diff --git a/services/domain/Big.Domain/Registration.cs b/services/domain/Big.Domain/Registration.cs
index 51f20cf..a3b3503 100644
--- a/services/domain/Big.Domain/Registration.cs
+++ b/services/domain/Big.Domain/Registration.cs
@@ -133,8 +133,24 @@ public sealed class Registration
         Status = RegistrationStatus.Ingetrokken;
     }
 
-    // A decision (or withdrawal) is only valid while the registration is still open (INGEDIEND or
-    // IN_BEHANDELING).
+    /// <summary>
+    /// Expire the registration — the 30-day document-wait timer fired before the required documents
+    /// were supplied, so the registratie process cancels the case (S-10a). Allowed while it is still
+    /// open (INGEDIEND or IN_BEHANDELING) and needs no zaak; a decided (INGESCHREVEN/AFGEWEZEN) or
+    /// withdrawn (INGETROKKEN) registration can no longer expire. Re-expiring one already
+    /// <see cref="RegistrationStatus.Verlopen"/> is a no-op — the worker job may be redelivered (§8.6).
+    /// </summary>
+    public void Expire()
+    {
+        if (Status == RegistrationStatus.Verlopen)
+            return;
+
+        RequireOpenForDecision(nameof(Expire));
+        Status = RegistrationStatus.Verlopen;
+    }
+
+    // A decision (or withdrawal, or expiry) is only valid while the registration is still open
+    // (INGEDIEND or IN_BEHANDELING).
     private void RequireOpenForDecision(string decision)
     {
         if (Status is not (RegistrationStatus.Ingediend or RegistrationStatus.InBehandeling))
diff --git a/services/domain/Big.Domain/RegistrationStatus.cs b/services/domain/Big.Domain/RegistrationStatus.cs
index fccfac7..5e8a28c 100644
--- a/services/domain/Big.Domain/RegistrationStatus.cs
+++ b/services/domain/Big.Domain/RegistrationStatus.cs
@@ -21,4 +21,8 @@ public enum RegistrationStatus
 
     /// <summary>Withdrawn by the zorgprofessional before a decision (S-11). Terminal.</summary>
     Ingetrokken,
+
+    /// <summary>Lapsed: the required documents were not supplied within the 30-day window, so the
+    /// registratie process cancelled the case (S-10a). Terminal.</summary>
+    Verlopen,
 }