed engineer’s stamp is typically required whenever a project involves new or modified structure: a new foundation, a detached unit, a second story, new openings in load-bearing walls, or a garage slab being repurposed for habitable use. In jurisdictions such as Arvada and unincorporated Adams County, foundation and structural plans must be stamped by a professional licensed in the State of Colorado before a permit is issued. Requirements vary from one city to the next, so we confirm the current standard for your address at the start of every project.
What the building department checks
Reviewers look closely at the structural fundamentals that keep an ADU standing through a Colorado winter:
- Snow load ground snow loads vary dramatically between the metro area and the high country, and the roof and framing must be designed for the local value.
- Wind and seismic loads the lateral system has to resist horizontal forces, not just gravity.
- Frost-depth footings foundations must bear below the frost line to prevent heave.
- Soil conditions expansive clay soils common along the Front Range can require specific foundation detailing.
What’s Included in Our ADU Permit Set
Our goal is a single, coordinated package that a plan reviewer can approve without a round of corrections. A typical ADU structural permit set from ISTA includes:
- Site & soils review. We assess the lot, existing structures, and available geotechnical information to inform the foundation approach.
- Structural load calculations. Dead, live, snow, wind, and seismic loads calculated to the adopted building code for your jurisdiction.
- Foundation design. Footings, stem walls, slabs, and details engineered for frost depth and local soil behavior.
- Framing plans. Floor, wall, and roof framing, including beams, headers, and connections.
- Lateral / shear design. The bracing and load path that resist wind and seismic forces.
- PE-stamped drawings & calc package. A permit-ready set sealed by a Colorado-licensed Professional Engineer.
- Construction administration (optional). RFI responses, submittal review, and structural observation during the build.
For a deeper breakdown of what a complete package contains, see our guide to what’s in an ADU permit set in Colorado.
ADU Types We Engineer
Every ADU type carries its own structural questions. We engineer all of them:
Detached and new-build ADUs
A standalone unit needs a complete structural system from footings to roof. This is the most flexible type to engineer, but it also carries the full scope of foundation, framing, and lateral design. Learn more about detached ADU structural engineering.
Garage conversion ADUs
Converting a garage is rarely as simple as adding walls. The existing slab and foundation were often designed for vehicles, not for habitable, conditioned living space, and frequently need evaluation or reinforcement. New openings, header changes, and floor framing all require engineering. This is one of the most common and most underestimated ADU projects we see. See garage conversion structural engineering.
Basement and internal ADUs
Finishing a basement into a separate unit introduces egress requirements, beam and header modifications, and sometimes foundation underpinning. Our basement ADU structural plans address these together so the unit is both legal and safe.
Attached, above-garage, and mountain guest houses
Attached and above-garage ADUs require careful coordination with the existing home’s structure. In mountain communities, guest houses must be engineered for substantial snow loads and the design-review standards common in those towns.
Our Process & Turnaround Time
We keep the path from idea to permit as direct as possible:
- Feasibility & site review we confirm zoning fit, the adopted code, and any soils considerations.
- Structural calculations loads and member sizing for your specific design.
- Stamped plan set drawings and the calc package, sealed by a licensed PE.
- Permit support we respond to plan-review comments so the project keeps moving.
- Construction administration optional RFIs, submittal review, and observation during the build.
Turnaround depends on the scope and the completeness of the design when we start, but a typical detached or garage-conversion ADU permit set is delivered within a few weeks. Because reviewing departments set their own timelines, we design to anticipate their comments and reduce back-and-forth.
How Much Does a Structural Engineer Cost for an ADU?
For most ADU projects in the Denver area, structural engineering runs about $2,000 to $5,000 roughly 1 to 3 percent of total project cost. That covers site evaluation, structural calculations, the stamped plan set, and consultation through permitting.
Where a project lands in that range depends on a few factors: whether the unit is attached or detached, its size and number of stories, the complexity of the foundation, and the soil conditions on your lot. Compared with the cost of correcting a structural problem after construction or a permit that gets stuck in review sound engineering is one of the better investments in any ADU. For a fuller breakdown, see our guide to the cost of a structural engineer for an ADU.
Colorado ADU Rules: Where Stamped Plans Are Mandatory
Colorado’s ADU landscape has shifted significantly, and the rules differ by jurisdiction. A few points matter for almost every project:
Statewide: HB24-1152
Colorado’s statewide ADU law, effective June 30, 2025, requires “subject jurisdictions” to allow at least one ADU on lots where single-family homes are permitted, generally through an administrative approval process, and prevents homeowners’ associations from banning ADUs outright. The practical result is a wave of new ADU activity and a corresponding need for engineered plans. We cover the details in our guide to Colorado’s ADU law (HB24-1152).
Denver
Denver now allows ADUs in all single-family zone districts, with no minimum lot size and no required parking space. The technical rules that affect structure height limits around 24 feet, the bulk-plane envelope, and placement within the rear portion of the lot all shape the design. See Denver ADU permit requirements for specifics.
Front Range jurisdictions
Cities such as Arvada and counties such as Adams require foundation and structural plans stamped by a Colorado-licensed engineer, with design values for snow load, wind speed, and frost depth shown on the plans. Boulder applies its own building, fire, and change-of-use requirements, including sprinkler provisions for some detached units. We confirm Boulder’s ADU permit standards and your local code at the outset.
A note on changing rules. ADU regulations are updated frequently and vary by city and county. Treat the points above as a starting orientation, not legal advice we verify the current code adopted for your specific address before we design.
ADUs We Engineer Across Colorado
Talk to a Licensed Structural Engineer
Get permit-ready ADU plans for your Colorado project — fast, stamped, and review-ready.
Our ADU structural engineering services cover the Denver metro area and Colorado’s mountain communities. Choose your city for local requirements and examples:
Denver · Aurora · Lakewood · Boulder · Westminster · Thornton · Broomfield · Golden · Aspen · Vail · Breckenridge
Why Homeowners, Builders & Architects Choose ISTA
ISTA Engineers brings licensed structural expertise to every ADU, whether you are a homeowner navigating your first permit or a contractor who needs a dependable engineering partner on every project.
- Colorado-licensed Professional Engineers who seal every plan set.
- Certified WBENC, WOSB, MWBE, and DBE a women-owned, minority-owned firm, which matters for public projects and for teams pursuing DBE goals.
- Permit-focused plans designed to clear review, reducing costly corrections and delays.
- Responsive coordination with your architect, builder, or design team throughout the project.
ADU Structural Engineering FAQ
Do I need a structural engineer for an ADU?
In most cases, yes. An ADU is an independent dwelling, so cities require engineered foundation and structural plans usually stamped by a licensed PE before issuing a permit.
Are stamped structural plans required for an ADU permit in Colorado?
Most Colorado jurisdictions, including Arvada and Adams County, require foundation and structural plans stamped by a Colorado-licensed engineer, with snow load, wind, and frost-depth design shown on the plans.
How much does a structural engineer cost for an ADU?
For most Denver-area ADUs, structural engineering runs about $2,000 to $5,000 roughly 1 to 3 percent of total project cost covering site review, calculations, stamped plans, and permit support.
What’s included in an ADU structural permit set?
A site and soils review, load calculations, foundation design, framing and lateral plans, and a PE-stamped drawing and calculation package ready for city submittal.
How long does it take to get ADU structural plans?
It depends on scope, but a typical detached or garage-conversion ADU permit set is delivered within a few weeks once the design and site information are in hand.
Can you engineer a garage conversion or basement ADU?
Yes. Conversions often require structural modification design and foundation evaluation. We engineer detached, attached, garage-conversion, and basement or internal ADUs.