The default OpenCart functionality has support for maintenance mode, but unfortunately this setting applies to both the default store as well as all sub-stores.

This VQMOD plugin now gives you more fine-grained control over which sub-stores you would like maintenance mode applied to.

So assuming you have a default store with 3 sub-stores, you can have:

1. The default store in maintenance mode and all other stores accessible
   
2. The default store accessible and all sub-stores in maintenance mode.

It's recommended to install VQMod Manager to enable or disable the extension.

The plugin ships with vqmods for both opencart 1.5.x.x and opencart 2.x.x.x.

No files are overwritten and it's required to have vqmod installed.