A special assessment funds a public improvement by charging the properties specially benefited, in proportion to that benefit. It differs from a tax, which is levied for general purposes without regard to benefit.
The benefit requirement. An assessment must confer a special benefit on the assessed property, distinct from the general benefit to the public, and may not exceed the value of that benefit. Assessments for improvements that benefit the community generally are vulnerable.
Apportionment methods. Front footage, area, unit count, or a formula combining them. The method must bear a reasonable relationship to benefit; mathematical precision is not required, and courts are deferential where a rational basis appears in the record.
Procedure. Statutes prescribe the sequence — petition or resolution of necessity, plans and estimates, notice, hearing on necessity, hearing on the assessment roll, confirmation, and appeal. Each notice and hearing is a jurisdictional prerequisite in many states, and defects invalidate the roll.
Objection and appeal. An owner generally must object at the hearing to preserve a challenge, and appeal within a short statutory window.
Financing. Assessments are commonly bonded, with the obligation running with the land and collected on the tax roll, which means unpaid assessments carry the same enforcement consequences as unpaid taxes.