My current stay on a retirement is due again on 19 March. I was planning a trip out of town. How many days late can I go to immigration to do the extension of stay for one year.

If you go late you will pay an overstay fine of 500 baht a day. Wait to long and you will have to start all over again.

You can do it up to 45 day days early at some offices.

As said, go early. By applying early you do not lose any days, the extension will start on March 19 and not from the date you apply.

I will renew my extension in mid-June, 2015. My passport expires mid March 2016. Do I have to get a new passport before I renew my extension?

I will renew my extension in mid-June, 2015. My passport expires mid March 2016. Do I have to get a new passport before I renew my extension?

Yes - if you dont want to lose 3 months of your extension. They will only give you an extension to the expiry date of your passport.

You don't have to but you will only get 9 months extension if you do not.

I will renew my extension in mid-June, 2015. My passport expires mid March 2016. Do I have to get a new passport before I renew my extension?

You do not have to get new passport before you apply for your extension but it will only be done to the date in March when you passport expires. Then you would have to apply for another extension when the shortened extension ends.

It would be best to get a new passport before applying for the extension.