I don't have water or sewer in my barn - that would require a different permit and a septic review. I have what the county calls a 'dry barn'.
However, I am allowed to have hose bibs. Theoretically I could attach a sink/faucet to one of these bibs by hose. I could also, theoretically, run the drain from this sink to a more convenient spot, outside the barn. After all, I can use the hose anywhere it is convenient so why can't I pipe the water to some place more convenient, after it exits the hose.
This theoretical arrangement does require great care in determining what goes into the sink. It also requires high quality hose for the jumper - the cheap stuff blows out if left under pressure for long periods.
For other, comfort related, plumbing I have a nice quiet spot behind the barn or a short (300') walk to the house.
Similar permit restrictions mean the electrical system in my nearby tool shed is plugged into an outlet in the barn and not hardwired. Keeps it a portable building and not an 'improvement'.
I know it sounds petty but they make the rules. The supreme court agrees that there is no reason for a person to pay more tax $$ than actually 'required' by the law.