Order a soils or geotech report if conditions require it

When you need a soils report

You need a soils or geotechnical report when one of these is true:

• The lot is in an Environmentally Critical Area (ECA) under SMC 25.09.
• The lot has any natural slope greater than ~15%.
• The structural engineer's tie-in assessment said the soil-bearing assumption needs verification.
• The site has visible water issues (standing water, springs, prior settlement at the existing foundation).

If none of those apply — flat lot, no ECAs, sound soil — you can skip the report and the structural engineer can use the conservative default soil-bearing values from the Seattle Building Code.

What the report does

A soils report (or full geotech for ECA / slope cases) does three things:

1. Confirms or sets the allowable bearing pressure for the new addition foundation.
2. Recommends footing depth and any drainage details.
3. Identifies any unusual conditions — fill, organic soil, perched groundwater — that change the foundation design.

Why this matters for the tie-in

If the existing foundation was built on assumptions that no longer match current code (common on pre-1950 houses), the new addition foundation has to either match the existing assumption (and risk underperforming) or be designed to current code (which can mean different bearing depth, which can mean differential settlement). The geotech report is what tells the structural engineer how to handle that.

When you can skip it

A flat inner-Seattle lot with no ECAs, a sound existing foundation, and an experienced engineer using conservative defaults — most projects qualify, and skipping saves $3K and three weeks. Default to skipping unless the structural engineer or SDCI specifically asks for it.