diff --git a/src/lib/invoice-pdf.ts b/src/lib/invoice-pdf.ts
index b4bc7ad..7c3a953 100644
--- a/src/lib/invoice-pdf.ts
+++ b/src/lib/invoice-pdf.ts
@@ -18,12 +18,23 @@ export interface InvoiceLineItem {
   billable?: boolean;
 }
 
+export interface InvoiceSubsection {
+  key: string;
+  label: string;
+  billable: boolean;
+  items: InvoiceLineItem[];
+  subtotal: number;
+}
+
 export interface InvoiceCaseGroup {
   caseNumber: string;
   caseTitle: string;
   practiceArea?: string | null;
   items: InvoiceLineItem[];
   subtotal: number;
+  /** Optional subsections (Billable Fees, Billable Expenses, Non-Billable Fees, Non-Billable Expenses).
+   *  When provided, the renderer groups items by subsection with a header and subtotal. */
+  subsections?: InvoiceSubsection[];
 }
 
 export interface InvoicePdfInput {