For most people, this year's deadline for filing Form 1040 is Thursday, April 15. Miss the deadline and you could get slapped with a substantial, nondeductible penalty. Generally, the penalty is 5 percent of the balance due (the amount that remains unpaid after subtractions for taxes previously paid through withholdings from wages during 2009 and payments of estimated payments) for each month, or portion of a month, that a 1040 is late.

The maximum penalty is 25 percent of the balance due -- a truly steep charge. On a balance due of, say, $10,000, that works out to $500 a month. The penalty can reach as much as $2,500 when more than four months elapse before your return reaches the Internal Revenue Service.

Sometimes, the IRS will forget about a late-filing penalty. The agency will waive the penalty only if you can convince it that the delay was "due to reasonable cause and not due to willful neglect." An example: You're unable to complete the return by the deadline because your residence, place of business or records are destroyed due to a fire, flood, other casualty or civil disturbance, or burglary. Another acceptable excuse is the death or acute illness of an immediate family member, or your own serious illness.

What if you don't have sufficient cash on hand to pay the balance due at filing time? Even if you're able to prove it, that isn't reasonable cause that will relieve you of the penalty.

There's no problem if you need extra time to complete your return or just to avoid the late-filing penalty. It's easy to obtain a six-month automatic filing extension, moving the deadline back to Friday, Oct. 15. By April 15, submit simple-to-complete Form 4868 (Application for Automatic Extension of Time to File U.S. Individual Income Tax Return), available at here, or file it by phone (call toll-free 888-796-1074), using computer tax preparation software, or through a tax professional.

Form 4868 extends only the time to turn in Form 1040, not the time to pay any taxes owed. While the IRS doesn't require payment by April 15 of the tax you estimate as due, failing to do so means you'll owe nondeductible interest, which runs until payment of the tax. It makes no difference that you had a good reason for not paying on time; you'll still owe interest. Moreover, you might be assessed a nondeductible late-payment penalty on the unpaid tax. When you finally file your return, be sure to enter any extension-related payment on Line 68 of Form 1040 as "amount paid with request for extension to file."

The IRS relaxes the rules for those who are unable to fully pay the balance due by April 15. Usually, it's easy to arrange for partial payments in installments by submission of Form 9465 (Installment Agreement Request), found here. It allows you to request a monthly payment plan and specify the amounts you can pay each month and the monthly due date.

STATE TAX RETURNS. Some states accept Form 4868 for extending their due date; some require their own extension forms. Check the rules of the state in which you have to file returns, including the penalties for any underpayments of taxes.

Published: March 17, 2010

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Julian Block is an attorney and author based in Larchmont, N.Y. He has been cited as “a leading tax professional” (New York Times), "an accomplished writer on taxes" (Wall Street Journal) and "an authority on tax planning" (Financial Planning Magazine).More of his articles are at julianblocktaxexpert.com. His books include “Julian Block’s Home Seller’s Guide to Tax Saving.” Law professor James Edward Maule of Villanova University praised it as “an easy-to-read and well organized explanation of the tax rules. Home sellers would be well advised to buy this book.” It is available from PassKey Publications, as well as at outlets such as Amazon and Barnes & Noble.