A Special Assessment occurs in a condominium when there are expenses that exceed the funds raised by monthly dues payments. The condo association will levy a one-time special assessment against each unit to cover the shortfall. For example, if a 50-unit condo association needs $100,000 to replace their roof, but only has $50,000 in reserves, they will charge a special assessment of $1,000 per unit to cover the shortfall.
A properly-run homeowners association will budget for large expenses that are anticipated in the future such as a roof, siding and window replacement. When a homeowners association does not collect enough in regular dues, there can be large budget shortfalls when important repairs are needed. It is not unusual to have a special assessment of $10k-$40k per unit in a condo, which is an unwelcome and potentially unaffordable surprise for many homeowners.
One other form of special assessment may come from a local government to support local utilities or roads. This most often occurs when a city decides to bring sewer service to a neighborhood that previously did not have it. They will create a local improvement district of the homes affected by the change and charge a special assessment to each of the homes to pay for the cost of bringing the new utility to the houses.