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Project GuideResidential

Building a Addition or Remodel?

Adding to an existing structure? Here's when you need new engineering and what's typically required.

Not Every Addition Triggers New Engineering

Whether you need geotechnical or civil engineering for an addition depends on the scope of work, your jurisdiction's requirements, and what engineering already exists for the property. Some additions require a full soils report and grading plan. Others can reference existing reports or don't trigger engineering at all.

The key factors: Are you adding new foundation elements? Are you modifying site drainage? Are you adding significant impervious area that triggers stormwater requirements? Are you doing enough work that the jurisdiction requires frontage improvements? The answers determine your engineering scope.

We scope additions individually. Some are straightforward (minor addition on a flat lot with an existing report on file). Others are complex (second-story addition on a hillside that requires slope stability evaluation). We research the jurisdiction requirements and advise on what's actually needed — not what might be nice to have.

What You'll Need

Engineering deliverables for addition or remodel projects:

Often Required

🔩

Soils Report or Update Letter

Required if you're adding new foundation elements and don't have a recent soils report covering the addition area. An update letter may be acceptable if an existing report is on file and compatible with the new work.

Varies

📐

Grading Plan

Required when the addition involves site grading — new retaining walls, drainage modifications, or significant cut/fill. Minor additions on flat lots may not trigger grading plan requirements.

Varies

💧

LID / WQMP (Stormwater)

Triggered when you're adding or replacing significant impervious area (typically 500+ SF). Requirements vary by jurisdiction — some exempt additions, others require full compliance.

Varies

🛣

Street Improvements

Many jurisdictions trigger frontage improvement requirements when additions exceed 50% of the existing structure's floor area. This can add unexpected costs — sidewalk, curb, ADA ramps.

If Grading

🏗

Compaction Testing

Required during construction if the addition involves engineered fill, new building pads, or utility trenches under or adjacent to the new foundation.

Why Addition Scope Varies So Much

Jurisdiction requirements for additions are all over the map. Some cities waive geotechnical requirements for additions under a certain size. Others require full investigation for any new foundation work. Some trigger stormwater compliance at 500 SF of new impervious area. Others don't trigger until 2,500 SF. There's no universal standard.

This is why we research the specific jurisdiction before scoping. An addition that requires zero engineering in one city might need a soils report, grading plan, LID compliance, and street improvements in the city next door. Getting the scope right upfront prevents surprises during plan check.

50%

Common threshold where additions trigger frontage improvement requirements (varies by city)

500 SF

Typical impervious area threshold where LID/WQMP requirements are triggered (San Diego)

Varies

Geotechnical requirements for additions vary dramatically between jurisdictions — research is critical

What to Expect

Scope Review & Jurisdiction Research

We review your addition plans and research the specific jurisdiction's requirements for geotechnical, civil, and frontage improvements. Send proposal with anticipated scope.

Field Work (If Required)

If new geotechnical investigation is needed, we drill borings or excavate test pits at the addition location. If an existing report is acceptable, we prepare an update letter instead.

Engineering & Plans

We prepare required deliverables — soils report or update letter, grading plan if needed, stormwater compliance if triggered. Coordinate with your structural engineer and architect.

Plan Check & Construction

Support your submittal through plan check corrections. During construction, provide compaction testing if grading is involved.

Building Something Else?

ADU Engineering Guide

Engineering deliverables for ADU construction in California.

Read Guide →

Hillside Development Guide

Slope stability, specialized foundations, and hillside engineering requirements.

Read Guide →

Retaining Wall Engineering

When you need geotech for walls, and what's involved.

Read Guide →

Common Questions

Maybe. If the existing report covers the addition area and the foundation type is compatible, some jurisdictions will accept it — possibly with an update letter. We review the existing report and advise whether new investigation is needed.

It depends on the jurisdiction and how much new impervious area you're adding. San Diego triggers LID at 500 SF. Other cities have different thresholds or may exempt additions entirely. We check your specific jurisdiction during scoping.

Then you'll need to install or upgrade sidewalk, curb and gutter, and possibly ADA ramps as a condition of the building permit. This can be expensive and is often a surprise to homeowners. We flag this during scoping if applicable.

Probably not, unless the jurisdiction requires a drainage plan showing how the new roof area ties into existing drainage. Minor additions on flat lots often don't require civil plans.

Depends on scope. If we can use an existing report with an update letter, just a few days. If new investigation is required, 2-3 weeks for geotech plus additional time for civil design if needed.

Ready to Get Your Addition or Remodel Started?

Tell us about your addition or remodel project and we'll send a proposal with every deliverable you need — scope, fee, and timeline.

Request a Proposal Talk to an Engineer