California Child Support Modification Guide

Under California Family Code § 3651, a child support order can be modified upon a showing of a "material change of circumstances" since the last order. While not explicitly defined by statute, courts generally consider a change material if it results in a change of at least 20% in the guideline support amount, or $100 per month, whichever is lower. This can result from changes in either parent's income, changes in parenting time (timeshare), or changes in mandatory add-on costs such as childcare or uninsured medical expenses. The party seeking modification must demonstrate that the change is substantial, continuing, and involuntary.

SB 343, effective September 1, 2024, fundamentally reformed California's child support guideline formula under Family Code § 4055. The legislation restructured the "K-factor" calculation using new income bands and modified how mandatory add-on expenses (childcare, uninsured healthcare) are allocated between parents. Previously, add-ons were often split 50/50; SB 343 mandates proportional allocation based on each parent's net disposable income. For modification purposes, this means that a recalculation using the new formula may show a material change even if neither parent's income has changed, simply due to the formula restructuring. Any modification filed after September 1, 2024 must use the new guideline.

DissoMaster (and its alternative, X-Spouse) are state-certified computer programs used by California courts and attorneys to calculate guideline child support under Family Code § 4055. These programs perform the complex algebraic calculations required by the guideline formula, including tax computations, income shares allocation, and add-on expense prorations. California courts rely heavily on DissoMaster printouts as evidence of the correct guideline amount. When filing a modification, it is critical to provide a current DissoMaster calculation showing the new guideline amount. Many courts have self-help centers with computers where parties can run DissoMaster calculations, or you can work with an attorney or court facilitator.

Generally, no. Under Family Code § 3653, a modification is effective only from the date the Request for Order (FL-300) is filed with the court, not from any earlier date. This is a critical timing issue. If you experience a job loss or other material change in March but don't file your modification petition until September, you will owe the full original amount for March through August. There is a very limited exception: if the parent seeking modification was prevented from filing earlier due to the other parent's fraud or concealment, the court may grant retroactive relief. The practical takeaway is to file immediately upon a material change.

Under Family Code § 4057(b)(3), California provides courts with discretion to order child support below the guideline amount if a parent has an "extraordinarily high income" and applying the guideline would exceed the children's reasonable needs. This is a fact-based determination. In modification proceedings involving high earners, the court will first calculate the guideline amount using DissoMaster. If the payor demonstrates that this amount exceeds the children's actual needs (considering their station in life), the court may deviate downward. Conversely, if the custodial parent demonstrates increased needs (e.g., private school, extracurricular activities commensurate with the family's pre-separation lifestyle), the court may maintain or increase support.

California Family Code § 4058(b) grants courts the authority to "impute" income to a parent who is voluntarily unemployed or underemployed. If the court finds that a parent has reduced their income in bad faith to avoid support obligations, it will base the child support calculation on the parent's earning capacity rather than actual earnings. Factors considered include education, work history, skills, age, health, and local job market conditions. In a modification proceeding, if the paying parent claims reduced income but the evidence shows voluntary underemployment, the court may deny the modification or impute a higher income. This is a common defense raised by custodial parents opposing modification requests.

Parenting time percentage (timeshare) is a key variable in California's guideline formula under Family Code § 4055. A substantial change in timeshare—typically a shift of 20% or more—constitutes a material change warranting modification. For example, if a parent's timeshare increases from 25% to 45%, this affects the child support calculation in two ways: (1) the "H%" variable in the formula changes, and (2) the higher-time parent incurs greater direct costs, reducing the need for cash transfer. Courts require documentation of the changed timeshare, such as a modified parenting plan or proof of the actual schedule being followed. Important: the change must be ongoing and substantial, not temporary (e.g., a one-time summer with the child doesn't qualify).

California offers a streamlined "stipulation and order" process if both parents agree on the modification. Instead of going to a hearing, the parents can complete a Stipulation to Establish or Modify Child Support (FL-350) and submit it to the court with a proposed order. The judge reviews the stipulation, and if it complies with the guideline (or includes a written justification for deviation), can approve it without a hearing. This process is significantly faster—typically 2-4 weeks instead of 3-6 months. However, both parents must genuinely agree, and the order must meet the child's best interests and guideline standards. Courts closely scrutinize stipulations that deviate significantly from the guideline.

If your child support case is being managed by the California Department of Child Support Services (DCSS), the agency has the authority to initiate modification proceedings on behalf of the custodial parent receiving public assistance or enrolled for enforcement services. DCSS reviews cases periodically and can petition for modification if there's evidence of a material change. If you have a DCSS case, you can also request a review and modification through the local child support agency. DCSS cases use the same legal standards (FC § 3651) but follow administrative procedures. You have the right to request a court hearing if you disagree with DCSS's proposed modification.

Self-employment income presents unique challenges in modification proceedings. California Family Code § 4058(a)(2) defines self-employment income as gross receipts minus expenses "required for the operation of the business." Courts can "add back" personal expenses paid by the business (such as personal vehicle use, meals, or travel) even if they are legitimate tax deductions. To prove a material income change, self-employed parents must provide: (1) business profit and loss statements, (2) tax returns for the past 2 years, (3) bank statements showing business account activity, (4) contracts or invoices demonstrating current revenue, and (5) an explanation of any add-backs. Courts often require a 12-month average to smooth out income volatility typical in self-employment.