When the IRS cancels an IA, they send out a letter, CP523 (upper rt corner). The default can be from a missed payment , or from being assessed additional taxes It does Not default on the date of the letter but 30 days later. There is another 10 day grace period after that to allow any payment to post. If the additional tax is small, sometimes the IRS will roll the IA over automatically. The options are an OIC, a new IA, or CNC (currently non-collectible). The easiest solution would be a streamlined Installment Agreement (IA). The balance due must be less than $50,000. If it is over that, the IRS will usually allow 30 days to pay it down. To determine the minimum payment, divide the amount due by 72 (months). If the client needs more time for the paydown, there are strategies for getting additional time. OIC, CNC, or a regular installment agreement will require documentation of income and allowable expenses.