Showing not authorised message by default to non-admin users (for maintenance/offline mode)

[eluser]RichLove[/eluser]
Hi

This is my first post here.. I have done a bit of Googling about this and have come across a few ideas but I want to make some things live on a site today and was hoping for some quick help if possible.

I would like to put my site in to offline/maintenance mode to put some changes live (code, database and content (stored in database). I would like for users with the admin role (using dx auth) to be able to get access the site and for any other users who log in to be shown a custom error message (e.g. site undergoing maintenance)

Could somebody please suggest a sensible way of doing this please (and if this sounds sensible)?

My thoughts are to try and interrupt the application flow quite early on to check if the page is not the login page (still want to allow access to this so admin can login) and check if the user is admin - if not, redirect to an error html page or view.

Richard