There’s a particular conversation that happens every year with people in their first profitable year of self-employment. They’ve set money aside. They’ve been careful. And they’re still short, because nobody told them the liability was due in instalments throughout the year rather than in one payment afterward.
The underlying principle is simple and widely misunderstood: the tax system operates on a pay-as-you-earn basis. Not pay-at-the-end.
Where withholding comes from
If you’ve only ever had employment income, this is invisible by design. Your employer withholds tax from each paycheck and remits it on your behalf. By the time you file, most or all of what you owe has already been paid — filing reconciles the estimate against the actual, producing a modest refund or balance.
You were paying throughout the year. You just weren’t doing it yourself.
When it becomes your problem
Income arriving without withholding shifts that responsibility to you. Common sources:
- Self-employment and freelance income
- Business profit flowing through to you as an owner
- Investment income, including gains from sales
- Rental income
- Retirement distributions where withholding wasn’t elected
- Certain benefit income
The obligation isn’t triggered by having this income — it’s triggered by the resulting liability being large enough that the system expects instalments rather than a lump sum at filing.
The penalty is for timing, not amount
This is the part that surprises people most.
You can pay everything you owe, on the filing deadline, in full, and still face an underpayment charge. Not because you paid too little overall, but because you paid it too late in the year relative to when you earned it.
The mechanism treats each instalment period separately. Income earned early in the year was supposed to generate a payment early in the year. Paying it all at the end doesn’t retroactively fix the earlier periods.
It’s charged as interest rather than as a penalty in the punitive sense, and it’s not usually enormous. But it’s entirely avoidable, and it irritates people considerably once they understand it was avoidable.
Safe harbours
The system provides a sensible accommodation: you’re not expected to predict your final liability perfectly.
Provisions generally exist that treat you as having paid enough if your payments meet a defined threshold — typically framed relative to either the current year’s liability or the prior year’s. Meeting one of these means no underpayment charge even if you ultimately owe more.
The prior-year basis is particularly useful because it’s a known number. You aren’t forecasting; you’re referencing something already determined. For businesses with volatile income, this is often the most practical approach — it removes the guesswork entirely.
The specific thresholds and how they apply depend on your circumstances and can change. Worth confirming for your situation rather than assuming.
Two workable approaches
Base it on last year. Take the prior year’s liability, divide by four, pay that. Simple, predictable, and generally protective regardless of how the current year develops. Best for people whose income is unpredictable or rising.
Project the current year. Estimate this year’s liability and pay accordingly. More accurate, avoids overpaying, requires actually doing the projection and updating it as the year develops. Better for people with reasonably stable income, or those whose income has dropped and who don’t want to fund payments based on a better prior year.
Most businesses we work with use the prior-year approach for its predictability, with a mid-year review to catch any large divergence.
The practical habit
The single most useful thing is separating tax money as income arrives, rather than at payment deadlines.
A separate account. A percentage moved across every time you’re paid. It’s unglamorous and it works, because the problem is almost never that people don’t want to pay — it’s that the money was in the operating account and got spent on something that felt urgent at the time.
The percentage depends on your situation; a preparer can give you a realistic figure rather than a rule of thumb. What matters is that it moves automatically and lands somewhere you don’t casually draw from.
Where people get caught
The first profitable year. No prior-year liability to reference, income arriving without withholding, and no established habit. This is the most common failure point by some margin.
A significant one-off event. Selling property, a large capital gain, an unusual distribution. These generate liability in a specific period, and the instalment for that period needs to reflect it.
Income that grows sharply. Payments based on a much smaller prior year may satisfy a safe harbour, but they’ll leave a substantial balance at filing. Protected from penalty, still a large cheque to write. Worth knowing in advance.
Assuming a spouse’s withholding covers it. Sometimes it does, particularly if withholding was adjusted deliberately. Often it doesn’t, and the assumption goes unexamined until filing.
The short version
If income arrives without tax withheld from it, and the resulting liability is more than trivial, you probably owe instalments through the year. Missing them costs interest even if you eventually pay in full.
Working out your position takes one conversation. Doing nothing about it costs a small amount every year, quietly, and creates a cash flow problem at filing that nobody enjoys.
This article is general information, not tax advice. Thresholds and safe-harbour rules depend on your circumstances and change over time — get in touch to work out where you stand.