What needs a geotechnical investigation? I will answer that by saying that for rational design of a foundation, we must know what the soil is verses depth and how strong it is for the depth of the foundation. Houses in a known subdivision, here, imply a competent soil at 5 feet down, so we could design piles adjacent without much risk.
Where there is no basement and foundations adjacent to look at, we need a geotechnical investigation or at least a few testholes to verify the soils are suitable. I personally would not plan an outhouse without a testhole. For an outhouse a hand auger hole would be suitable, but for a duplex, 4- 24 foot deep macro core holes with moistures and pocket pen readings ever two feet would be suitable. One in each corner, for information.
Peat, topsoil, fill, garbage, or a good lot have different values and issues. With organic materials, there is also outgassing to consider.
Negligence is such an ugly thing to be involved in, I think I will just decline to be involved, unless we know what the soils are.