May or may not need a geotech survey
Determined by the county geologist
The site may need to have a geological survey to ensure it is ok to use a conventional foundation, rather than needing a more costly piled foundation. It's up to the geologist in the local county planning department to determine whether a geotech survey is required for your plot. Getting a geotech survey done will cost you something like up to $4,000. The good news is that a geotech survey isn't usually required.
Some benefits to having a geotech survey
Reducing critical slope setback
If you have a critical slope on your property then the default is that you will need a 50 foot setback plus an additional 15 feet to your building. A critical slope is defined as a slope that goes more than 10 feet down for every 25 feet horizontally. If you want to build closer to the top of the critical slope than the default figure of 65 feet, then a geotech survey may let you do it if the ground is found to be stable enough. A geotech survey may reduce the 50 foot setback to say 10 feet. You'll still need the 15 foot from there to the building line, but that will mean your house can be 25 feet from the top of the slope, instead of 65 feet.
In my case, the county geologist thought the ground was stable and so a geotech survey was not required. I opted to do without the geological survey and instead put the money towards building really good foundations. The slope on my plot is not steep enough to be classed as a critical slope, but even so I wanted to build comfortably away from it. I was happy to build more than 65 feet from the slope.