how do you decide if a soils report is required?

I am biased, but I really believe your supervisor. You should get a soils report for every project.

Most of the cost over-runs on a project are soils related.

Actually, it turns out we had one done on the same section of the same street already, so I remembered the reason for not doing a new one incorrectly.

Thanks.

I'm inclined to agree with your supervisor and eric1037 -- every project requires a geotechnical report. Such reports are usually inexpensive and are a pretty cheap way to ensure that the correct design is implemented.

I wonder what your firm's insurer thinks about you inferring data from other sites? I think they'd consider a geotech report to be a pretty cheap way of mitigating their risk.

Yes, you're right.

I was wrong about the reason we weren't getting a geotech report for this project. I thought it was because we had one for the adjacent section of street, but it was because we actually already had one for the exact same section of street, for another project.

Thanks.

If the other project is different from the intended development, you may need at least a review of the old Geotechnical report.
One can not use a Geotechnical report done for a site and a proposed 2 storey building to design/build a 10 storey building on the same site.

How much is the cost of the Geotechnical report as a fraction of the development cost?
I just find hard to believe that a client wouldn't spend few thousand dollars at the risk of spending few hundred thousand in remedial works!

I'm structural engineer so I don't make money from Geotechnical reports, but I strongly recommend my clients to seek professional Geotechnical advice. And they do. Just explain them what the risk is. They will appreciate your advice and professionalism.