diff --git a/common/dynamic_partition_control_interface.h b/common/dynamic_partition_control_interface.h
index c171fb5..19bb523 100644
--- a/common/dynamic_partition_control_interface.h
+++ b/common/dynamic_partition_control_interface.h
@@ -64,10 +64,15 @@
   // This is needed before calling MapPartitionOnDeviceMapper(), otherwise the
   // device would be mapped in an inconsistent way.
   // If |update| is set, create snapshots and writes super partition metadata.
+  // If |required_size| is not null and call fails due to insufficient space,
+  // |required_size| will be set to total free space required on userdata
+  // partition to apply the update. Otherwise (call succeeds, or fails
+  // due to other errors), |required_size| is set to zero.
   virtual bool PreparePartitionsForUpdate(uint32_t source_slot,
                                           uint32_t target_slot,
                                           const DeltaArchiveManifest& manifest,
-                                          bool update) = 0;
+                                          bool update,
+                                          uint64_t* required_size) = 0;
 
   // After writing to new partitions, before rebooting into the new slot, call
   // this function to indicate writes to new partitions are done.
diff --git a/common/dynamic_partition_control_stub.cc b/common/dynamic_partition_control_stub.cc
index bc792c8..974cd1b 100644
--- a/common/dynamic_partition_control_stub.cc
+++ b/common/dynamic_partition_control_stub.cc
@@ -43,7 +43,8 @@
     uint32_t source_slot,
     uint32_t target_slot,
     const DeltaArchiveManifest& manifest,
-    bool update) {
+    bool update,
+    uint64_t* required_size) {
   return true;
 }
 
diff --git a/common/dynamic_partition_control_stub.h b/common/dynamic_partition_control_stub.h
index 1704f05..09990a7 100644
--- a/common/dynamic_partition_control_stub.h
+++ b/common/dynamic_partition_control_stub.h
@@ -35,7 +35,8 @@
   bool PreparePartitionsForUpdate(uint32_t source_slot,
                                   uint32_t target_slot,
                                   const DeltaArchiveManifest& manifest,
-                                  bool update) override;
+                                  bool update,
+                                  uint64_t* required_size) override;
 
   bool FinishUpdate() override;
 };