Does a 20-day visa overstay affect my eligibility for the DTV visa in Thailand?

The question o long overstay is down to the immigration I.e 20 days isn’t really long as they start the penalty from the 90 day mark BUT it depends on so many factors like the reason, the amount of overstays etc , the embassy is the only place that can give a defined answer for you not anyone here

I'd email the visa section email address first, explain that you had a twenty day overstay and the reason you were waiting for a new passport to arrive, therefore I presume you couldn't of left anyway. I also presume that you declared your overstay when leaving at the airport, paid the fine and left. In my opinion it would only have a bearing on your application for a DTV if you'd been arrested by police or immigration police. I reckon once you explain in an email, you'll be ok. Better to get confirmation before you apply and pay over £200, which if refused they don't refund for failed applications.

No one can say, apply and find out.

You do know that is not an official website or test.

This website is not official. It's created by individual in Thailand

It says overstays, plural, you have one overstay and not a history (!) of overstays(!), so you should be fine.

Long + overstay(s) not just one overstay … you are just fine

I just got approved in London, I have an 11 day overstay from a year ago and they said nothing about it.

It is up to the embassy's discretion.

I almost didn't allow this post, because the ONLY people that can answer this is the embassy you are applying at. Regardless of if they are replying to you or not, they are the ones that make the determination of how much is too much.