Silverstripe extensions not registering

Question

I've got a simple extension which I'm planning to use in order to add extra fields.

<?php
use SilverStripe\Forms\FieldList;
use SilverStripe\Forms\TextField;
use SilverStripe\ORM\DataExtension;


class ColumnExtension extends DataExtension
{
    private static $db = [
        'SectionID' => 'Varchar(255)'
    ];

    public function updateCMSFields(FieldList $fields)
    {
        dd('sdsd');
        $fields->addFieldToTab('Root.Content', TextField::create('SectionID', 'Section ID'), 'SubHeading');
    }
}

And I've added the below to a yml file in the app/_config folder

PackageNameSpace\Blocks\Column:
  extensions:
    - ColumnExtension

FYI: I haven't added any namespacing. I've got a few other extensions working without the namespacing. My issue is that my extension doesn't seem to be registering. The code doesn't seem to reach the dd() command.