Residential

ADU Site and Structural Engineering

Site, grading, drainage and utility design for accessory dwelling units, backyard cottages and garage conversions. Typical US fees run $2,000 to $10,000, and most projects take 3 to 8 weeks. Our drawings, calculations and reports are sealed by a Professional Engineer (PE) licensed in the state where your project is located, ready for permit submission.

Typical US fee$2,000 to $10,000
Typical timeline3 to 8 weeks
Permit drawingsSealed by a PE

What the work involves

An accessory dwelling unit needs more than a floor plan. A civil engineer checks setbacks, grading, drainage, sewer or septic capacity, and water service size, then prepares site and utility plans for permitting. Structural engineering covers the foundation and framing, and garage conversions often need slab and footing evaluation. On septic properties the system may need to be evaluated or expanded for the added bedrooms.

When you need it

  • Building a detached ADU or backyard cottage
  • Converting a garage into living space
  • Lot has a slope, drainage issues or limited access
  • New sewer lateral, water line or septic expansion needed
  • Jurisdiction requires a stamped site or grading plan

What you receive

  • Site plan with setbacks
  • Grading and drainage plan
  • Utility connection plan
  • Structural plans and calculations
  • Septic capacity evaluation if applicable

What drives the cost

Estimate built from HomeAdvisor residential engineering ranges ($2,000 to $8,500 per project) plus site and utility work. Septic upgrades, sewer extensions and slopes increase cost.

Related calculators

Concrete Slab Cost CalculatorEstimate the cost of a concrete slab from its size, your local ready-mix price per yard and an installed labor rate per square foot.

Frequently asked questions

How much does aDU Site and Structural Engineering cost?

Across the US, aDU Site and Structural Engineering usually costs $2,000 to $10,000. Estimate built from HomeAdvisor residential engineering ranges ($2,000 to $8,500 per project) plus site and utility work. Septic upgrades, sewer extensions and slopes increase cost.

How long does aDU Site and Structural Engineering take?

Plan on 3 to 8 weeks from signed proposal to stamped deliverables, longer if a site visit, soil testing or a permit review cycle is involved.

Do I need a licensed PE for aDU Site and Structural Engineering?

In most US jurisdictions, yes. When the work needs a permit, the plans, calculations or report usually must be sealed by a Professional Engineer licensed in the state where the project is located. Every deliverable we issue for permit is sealed that way.

Will I also need a structural engineer?

Sometimes. Depending on the site, aDU Site and Structural Engineering can also involve a structural engineer, architect, land surveyor or septic designer. We tell you up front which other professionals your project needs and coordinate with them so the drawings agree.

Who do you provide aDU Site and Structural Engineering for?

Most of our clients for this service are homeowners, contractors and architects. Describe your project and one of our engineers will reply with scope, fee and schedule.

Do ADUs need a civil engineer?

Not always. Flat lots with simple utility connections may only need a site plan from the designer. Slopes, drainage problems, septic systems and new utility runs usually call for a civil engineer.

Can my septic system handle an ADU?

Septic systems are sized by bedrooms or flow. Adding a unit often requires an evaluation and sometimes a larger tank or drain field, approved by the local health department.

Is a garage slab good enough for living space?

Garage slabs often lack insulation, vapor barriers or footings sized for new walls. An engineer evaluates whether it can stay or needs work.

Who draws the site plan?

A civil engineer, surveyor or the architect depending on your jurisdiction. Some cities require a licensed surveyor to show property lines.

Where we provide this service

Alabama Alaska Arizona Arkansas California Colorado Connecticut Delaware District of Columbia Florida Georgia Hawaii Idaho Illinois Indiana Iowa Kansas Kentucky Louisiana Maine Maryland Massachusetts Michigan Minnesota Mississippi Missouri Montana Nebraska Nevada New Hampshire New Jersey New Mexico New York North Carolina North Dakota Ohio Oklahoma Oregon Pennsylvania Rhode Island South Carolina South Dakota Tennessee Texas Utah Vermont Virginia Washington West Virginia Wisconsin Wyoming

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