ADU Structural Engineering, From Foundation to Roof.
Complete structural packages for detached ADUs, garage conversions and units over garages — sized for California's ADU review process.
Upload plans, photos or permit comments. Get a defined scope and quote before any commitment.
California's ADU rules have made the planning side faster, but the structural submittal still has to stand on its own. A detached ADU is a small new house: it needs a foundation plan, framing plans, a lateral system that works for its wall layout, and calculations that show it. Garage conversions look simpler on the surface, but they usually involve infilling the garage door opening (a significant lateral change), verifying a slab that was poured for cars, and cutting openings in walls that were never intended to have them.
We engineer ADUs every week — detached units, attached units, conversions and above-garage units — and we know what plan checkers in ADU-heavy cities ask for. If you are working from a pre-approved or standard plan set, we provide the site-specific foundation and lateral engineering the jurisdiction requires. Upload your plans and we will scope it.
Common projects
- Detached new-construction ADUs, studio to three bedrooms
- Garage conversions with door-opening infill
- Attached ADUs sharing a wall with the main house
- ADUs above existing or new garages
- Junior ADUs carved from existing floor area
- Two-story detached ADUs
- Site adaptation of pre-approved or standard ADU plans
- ADUs on sloped lots or expansive soils
- Foundations for prefab and modular ADUs
What to upload
Submit what you have — we'll tell you what else is needed before engineering begins.
- Architectural plans for the ADU: floor plan, elevations, sections and roof plan
- Site plan with the ADU location, setbacks and distance to the main house
- Photos of the existing garage or structure for conversions and build-overs
- Geotechnical report, if the lot has one
- Existing structural drawings for garages being converted or built over
- Pre-approved plan set, if you are adapting one
- Utility and drainage information affecting the foundation, if known
Deliverables
What you receive
Every set goes through structural design, drafting and review by the licensed engineer responsible for the work before it is released to your dashboard.
- Foundation plan and details: slab-on-grade, raised floor or piers
- Floor and roof framing plans
- Shear wall schedule, hold-downs and lateral calculations
- Garage door infill and existing-structure evaluation for conversions
- Beam, header and post schedule
- Structural details keyed to plan locations
- Bound calculation package
- Signed and sealed documents where required by the jurisdiction
- Plan-check correction responses as included in your proposal
Example project
Detached 750 sq ft one-bedroom ADU — Long Beach
New single-story detached ADU at the rear of a flat lot with a slab-on-grade foundation, 9-foot plate height and a low-slope roof carrying a rooftop mechanical unit.
Jurisdiction
City of Long Beach
Turnaround
10–15 business days
Engineering scope
- Slab-on-grade foundation with turned-down footings
- Roof framing with mechanical unit support
- Wood shear wall lateral design with hold-downs
- Header and beam schedule at the large patio slider
- Full gravity, lateral and foundation calculations
How it works
From Plans to Engineering in Four Steps
- 1
Submit Your Project
Tell us what you're building and upload whatever you already have. Plans, photos, sketches and permit comments can all be submitted.
- 2
Receive Your Scope & Quote
We review the project and send you a defined engineering scope, price and estimated turnaround.
- 3
Engineering Begins
Your project moves through structural design, calculations, drafting and engineer review.
- 4
Receive Your Documents
Final engineering documents are delivered through your project dashboard. Permit comments can be submitted back through the same project.
Frequently asked questions
Does a garage conversion really need structural engineering?+
Usually. Removing the garage door and infilling the opening changes the building's lateral system on that wall line; new windows and doors add openings; and a garage slab may need verification or a new stem wall for the floor and wall framing. Some jurisdictions also require the header over the former door to be checked. We scope only what your conversion needs.
Can you provide the engineering for a pre-approved or standard ADU plan?+
Yes. Pre-approved plans still require site-specific items — a foundation designed for your soil, lateral design for your seismic parameters, and adaptation for any changes to the standard plan. Upload the plan set and the site plan and we will define the site-specific scope.
What kind of foundation will my ADU use?+
A slab-on-grade with thickened-edge footings is the most common choice for flat lots and suits most single-story ADUs. Raised floors, stem walls and drilled piers come into play on slopes, poor soils or where the design calls for a crawlspace. The foundation type is part of the scope review, and we coordinate it with your plumbing layout.
Will I need a soils report for an ADU?+
Many single-story ADUs on flat lots are designed with the code's presumptive soil values. Sloped sites, known expansive or fill soils, and some jurisdictions' local requirements can trigger a geotechnical report. We tell you before quoting if your site is likely to need one.
Can you engineer an ADU above my existing garage?+
Yes, with a caveat: most existing garages were not built to carry a floor and a second story. We evaluate the existing walls, footings and roof from your photos and drawings. Often the design reinforces or replaces portions of the garage framing; occasionally the most economical answer is a new structure. We will tell you which after scope review.
How does a two-story ADU differ structurally?+
Higher lateral demand per wall, floor framing to design, larger footings under bearing walls and posts, and stair and opening framing. It is still a small building and the process is identical; the calculation package is just longer.
Related services
Room & Second-Story Additions
Foundation, framing and lateral design for room additions, second stories and bump-outs — coordinated with your architectural set and the existing structure.
Foundation Engineering & Repair
New foundations, underpinning, seismic retrofits and repair details — engineered from your plans, photos and inspection reports.
Structural Plans & Calculations
Foundation and framing plans, lateral design, details and a bound calculation package — engineer reviewed and organized the way building departments expect to read them.
Have a project that needs engineering?
Upload what you have and we will define the engineering scope and send a quote.
No commitment required to submit a project for review.