You and your spouse are considered to be separated as soon as one of you decides that the relationship is over, and that there is no ‘reasonable chance of reconciliation’.
You can still physically live in the same house as long as you are living ‘separate and apart’, meaning that you are both living independent lives while sharing common accommodation, and there is no chance that you will ‘get back together’. In Ontario, there is no legal formality to go through to be ‘separated’ so long as the above threshold is met.
Once you have separated, you and your spouse can negotiate and enter into a separation agreement to settle any issues arising from your separation. You can learn more about preparing separation agreements and resolving family law disputes.