Filing your 2011 Illinois state tax return after the initial filing deadline of April 18, 2011, will cause you to be charged a late filing penalty and compound interest on any outstanding tax balance you have. However, if you request an approved extension before the filing deadline, you will receive an extra six months to submit your return. This means you will have until October 18, 2011, to file your state tax form.http://www.revenue.state.il.us/individuals/FilingRequirements/extension.htm   

A state tax extension is an extension to file your tax return, it is not an extension of the time to pay any balance owed. Even if you receive an extension, any balance unpaid after April 18, 2011, will still accrue compound interest until it is paid in full.http://www.revenue.state.il.us/individuals/FilingRequirements/extension.htm  

Before you get started on your Illinois extension, take a few moments to gather all of your essential tax paperwork. This will be the same information you will need to prepare your Illinois tax return, so keep it all together for easy reference later. You will need the following: 

1. The total amount of earned income you received this past year––If you are an employee, use the total taxable wages as reported on your Form W-2 or Form 1099. If you are self-employed, calculate your gross earnings and then subtract any applicable business expenses you are eligible to claim.
2. Your total unearned income that is subject to state tax––Examples of this include: unemployment compensation, pension payments, lottery winnings, and bank interest.
3. The amount of state income taxes you have paid––For employees, this will appear on Form W-2 as state taxes withheld. Self-employed individuals will use the total of any estimated state tax payments made during the year.
4. A copy of your completed federal income tax return, if you will be filing one at this time
   

You will use the amount of your federal adjusted gross income as a starting point to calculate your state tax liability, so be sure to complete your federal return before you begin this process. If you will also be filing a federal tax extension, you may wish to consult a tax professional for help figuring the amount of your adjusted gross income.http://www.revenue.state.il.us/taxforms/IncmCurrentYear/Individual/IL-1040.pdf 

To complete the payment voucher, you will also need the following:

1. Your Social Security number
2. Your current mailing address
3. Your spouse's full name and Social Security number, if applicablehttp://www.revenue.state.il.us/taxforms/IncmCurrentYear/Individual/IL-1040.pdf
   

The next step to getting your Illinois extension is to assess the amount of any additional state taxes you will owe, if any. To do this, you will need to get a paper copy of Form IL-1040 "Illinois Individual Income Tax Return" and the tax form instruction booklet. If you have Internet access, you can download these forms from the Illinois Department of Revenue website. You can also obtain these at your local library during tax season.http://www.revenue.state.il.us/taxforms/IncmCurrentYear/Individual/IL-1040.pdf 

Begin by reading the entire tax instruction booklet carefully. Pay particular attention to income adjustments you may need to make or state tax credits you are eligible to claim. Enter the amount of your federal adjusted gross income on line 1 of your Illinois return. Then make any necessary adjustments to income on lines 2 through 8. This will figure your Illinois base income on line 9.http://www.revenue.state.il.us/taxforms/IncmCurrentYear/Individual/IL-1040.pdf

Subtract your total exemption allowances to arrive at your total taxable income on line 11. Multiply the total by the state tax rate of 3 percent to figure the amount of your state tax liability. Enter this amount on line 13. Deduct your eligible state tax credits and the amount of state income taxes you have already paid through withholding or estimated tax payments. What remains will be either the amount you overpaid (refund) or the amount you underpaid (balance due).http://www.revenue.state.il.us/taxforms/IncmCurrentYear/Individual/IL-1040.pdf 

If you are entitled to receive a refund or if you have a zero balance, you can stop here and simply file your return when you can. But if you have a balance due, you will need to remit payment of your tax before the April 18, 2011, filing deadline to avoid being charged late filing penalties and compound interest. If possible, try to make your payment electronically through the Illinois Department of Revenue website.http://www.revenue.state.il.us/taxforms/IncmCurrentYear/Individual/IL-505-I.pdf 

To make your payment by mail, you will need to complete a written payment voucher entitled Form IL-505-I "Automatic Extension Payment for Individuals Filing Form IL-1040." This voucher is also available for download from the state revenue website.http://www.revenue.state.il.us/taxforms/IncmCurrentYear/Individual/IL-505-I.pdf 

Enter your full name and Social Security number as well as your spouse's, if applicable. Write in your complete mailing address and the amount of the payment you are making with the voucher. Make out a check or money order for your total tax balance and send it with the voucher to the address listed on the form. Be sure to submit both by April 18, 2011.http://www.revenue.state.il.us/taxforms/IncmCurrentYear/Individual/IL-505-I.pdf

If you cannot pay your entire balance at once, send in as much as you can afford. Then contact the Department of Revenue about establishing a payment plan. As soon as you remit your payment, begin preparing your state return as soon as you can.