The major power of the HOA is the ability to compel property owners to pay a share of common expenses for the overall maintenance of the HOA and the amenities, usually proportionate to the ownership interests (either by unit or based on square footage). These expenses generally arise from the operation and maintenance of common property, which vary dramatically depending on the type of association. An HOA may have, in addition to a regular assessment, a "special" assessment for unexpected expenses (such as for road maintenance).