How to override and extend basic Django admin templates?

Question

How do I override an admin template (e.g. admin/index.html) while at the same time extending it (see https://docs.djangoproject.com/en/dev/ref/contrib/admin/#overriding-vs-r

Answer 1

Chengs's answer is correct, howewer according to the admin docs not every admin template can be overwritten this way: https://docs.djangoproject.com/en/1.9/ref/contrib/admin/#overriding-admin-templates

Templates which may be overridden per app or model

Not every template in contrib/admin/templates/admin may be overridden per app or per model. The following can:

app_index.html
change_form.html
change_list.html
delete_confirmation.html
object_history.html

For those templates that cannot be overridden in this way, you may still override them for your entire project. Just place the new version in your templates/admin directory. This is particularly useful to create custom 404 and 500 pages

I had to overwrite the login.html of the admin and therefore had to put the overwritten template in this folder structure:

your_project
 |-- your_project/
 |-- myapp/
 |-- templates/
      |-- admin/
          |-- login.html  <- do not misspell this

(without the myapp subfolder in the admin) I do not have enough repution for commenting on Cheng's post this is why I had to write this as new answer.