I will be going to Samui to apply for a 30 day visa extension. Will this 30 day extension start from the date my original visa expires or will it start 30 days from the day that I apply for the extension?

Any help on this would be very much appreciated.

30 days from the date your current persmission to stay ends. You only get 30 days if you have a tourist visa.

I believe I read somewhere yesterday on this forum that it's okay to apply for a 30 day extension a few days before your visa expires, as that additional 30 days doesn't start until the last day of the current visa ends.

You can apply early and do not lose any days. The extension will start from the end of your current permission to stay.