What Does an ADU Engineer Do?

Key takeaways

• An ADU engineer designs what holds the unit up: foundation, beams, posts and bracing.
• Most ADUs need a structural engineer. Some also need civil or soils work.
• In California, anyone may draw a simple wood-frame home, but non-standard parts need a licensed engineer or architect ¹.
• Final civil engineering plans must be signed and stamped, on every sheet ².
• Only a qualified licensee may call themselves a “structural engineer” in California ³.
• You can check any California engineer’s license online in a minute ⁶.

What an ADU engineer is, in plain English

A house has to hold itself up. It also has to stand through wind and earthquakes, and sit on ground that will not shift. An engineer does the math that proves it will.

For an ADU, that math becomes a set of calculations, often called “calcs.” It also becomes drawings: a foundation plan, a framing plan and details for how parts connect. The engineer signs and stamps them. The city’s plan checker then reviews them.

The engineer is usually not the person who designs the layout. That is the architect or designer. The engineer takes their design and makes it stand up.

Why it matters to you

The structure is the part you cannot fix later without tearing things out. Getting it right on paper is cheap compared with fixing it in the field.

Engineering also changes your bid. Footing size, steel, hold-downs and beam sizes all come from the engineer’s design. Two bids built from vague plans can differ widely for this reason.

Last, missing engineering is a common reason plans come back from plan check. Bringing an engineer in early can save a round of corrections.

Which kind of engineer does what

How engineering fits into an ADU project

1. Your designer sets the layout and size.
2. If the lot is hilly or the soil is in question, a soils engineer may test the ground and write a report.
3. The structural engineer reads the layout and the soils report.
4. They size the foundation, framing and bracing, and write the calcs.
5. They draw or mark up the structural sheets of the plan set.
6. They sign and stamp the final sheets and calcs ².
7. They answer structural comments from plan check.
8. Some cities or designs need the engineer to visit during the build. Ask early.

California’s rules on engineers

California requires a license to practice civil, electrical or mechanical engineering in the state, unless the law makes an exception ⁵. Civil engineering includes work on foundations, grading, framed structures and buildings ⁴.

There is a key exception for small homes. The engineering law does not stop any person from preparing plans for a single-family wood-frame home of up to two stories and basement, or for a wood-frame building of up to four homes of the same height ¹.

A building official may still require an engineer if the official finds an undue risk to public safety ¹.

The exception has a limit. If any part of the home departs from conventional wood framing in Title 24 or the local code’s framing tables, the building official must require that part to be prepared by a licensed engineer or architect. Those sheets must carry the licensee’s stamp and signature ¹.

Stamping is strict. Civil engineering plans and specifications that are permitted or released for construction must bear the licensee’s signature, seal or stamp, and the date. On multi-sheet plans, it must appear on every sheet. Preliminary documents must be marked with their purpose, such as “for plan check only” ².

The title is protected. Only a licensed civil engineer the board has qualified as a structural engineer may use that title ³. Many licensed civil engineers design ADU structures without that title. Check the license, not the business card.

This mirrors the architect rule on the ADU architect page, which covers the design side. The guide below turns the small-home exemption into four plain cases.

Sources: [5] [4] [1] [2] [3]

Rules in other states

Every state licenses professional engineers. The line for when a home needs one varies. Some states exempt small wood-frame homes much like California. Others leave it to the building official.

Seismic and wind zones matter too. A city in high-wind or high-earthquake country may ask for engineering on more homes. Ask your local building department, and see ADU laws by state for ADU rules.

Worked example: does this ADU need engineered parts?

A hypothetical one-story wood-frame ADU in California. The rule is real ¹; the design choices are made up to show how the line works.

The building official decides what counts as conventional. Many cities ask for structural calcs on every new ADU anyway. Treat the exemption as a floor, not a plan.

Where you will see the engineer’s work

• Structural sheets in the plan set, often numbered “S-1,” “S-2.”
• A bound calc package submitted with the plans.
• A soils or geotechnical report, if your site needs one.
• A stamp, signature and date on each final structural sheet ².
• Plan-check comments marked “structural.”
• A contractor’s bid that lists hold-downs, steel or special footings.

