It is trus that a parent's "circumstances" may change because a third party's income is contributed to the parent's basic living expenses, in turn increasing the parent's "net disposable income" available to pay child support., But except in a specified "extraordinary case" that would otherwise result in "extreme and severe hardship" to the child, courts cannot consider the income of either parent's "subsequent spouse" or "nonmarital partner" when "determining or modifying child support." [Ca Fam § 4057.5] Thus, a motion to modify child support based solely on the fact income from a parent's new spouse or nonmarital partner has increased the parent's ability to pay must be denied absent the requisite showing of extreme and severe hardship to the child.