There's usually nothing stopping you from filing a complaint for separation even if your spouse is still living with you. Living together might defeat the purpose of filing, however. A separation judgment or decree defines the terms by which you'll live apart. The same applies to marital settlement agreements. If you're living together, custody probably isn't an issue yet -- and you may not have to deal with financial issues yet either. Legal separation in any form typically paves the way for you and your spouse to live apart.
Separation as Grounds
If you and your spouse are separating to establish grounds for divorce, this may change the picture. In some no-fault states, such as North Carolina, spouses must live in two different homes for a full year before they can file for divorce. The state doesn't recognize fault grounds such as adultery or cruelty. In other states, such as Maryland, you have a choice: You can file for divorce on fault grounds, or you can live separately – in two different homes – for a year. Illinois offers similar options, but spouses may be considered separated even if they continue to reside in one dwelling.