What drives engineering cost

No dollar figures; fees vary by firm, city and design. These are the drivers.

• Design complexity: two stories, steel, large openings, odd shapes.
• Site: slopes, retaining walls and poor soil add civil or soils work.
• Conversions: an existing garage may need its foundation checked or upgraded.
• City demands: some ask for calcs on every ADU, others only for non-standard parts.
• Plan-check rounds: more comments mean more engineer time.
• Site visits during construction, if needed.

Plan the full budget

• ADU soft costsWhere engineering sits among design and permit costs
• ADU cost calculatorEstimate a full project budget

Benefits and limits of hiring an engineer early

Engineer compared with related roles

Common misreadings

Myth “Every ADU must have a structural engineer by law.”

Reality California’s law lets anyone prepare plans for conventional wood-frame homes. Non-standard parts need a licensee. Many cities ask for calcs anyway ¹.

Myth “A civil engineer cannot design my ADU structure.”

Reality Civil engineering includes framed structures and buildings ⁴. The “structural engineer” title is a further credential ³.

Myth “The architect handles the structure.”

Reality Some do. Many hire an engineer for calcs. Ask who signs the structural sheets.

Myth “A soils report is only for big buildings.”

Reality Hillside lots, fill soil and some hazard zones can call for one on an ADU.

What to ask an ADU engineer

• What is your license type and number? I will check it ⁶.
• Have you engineered ADUs in this city?
• Do I need a soils report for this lot?
• What is included: calcs, sheets, plan-check replies, site visits?
• How will changes to the layout affect your fee?
• Will the foundation of my existing garage work for a conversion?
• Which parts of this design go beyond conventional framing?

Checklist: engineering for your ADU

1. Ask the building department whether it requires calcs on every ADU.
2. Check the lot for slope, fill, or hazard maps.
3. Settle the layout before engineering starts.
4. Verify the engineer’s license online ⁶.
5. Get a written scope with plan-check replies included.
6. Confirm stamps and signatures on every final sheet ².
7. Share the structural sheets with every bidder.
8. Keep the calcs with your permanent records.

Related guides, tools and research

• ADU construction guideHow an ADU gets built, stage by stage
• ADUs in flood zonesSite rules that bring in more engineering
• Garage conversion ADUsExisting slabs and walls
• Building codeThe rules the engineer designs to
• Feasibility checkerSpot site issues early

Related terms

ADU Architect

An ADU architect is a state-licensed architect who designs an accessory dwelling unit and prepares the drawings the building department reviews, often managing plan-check corrections as well.

ADU Plans

ADU plans are the set of scaled drawings and documents that describe an accessory dwelling unit well enough for a building department to check it against zoning and building codes and for a contractor to build it.

Building Code

A building code is a set of minimum rules for how buildings must be designed and built so they are safe to live in, covering structure, fire safety, exits, plumbing, wiring, ventilation and energy use.

California Residential Code

The California Residential Code (CRC) is Part 2.5 of Title 24 of the California Code of Regulations: the state building code for detached one- and two-family dwellings and townhouses, which is the code most ADUs in California are designed under.

ADU Contractor

An ADU contractor is a licensed construction business that contracts to build an accessory dwelling unit, either doing the work itself or running the subcontractors who do the trades.

ADU Soft Costs

ADU soft costs are the parts of an accessory dwelling unit budget that pay for paperwork, professional services and money rather than for physical construction: design, engineering, surveys, permits and government fees, utility charges, insurance and financing costs.

Garage Conversion ADU

A garage conversion ADU is an accessory dwelling unit created by turning an existing garage, attached or detached, into a complete home with its own kitchen, bathroom, entrance and living space.

Two-Story ADU

A two-story ADU is an accessory dwelling unit with living space on two levels, usually with the living area and kitchen below and bedrooms above, joined by an inside stair.

Also referenced from: ADU Soft Costs, Building Code, California Building Code, ADU Design-Build, ADU Foundation, ADU Architect