Ownership of files not set to apache user after installing extension

[AronM]

I'm quite new to Magento and have no idea if this is regular behaviour, but it doesn't seem to me like it is.

Whenever I try to install an extension (through the extension manager), it successfully runs through all the tests, creates a backup and starts installing the extension. Whenever it completes, however, I get thrown out and all there is to see is an HTTP 500 error. 

This is then fixed by me running a chown command in terminal, setting the ownership of my Magento folder (and all underlying sub-folders/files) to the usergroup of www-data (the user running apache2).

After running this command everything works like it's supposed to and I get welcomed back into the system by a page stating the installation of the extension has succeeded. 

Is there any way to make it so that the ownership of the added files/folders is automatically set to the usergroup of the apache2 user upon installation?

I'm running the latest version of Magento (2.2.2) on a Ubuntu 16.04 dedicated server.

[Tom Robertshaw]

Have you checked the instructions provided in this official guide? http://devdocs.magento.com/guides/v2.0/install-gde/prereq/file-system-perms.html

[AronM]

I have and, to my knowledge, all these permissions are set correctly.

[Tom Robertshaw]

What user and group to the files that get installed by Magento get added as?

[AronM]

Oddly enough, the extension files themselves don't seem to be the problem.

Their ownership gets assigned to my Magento admin user and the www-data group. Yet, I still get the HTTP 500 error, which is fixed by chowning the entire Magento installation folder.

[Tom Robertshaw]

Narrowing down precisely which files have the problematic permissions would be useful. It may be that it's still the extension files. Is there a group www-data that the www-data apache user is also in? 

Can you also go into the installed extension directory and provide a list of the permissions that are assigned, please?