How do I get Drupal to 'discover' a new layout .tpl? I wanted to make a slight variation of a .tpl layout a plugin uses. To do that I simply when into the plugins file structure and duplicated one of its layout folders and renamed the files within (.css, .tpl and .inc). I then made the changes to the .tpl file.

In the .inc file I find:

$plugin = [
  'title' =&gt; t('Two Col 50/50'),
  'category' =&gt; t('Foundation'),
  'icon' =&gt; 'icon.png',
  'theme' =&gt; 'foundation_twocol_50_50',
  'admin css' =&gt; 'foundation-twocol-50-50.css',
  'regions' =&gt; [
    'left' =&gt; t('Left'),
    'right' =&gt; t('Right'),
  ],
];


All I did here was alter these names slightly to:

   $plugin = [
      'title' =&gt; t('Two Col 70/30'),
      'category' =&gt; t('Foundation'),
      'icon' =&gt; 'icon.png',
      'theme' =&gt; 'foundation_twocol_70_30',
      'admin css' =&gt; 'foundation-twocol-70-30.css',
      'regions' =&gt; [
        'left' =&gt; t('Left'),
        'right' =&gt; t('Right'),
      ],
    ];


I then refreshed my cache and theme registry, but the new 70/30 option does not appear when I go to swap layout for the mini-panel I'm working on. 

Is there something I must do to 'tell' drupal this new layout exists?
I assume this is a Panels layout and Drupal won't discover it. You have to manually add it to the theme's .info file like this:

plugins[panels][layouts] = your_new_layout_name