Thank you for your question.
I'm sorry to bring you bad news, but the situation you describe is a perfect example of the difference between "taxes" and "assessments". The way the state law is written, it is unclear from my perspective.
In Wisconsin, you have this statute in favor of taxability of property in general:
70.109 Presumption of taxability. Exemptions
under this chapter shall be strictly construed in every instance with a presumption that the property in question is taxable, and the burden of proof
is on the person who claims the exemption.
The exemption from taxation in 70.11 is for "general property taxes".
Special assessments fall under a different part of the law:
66.0701 Special assessments by local ordinance.
(1) Except as provided in s. 66.0721, in addition to other methods provided by law, the governing body of a town, village or 2nd, 3rd or 4th class city may, by ordinance, provide that the cost of installing or constructing any public work or improvement shall be charged in whole or in part to the property benefited, and make an assessment against the property benefited in the manner that the governing body determines. The special assessment is a lien against the property from the date of the levy.
(2) Every ordinance under this section shall contain provisions for reasonable notice and hearing. Any person against whose land a special assessment is levied under the ordinance may appeal in the manner prescribed in s. 66.0703 (12) within 40 days of the date of the final determination of the governing body.
The exception cited in there, 66.0721, applies only to farmlands and camps operated by non-profits, so I won't discuss them.
It would *appear* that being tax-exempt and thus "exempt" from "assessment" under the general taxation statute would make special assessments also taboo under the first option of such assessments, 66.701 just quoted above.
BUT an alternative authority for assessments here makes it less clear (look at (c.) at the end):
66.0703 Special assessments, generally.
(a) Except as provided in s. 66.0721, as a complete alternative to all other methods provided by law, any city, town or village may, by resolution of its governing body, levy and collect special assessments upon property in a limited and determinable area for special benefits conferred upon the property by any municipal work or improvement; and may provide for the payment of all or any part of the cost of the work or improvement out of the proceeds of the special assessments.
(b) The amount assessed against any property for any work or improvement which does not represent an exercise of the police power may not exceed the value of the benefits accruing to the property. If an assessment represents an exercise of the police power, the assessment shall be upon a reasonable basis as determined by the governing body of the city, town or village.
(c) If any property that is benefited is by law exempt from assessment, the assessment shall be computed and shall be paid by the city, town or village.
What gives me pause is that the local government is trying to levy the assessment. Under 66.0703 (12), anyone has a 40-day time limit to appeal the assessment. It may depend on whether the local government used 701 or 703 as authority for the assessment.
To get more definitive, I would have to research case law in Wisconsin citing these statutes to see whether any judges have clarified the interplay of these laws, or even eviscerating them in one direction or another. That happens. In interpreting the law, judges get to say what they mean under plain language, what they mean if they are inconsistent (one rule is that the last-enacted law trumps the earlier one(s)), what they mean if they could be interpreted more than one way, whether they are too vague to be valid, or if there is some other reason for the law to be invalid. "Judicial activism" is often the only way to reign in unruly lawmakers.
That research would take another half-hour. As you can tell, I'm into this pretty deep right now.
Part of the rationale here might be that, notwithstanding them being collected by the same tax minions of the local government, taxes benefit the general community (including servicing government bond debt even if for a specific thing NOT in that neighborhood), while assessments are specific to the neighborhood which gets the greatest benefit of the thing(s) being paid for.
In theory, these things are supposed to be kept under control because of citizen input into local government.