If you are legally married and your marriage breaks down you must get a divorce in order to remarry. It also is psychologically helpful for many people, when a marriage breaks down, to get a divorce as that helps to give closure to the failed relationship.
That said, as a legal matter there is no formal need to divorce and many couples separate and never divorce. In such circumstances the couple remains legally married and there is no time limit whereby a married couple that separates ceases to be married. I often hear people who tell me they split up, say, ten years ago and assume they are automatically divorced. There is no "limitation period" on a marriage.
The grounds for divorce are fairly limited. Either the relationship has come to an end and the parties have separated for a lengthy period or there is adultery or cruelty justifying the end of the marriage. In practice, marital breakdown and lengthy separation is the basis for divorce. Adultery and cruelty are legally available but almost never used – even where they might apply. Seeking a divorce for adultery or cruelty is fair more complex that seeking a divorce for marital breakdown and separation and so is very rarely seen.
If a marriage has broken down and the couple is living separate and apart, a divorce can be granted by the Court after one full year of separation. You do not have to wait until you have been separated for a full year in order to apply for a divorce. You can seek a divorce immediately upon separation but the court will not grant a divorce until there has been one year separation. The one year period is in place to make sure that divorce does not happen as a result of a marital spat – divorce is a serious matter and should only happen when there is a real and permanent marital breakdown.
Living separate does not require that the couple live at separate addresses. The reality of cost and limited housing means the Court recognizes you can be living apart but in the same house. To be separated the couple must be living separate lives. Separate homes are the most clear and uncomplicated way to complete a one year separation period. However, if you live in the same home so long as you can show you were living separate lives for the one year peiod the divorce will be granted.
If you try to reconcile during the one year period but the reconciliation does not last more than 90 days the one year period does not have to start again. This allows couples a chance to try and work on repairing their marriage without delaying a divorce if reconciliation does not work. Repairing a marriage, if possible, is better than a needless divorce.
As mentioned, divorce for adultery or cruelty is available but is costly, lengthy and difficult. I always recommend against it and have not seen a divorce based on adultery or cruelty for over twenty years.
Custody, access and support of children is not affected by the basis upon which you seek a divorce.