The possible issue you face when you prepare your income tax return is being "underpaid." That's a piece of jargon that has a specific meaning under the income tax laws.
The US income tax system is a "pay as you go" system, and most people "pay as they go" by having income taxes withheld from their paychecks. For people with significant amounts of income NOT subject to withholding there's an alternative way of paying taxes. Namely, you pay "estimated taxes" on a roughly quarterly basis, sending in checks to the IRS on 4/15, 6/15, 9/15 and 1/15 of the following year, accompanied by a "voucher" called Form 1040-ES.
If you are self-employed you owe income taxes on your net self-employment income, (amounts collected from clients minus expenses associated with that income), PLUS you owe self employment taxes, (Social Security and Medicare), on that income at roughly a 15% rate.
Simply because you have to send in a large check when you mail off your income tax return does not, in and of itself, mean you are "underpaid" and subject to underpayment penalties.
Most taxpayers will avoid being underpaid if they:
1) owe less than $1,000 in tax after subtracting their taxes WITHHELD and available tax credits,
OR
2) if they paid at least the lesser of
a) 90% of the tax for the current year, or
b) 100% of the tax shown on the return for the prior year. (If last year's return shows AGI over $150K (for married filing jointly) then change that "100%" figure to "110%.)
So, for example, if in 2013 you had little income and accordingly had just a small income tax liability, then paying that same amount in 2014 via quarterly estimated taxes would put you in that 2)b) "safe harbor" and no matter how large a check you paid with your 2014 income tax return, you'd not be subject to an underpayment penalty.
As far as what you can do at this point - maybe having missed the first 3 estimated tax deadlines - is to do a quick estimate of how much taxes you'll owe based on your 2014 income. If you see that you'll owe underpayment penalties then send in as much of that tax liability as soon as you can. You might still owe a penalty for "past due" quarters, (the penalty is calculated on a quarter by quarter basis), but a payment will at least mitigate or stop the clock on any further penalties.
Tom Young
