Child support is the exclusive responsibility of the two parents of a child.  If one parent moves in with someone else, the existence of a roommate (or boyfriend/girlfriend) will not affect the obligation-- because they are not obligated to support the child.  As such, the custodial parent could not seek an increase in child support because of the income of the new "cohabitant."If the two parents separated, and then moved back in together, then this could affect the support calculation.  However, until one party files a motion to modify, the support obligation set out in the custody orders will continue.  Many parents get frustrated when they reconcile and the court continues garnishing wages.  After one of the parents files a motion to modify the child support obligation, the court can take into consideration that both parents are living together and dividing the responsibilities for providing for the child.  Child support, hypothetically, is to help with the food, housing, and clothing expenses of the child.  If both parents are paying for housing and utilities-- the court can reduce or terminate the support obligation.