Child support is calculated using a complicated formula which is designed to equitably consider respective incomes and needs of the parties. In general terms, the most important factors are net disposable income and the custodial timeshare. The formula itself can be found in Family Code §4055.
To answer your question the income of a subsequent spouse (and certainly of a girlfriend) "shall not be considered" in ordering or modifying child support, "except in an extraordinary case where excluding that income would lead to extreme and severe hardship" to the child. Fam. Code § 4057.5, subd. (a)(1). Keep in mind that the focus is whether the calculation would pose an extreme hardship to the child, not to you. However, a statutory example of an 'extraordinary case' is where the spouse voluntarily quits work or reduces income, or intentionally remains unemployed or underemployed. §4057.5, subd. (b). Thus, if your former spouse can demonstrate to the court that you are intentionally remaining unemployed or unemployed in reliance on your girlfriend's income, the court could consider that new income in determining child support.
You should consult with a family law attorney for more specific advice on how to protect your rights.
I am not your attorney. This comment does not create an attorney-client relationship. I have not considered the circumstances of your case. Rather, this general information is being provided as a courtesy to you for informational and educational purposes only. It is no substitute for the advice of an attorney.
If you are "underemployed" because there is employment available in your field and you are not working, then income may be imputed to you. The rent and bills being paid by your live in girlfriend result in net income to you (as those are bills that you no longer have to pay with after-tax income). Most judges are not sympathetic to any individual who is not making a reasonable effort to contribute to the support of his / her minor children. The facts you have provided are not favorable unless there is some circumstance justifying your unemployment at this time.
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