pcorder – thanks for the question.
It sounds like you will have a building, as defined under the flood regulations, as collateral and if it sits in a flood zone, flood insurance would be required. Unless you plan to carve out the section of land with the well house from your collateral (not that I’m advocating this because it isn’t practical and in most cases would diminish the collateral) you would need to pull a SFHDF.
And since it isn’t part of a residential property the detached structure exemption would not apply.