diff --git a/upb/mini_table/decode.c b/upb/mini_table/decode.c
index b8adaf60c1..baaf1fcccb 100644
--- a/upb/mini_table/decode.c
+++ b/upb/mini_table/decode.c
@@ -992,6 +992,21 @@ const char* _upb_MiniTableExtension_Init(const char* data, size_t len,
                                                extendee, sub);
 }
 
+upb_MiniTableExtension* _upb_MiniTableExtension_Build(
+    const char* data, size_t len, const upb_MiniTable* extendee,
+    upb_MiniTableSub sub, upb_MiniTablePlatform platform, upb_Arena* arena,
+    upb_Status* status) {
+  upb_MiniTableExtension* ext =
+      upb_Arena_Malloc(arena, sizeof(upb_MiniTableExtension));
+  if (UPB_UNLIKELY(!ext)) return NULL;
+
+  const char* ptr = _upb_MiniTableExtension_Init(data, len, ext, extendee, sub,
+                                                 platform, status);
+  if (UPB_UNLIKELY(!ptr)) return NULL;
+
+  return ext;
+}
+
 upb_MiniTable* _upb_MiniTable_Build(const char* data, size_t len,
                                     upb_MiniTablePlatform platform,
                                     upb_Arena* arena, upb_Status* status) {
diff --git a/upb/mini_table/decode.h b/upb/mini_table/decode.h
index efcbe20414..385bda4661 100644
--- a/upb/mini_table/decode.h
+++ b/upb/mini_table/decode.h
@@ -80,7 +80,7 @@ UPB_API bool upb_MiniTable_SetSubEnum(upb_MiniTable* table,
                                       const upb_MiniTableEnum* sub);
 
 // Initializes a MiniTableExtension buffer that has already been allocated.
-// This is needed (for example) by upb_FileDef, which allocates all of the
+// This is needed by upb_FileDef and upb_MessageDef, which allocate all of the
 // extensions together in a single contiguous array.
 const char* _upb_MiniTableExtension_Init(const char* data, size_t len,
                                          upb_MiniTableExtension* ext,
@@ -96,6 +96,35 @@ UPB_API_INLINE const char* upb_MiniTableExtension_Init(
                                       kUpb_MiniTablePlatform_Native, status);
 }
 
+UPB_API upb_MiniTableExtension* _upb_MiniTableExtension_Build(
+    const char* data, size_t len, const upb_MiniTable* extendee,
+    upb_MiniTableSub sub, upb_MiniTablePlatform platform, upb_Arena* arena,
+    upb_Status* status);
+
+UPB_API_INLINE upb_MiniTableExtension* upb_MiniTableExtension_Build(
+    const char* data, size_t len, const upb_MiniTable* extendee,
+    upb_Arena* arena, upb_Status* status) {
+  upb_MiniTableSub sub = {.submsg = NULL};
+  return _upb_MiniTableExtension_Build(
+      data, len, extendee, sub, kUpb_MiniTablePlatform_Native, arena, status);
+}
+
+UPB_API_INLINE upb_MiniTableExtension* upb_MiniTableExtension_BuildMessage(
+    const char* data, size_t len, const upb_MiniTable* extendee,
+    upb_MiniTable* submsg, upb_Arena* arena, upb_Status* status) {
+  upb_MiniTableSub sub = {.submsg = submsg};
+  return _upb_MiniTableExtension_Build(
+      data, len, extendee, sub, kUpb_MiniTablePlatform_Native, arena, status);
+}
+
+UPB_API_INLINE upb_MiniTableExtension* upb_MiniTableExtension_BuildEnum(
+    const char* data, size_t len, const upb_MiniTable* extendee,
+    upb_MiniTableEnum* subenum, upb_Arena* arena, upb_Status* status) {
+  upb_MiniTableSub sub = {.subenum = subenum};
+  return _upb_MiniTableExtension_Build(
+      data, len, extendee, sub, kUpb_MiniTablePlatform_Native, arena, status);
+}
+
 UPB_API upb_MiniTableEnum* upb_MiniTableEnum_Build(const char* data, size_t len,
                                                    upb_Arena* arena,
                                                    upb_Status* status);