correct, that's exactly 90 days from June 4th thru September 1st, with June 4 being day number one.. . . . . so your 1-year extension should have started on September 2nd, 2024 and be valid until the same date one year later - by September 2, 2025. . . I have no idea why they made you lose one day. Thai logic?
if you arrived, let's say, on June 10th 2024, your 90-days stay permit got you admitted until September 9th, 2024. Any 1-year extension of stay will start on the day after the expiry of your initial 90-days stay permit. That would be September 10th 2024, and the one-year extension would be good until September 10th, 2025. Check your extension stamp, it should be accurately stamped. NOT September 1st but September 10th. Go back to your Immigration office and ask them to correct the stamp! . . .if you arrived on June 1st, 2024, then everything is correct and your EOS should expire on September 1st, 2025 . . you got a FULL year stamped
the amount of ill-informed people who are not realizing that you, as a Belgian citizen, can use your income as a financial proof both for the 90-days visa AND for the 1-year Extension, , and don't need money in the bank, is astounding
. . .and to add insult to injury, somebody recommended you get those 9 months stay out of visa-exempt entries, which is absolutely irrational, as this person cannot give you any guarantee that you will succeed
(I say you WON'T succeed to make more than two visa exempt entries within a 1-year period except maybe if you don't maximize each stay with a 30 days extension and stay abroad for some time in between the entries)
and of course, another good option that doesn't need any agent or any money in your bank account, neither in England nor in Thailand, you also could apply for the multi-entry 6 months Tourist Visa in England every other year. It allows unlimited entries during the 6-month visa validity. On each entry you get stamped in for 60 days. You can either extend them for 1900 THB with 30 more days, or you do a border run and get stamped another 60-days stay permit upon re-entering. If you do a border run shortly before the visa validity expires, you get stamped in for 60 days for a last time - and this would give you almost 8 months of stay. You might reach exact 180 days by this method, because as the 6-month visa validity starts on the day the visa gets issued, people mostly will lose a few days if not a week or two of the visa validity before they fly to Thailand - and you could catch up those days lost by doing a last border run before the visa validity expires.
Every year, you could come on a 90-days Non-Imm-O Family Visa, if you fulfill the financial requirements of the Royal Thai Embassy London. This visa is very easy to apply for. . . . You will get stamped in for a 90-days stay permit. Before these 90 days expire, apply on Immigration for the 60-days "family visit" extension. (for this your marriage needs to be registered in Thailand because Immigration wants to see a Kor Ror 22 marriage registry freshly printout from the Amphur) . . 90 plus 60 = 5 months in Thailand . . while it is not half/half, I thought you maybe like the simplicity of this visa option which doesn't need any money deposit in a Thai bank, you don't need to apply for the 1-year extension (nobody will enforce you do!) and you avoid all the paperwork that would come with applying for an extension
. . . or you get denied entry because you maximized your first visa-exempt 60 days with 30 days! . . . . . There is no guarantee! For a 60 plus 60, if one doesn't have too many stamps in the passport, there's a better chance of success