I understand your tax problems Dr. Ple and GD :

The drive for more government revenue comes from corruption, economic slow-down and inability to collect taxes from many sources. I think the current government can't rely on recovering money from corruption; can't find a solution to increase GDP; and it is much easier to collect taxes from middle-income people than trying to squeeze more juices from mega-businesses. So, the government look for every nook and corner (among the people ways of life), we can predict more complex tax laws describing more types of taxes (and more exemptions for some).

I don't have data on 'captital gain tax' on sales of properties in Thailand. (In Aus, any 'profit' after reasonable cost-of-sale made from sales of houses --bought or built-- are taxed at about 30% rate.)

I think the article on 'personal income tax' by Thai Publica is quite informative.

ภาษีเงินได้บุคคลธรรมดา
http://thaipublica.org/investigations/tax-system-t... 10 เมษายน 2015

We can try pointing to 'sources of tax revenue' and hope the government see that these sources are worth more than unpopular taxws on people who will vote in the next election.