You’re probably reading this because you already have a Let’s Encrypt certificate issued using certbot for a domain, and now want to add-on subdomains.

If you want to create a new certificate, have a look at the end of this article.

In my case I already had a certificate which covered

• golightlyplus.com
• www.golightlyplus.com

I have a directory on my server called dev that I want to reference via dev.golightlyplus.com.

So to do that, I issued the following command

certbot -d golightlyplus.com,dev.golightlyplus.com,www.golightlyplus.com --expand

Key takeaways:

1. re-include every domain name you want on that certificate again. I forgot www. the first time. No biggie, just re-run with all the domains you want.
2. don’t forget to add --expand to the end

After reading around on the net a bit, I found this solution on this GitHub thread.