My wife and I are married without a contract. She left the marital home several years ago. We don't want a divorce or legal separation. However, we would like to declare our income separately. Is this possible?

Yes, under certain conditions. In principle, when you are married, you are subject to joint taxation with your spouse for the income each of you receives (art. 6, 1, CGI).

However, it is possible to request separate taxation when one of the spouses has abandoned the marital home and each has separate income (art. 6, 4, a CGI), whether professional or property income (e.g. rental income from your own property).