5% Witholding Tax For Office Rent... Can Pay It 6-monthly?

[thaitanic]

So far our Thai Limited Company has been paying rent for our office monthly and filing form #3 every month reflecting the 5% tax that we're witholding.

If we start to pay our office rent every 6 months instead of every month, does this mean that we only have to file form #3 twice a year and pay the witholding tax twice a year? Or should we calculate it pro-rata per month and keep filing the form/paying each month?

It would be simpler to pay it just twice a year but the form #3 has tick boxes for each month, implying that it might have to be filed each month.

Thanks