diff --git a/ssl/t1_ext.c b/ssl/t1_ext.c
index 09f1044..eb7d8a7 100644
--- a/ssl/t1_ext.c
+++ b/ssl/t1_ext.c
@@ -54,11 +54,14 @@
 
 /* Custom extension utility functions */
 
+#ifndef OPENSSL_NO_CT
+# include <openssl/ct.h>
+#endif
 #include "ssl_locl.h"
 
 
 /* Find a custom extension from the list. */
-static custom_ext_method *custom_ext_find(custom_ext_methods *exts,
+static custom_ext_method *custom_ext_find(const custom_ext_methods *exts,
                                           unsigned int ext_type)
 {
     size_t i;
@@ -211,8 +214,12 @@
      */
     if (!add_cb && free_cb)
         return 0;
-    /* Don't add if extension supported internally. */
-    if (SSL_extension_supported(ext_type))
+    /*
+     * Don't add if extension supported internally, but make exception
+     * for extension types that previously were not supported, but now are.
+     */
+    if (SSL_extension_supported(ext_type) &&
+        ext_type != TLSEXT_TYPE_signed_certificate_timestamp)
         return 0;
     /* Extension type must fit in 16 bits */
     if (ext_type > 0xffff)
@@ -241,6 +248,12 @@
     return 1;
 }
 
+/* Return true if a client custom extension exists, false otherwise */
+int SSL_CTX_has_client_custom_ext(const SSL_CTX *ctx, unsigned int ext_type)
+{
+    return custom_ext_find(&ctx->cert->cli_ext, ext_type) != NULL;
+}
+
 /* Application level functions to add custom extension callbacks */
 int SSL_CTX_add_client_custom_ext(SSL_CTX *ctx, unsigned int ext_type,
                                   custom_ext_add_cb add_cb,
@@ -249,8 +262,25 @@
                                   custom_ext_parse_cb parse_cb,
                                   void *parse_arg)
 {
-    return custom_ext_meth_add(&ctx->cert->cli_ext, ext_type,
-                               add_cb, free_cb, add_arg, parse_cb, parse_arg);
+    int ret = custom_ext_meth_add(&ctx->cert->cli_ext, ext_type, add_cb,
+                                  free_cb, add_arg, parse_cb, parse_arg);
+
+    if (ret != 1)
+        goto end;
+
+#ifndef OPENSSL_NO_CT
+    /*
+     * We don't want applications registering callbacks for SCT extensions
+     * whilst simultaneously using the built-in SCT validation features, as
+     * these two things may not play well together.
+     */
+    if (ext_type == TLSEXT_TYPE_signed_certificate_timestamp &&
+        SSL_CTX_get_ct_validation_callback(ctx) != NULL) {
+        ret = 0;
+    }
+#endif
+end:
+    return ret;
 }
 
 int SSL_CTX_add_server_custom_ext(SSL_CTX *ctx, unsigned int ext_type,
@@ -280,6 +310,7 @@
     case TLSEXT_TYPE_signature_algorithms:
     case TLSEXT_TYPE_srp:
     case TLSEXT_TYPE_status_request:
+    case TLSEXT_TYPE_signed_certificate_timestamp:
     case TLSEXT_TYPE_use_srtp:
 #ifdef TLSEXT_TYPE_encrypt_then_mac
     case TLSEXT_TYPE_encrypt_then_mac: