Sometimes there are variations of separations. Perhaps you separated under one roof for three months and then one of you moved out for another 9 months. If you lived under one roof for any part of the 12 months required prior to filing for divorce, you need to file an affidavit along with your divorce application. An affidavit is a statement made by you or another person, serving as your testimony about particular issues.
In your affidavit, you need to show that you and your spouse separated even though you were under one roof (see above) and you need to explain why you remained living in the same house. You also need to explain what the living arrangement were for any of your children who were under 18 during the separation and what government agencies you told about your separation.