What a special assessment is
A one-time charge, beyond the regular assessment, to pay for something the operating budget and the reserves cannot: a roof that failed early, a lawsuit, an insurance deductible after a storm, a shortfall from delinquencies. It is levied on every home in the same proportion as the regular assessment unless the declaration says otherwise.
Who approves it
The declaration decides. Many documents let the board levy a special assessment up to a limit on its own, often a percentage of the annual budget, and require a member vote above it. Some require a member vote for any special assessment; some give the board full authority. The state act may add a cap or a vote requirement on top, and a few states exempt emergencies from both.
Read the section of the declaration titled special assessments or additional assessments before the first board discussion, and read the state act's section on assessments beside it. The HOA laws page for your state links the text.
How it is split
By the same allocation as the regular assessment: equal shares in most planned communities, allocated interests in most condominiums. A special assessment that benefits only some homes, a private road or a shared wall, may be levied on those homes alone only if the declaration allows it.
Notice and payment
The notice should say what the money is for, the total, each home's share, when it is due, whether it can be paid in installments, and what happens if it is not paid. Most states require written notice a set number of days before the first installment. Give owners a choice of a lump sum or installments; the collection rate is higher.
Record the levy in the minutes with the vote, post the notice to the owner portal, and add the charge to each owner's ledger as its own line so statements and resale certificates show it separately from regular dues.
Before you levy one
A special assessment is usually a symptom: reserves that were underfunded for years, or a budget that was kept flat to avoid an increase. Fund the reserve study's recommendation and raise the regular assessment with inflation, and special assessments become rare. If the need is a large capital project, compare an assessment with a bank loan repaid through the regular assessment over several years; many lenders offer them to associations.