This is often done through testimony of a close friend or family member who knows that you have been separated - they have visited in your home, called you on the telephone, etc. and never seen any evidence of the other party living or staying there.
Usually if parties get back together, even for one night, they tell a close friend about it. So the absence of such a discussion, if you are close to that friend, will also strengthen the assertion that you are separated.
"Separation" in Illinois is loosely interpreted. Courts have found that you can be "separated" from your spouse even if you are still living in the same house. This is very rarely an issue in a case and both parties typically just stipulate to the period of separation. If this becomes a contested issue, then your attorney will be able to advise you what, if anything, you need to prove the separation other than your own testimony.
In Illinois only two of the grounds for divorce require separation. These two grounds are desertion and the no fault ground of irreconcilable differences. Desertion requires an absence of at least one year and is rarely used in court. One of Illinois’ most common grounds for divorce is irreconcilable differences. Since irreconcilable differences do not allocate blame to either spouse for the marriage’s breakdown, it is the favored choice of grounds.
Generally, in order to obtain a divorce on the grounds of irreconcilable differences a husband and wife must be separated for two years or more. The two year period must be met by the time the divorce is finalized, thus the time can accumulate during the divorce case. An exception to the general rule is where the parties have lived separate for less than the two years, but at least six months, and they waive the two year period in writing. For purposes of irreconcilable differences, separation does not require the parties to live in two different residences. Spouses can live under the same roof and still qualify for irreconcilable differences.
In your case, you can qualify for irreconcilable differences only if you and your spouse waive the two year separation period by signing a written waiver. The written waiver is the only proof you will need to establish a separation of at least six months but less than two years.
If your spouse is not contesting the grounds of your divorce, then your testimony (what you tell the judge) is enough. If your soon-to-be ex is challenging the length of the separation, then you may need other evidence. This would include testimony from witnesses, or physical proof of separate residences, such as leases, utility bills, etc.
Perhaps of bigger concern is your reference to six months. This only applies to cases where no-fault (irreconcilable differences) is the ground.for the divorce and where both you and your spouse specifically waive the requirement that you be separated for at least two years.
The "six month" part of the law can be misleading. Be sure your spouse signs the six month waiver before you go to court. If he or she refuses to sign this, you must wait two years or use one of Illinois' other ten grounds for dovorce.
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