The same one-month-late return, ₩1.06M or ₩2.06M — did you file on your own before a tax audit began?

Two people made the same mistake: they did not file ₩10M of tax by the filing deadline. Both submitted a return one month later. One's penalty was about ₩1.06M; the other's, about ₩2.06M. Same date, same tax, a ₩1M gap. The only difference was whether they filed on their own before a tax audit began, or after learning it had begun. Penalty tax runs on a timetable: the sooner you move the more is cut, and once an audit has begun, nothing is cut.

Penalty tax is not a fine; it is an amount the tax law adds on top of the tax. The Framework Act on National Taxes spells out the kinds, the rates and the relief entirely in numbers. First, there are three kinds. Second, the one that grows every day, late payment. Third, the relief steps. Fourth, the moment relief disappears. Past all four, the reason to act today when you have missed a filing shows up as a number.

Step one, the three kinds. Filing-related penalties come in two kinds. If you did not file at all, the non-filing penalty: 20% of the tax due. If you filed but understated, the under-reporting penalty: 10% of the understated tax (Framework Act Arts. 47-2 and 47-3). Not filing at all is twice as heavy, so submitting a return has value in itself. In both kinds, fraud raises the rate to 40%: deceptive conduct such as double books or false vouchers, as determined by the tax office. On ₩10M of tax, non-filing is ₩2M, under-reporting ₩1M, fraud ₩4M. For double-entry bookkeepers and corporations, a revenue-based figure (14/10,000 for fraud, 7/10,000 otherwise) is compared on non-filing, and on under-reporting only where there is fraud, with the larger applying (Arts. 47-2(2) and 47-3(2)). The third kind is separate from filing: the late-payment penalty on simply paying late.

Step two, the late-payment penalty. It accrues at 0.022% per day on unpaid tax (Art. 47-4 · Decree Art. 27-4). On ₩10M that is ₩2,200 a day. Its menace is twofold. First, it never stops. It runs through weekends and holidays, every day until payment (or until the day before a payment notice is issued), and over a year it exceeds 8% of the tax, about ₩800,000 on ₩10M. Second, there is no relief. The relief steps cut only filing-related penalties; late payment is not reduced by a single day even if you file voluntarily. So filing and paying run on separate clocks. Even without the money right now, filing first lets you cut the big block, the filing penalty. Once a payment notice is issued the daily charge stops at the notice date; if the tax is still unpaid by the notice's due date, 3% is added at once, and after that 0.67% a month for up to 5 years where the amount per notice and tax is ₩1.5M or more (Art. 47-4(1)1-2 and 3, (7), (8)). Finishing before the notice is cheapest.

Step three, the relief steps. Even after the deadline, filing on your own before the tax office acts earns statutory relief (Art. 48 para. 2). For someone who never filed, a late return gets 50% off within 1 month, 30% within 3 months and 20% within 6 months; after 6 months the relief is gone. For someone who understated, an amended return has a longer staircase: 90% within 1 month, 75% within 3, 50% within 6, 30% within 1 year, 20% within 18 months, 10% within 2 years.

File within a month and the ₩2M non-filing penalty is halved to ₩1M; add the late-payment charge and it is about ₩1.06M. At six months, about ₩1.99M; at one year, with no relief, about ₩2.8M; at two years, about ₩3.6M. Same mistake, and the date you moved made a ₩2.5M difference. The staircase is highest today, and tomorrow it is lower than today.

Step four, the moment relief disappears. The relief article carries a proviso: no relief for a return filed knowing in advance that the tax office was going to determine or correct the tax base and amount. Decree Article 29 limits "knowing in advance" to two cases: an amended or late return filed knowing a tax official has begun an audit, and an amended return filed after the tax office's request to explain tax data. A return filed after an ordinary filing reminder or a pre-assessment notice is not on the list, so the relief steps still apply. That is what separated the two people. One filed on their own before any audit and had half cut, about ₩1.06M. The other filed after learning an audit had begun, with no relief, about ₩2.06M. Not one character of the return was different. Relief rewards correcting things yourself first, not submitting a form. The tax office already holds interest and dividend data, card sales and withholding records, so a missed filing tilts toward discovery the longer it sits.

There is an opposite side too. The law also lists cases where no penalty applies (Art. 48 para. 1). First, grounds for deadline extension such as natural disaster, or a justifiable reason for not meeting the duty, though "I did not know" is hard to get accepted, and each case is judged on its facts. Second, a return with no tax due: the non-filing penalty is in principle a rate applied to the tax payable, so someone due a refund has nothing to multiply, and a late filing only delays the refund. There are exceptions: a double-entry bookkeeper or corporation that fails to file income or corporate tax pays the larger of that and 7/10,000 of revenue, so a penalty can arise with no tax due, and VAT zero-rated sales add 0.5% of that tax base (Art. 47-2(2)). Claiming a larger refund than due draws the excess-refund penalty (Art. 47-3). Third, a simplified VAT taxpayer with under ₩48M in annual sales, exempt from VAT payment, is expressly excluded from the non-filing penalty on that VAT return. Penalty tax is the problem of someone who owes tax and is late; if you do not know whether you owe, finding out is what filing is.

Two things to do now. First, check today whether you have a missed filing. Hometax accepts late and amended returns, and the staircase is highest today. Second, file first even if you cannot pay yet: the filing penalty is reduced and only the late-payment charge remains. Penalties specific to each tax, fraud determinations and the arithmetic after a notice vary by case, so for large amounts or several tangled years, sort it out with a tax adviser. What separated the two people was not tax knowledge but the order in which they moved.