If I am staying here with a single entry o visa and have gained my retirement extension, can I leave the country and come back without any issues ? or loss of retirement extension ?

You need to get a re-entry permit at your local immigrations office.

1000 baht for single or 3800 baht for a multiple re-entry permit.

Without it your extension will be void.

You can if you obtain a Re Entry Permit from Immigration.

1,000 Baht for a single.

3,800 Baht for a multiple

Thanks very fast and helpfull

In another discussion, ppl also said you can get this at either airport in BKK (after you check in). You'll need a passport photo plus the baht.