diff --git a/doc/netplan-yaml.md b/doc/netplan-yaml.md
index b22755755..f12b7a5d4 100644
--- a/doc/netplan-yaml.md
+++ b/doc/netplan-yaml.md
@@ -1,5 +1,9 @@
 # YAML configuration
 
+```{note}
+Netplan configuration files must use the `.yaml` extension; `.yml` files will not be read.
+```
+
 ## Top-level configuration structure
 
 The general structure of a Netplan YAML file is shown below.