4 minutes ago, sometimewoodworker said:

That depends on if the rental income is remitted to Thailand, if it is it is subject to Thai income tax (since 1 January 2024), if it is held permanently overseas it is not subject to Thai tax.

Agreed, thanks, I'll make it more clear in the guide.

9 minutes ago, sometimewoodworker said:

That depends on if the rental income is remitted to Thailand, if it is it is subject to Thai income tax (since 1 January 2024), if it is held permanently overseas it is not subject to Thai tax.

That para (in my master copy) now reads:

 

Rental income from overseas property, owned by foreigners who are tax resident in Thailand, is not liable to Thai tax on that income, as long as that income is not remitted to Thailand.

 

The link below explains that it is a flat tax of 15% for Non Thailand Tax residents & taxed as personal income for Thailand Tax residents. For building they tax 70% of the rent as 30% is allowed for maintenance etc. More than 30% can be claimed if you got supporting documents.
 

So if your wife is a non working Thai under the age of 65 then all you get if you file your tax as a coupe is you can claim her 60,000 THB tax allowance but you can both file as singles. So she has 60,000 tax allowance. Then the first 150,000 is 0% tax. So she pays no tax paid up to 210,000 & as only 70% of the rent is taxable  that means you can get 300,000 (25,000/month) in rent and pay no tax. The next 150,000 (70% of rent = 214,000 full rent) you only pay 5% tax which is 7,500 on the full 150,000. So for rent of 514,000 (42,800/month) you paying 7,500 a year tax. After that it goes up as it goes through the higher tax rate bands.
 

https://phuketrealtor.com/blog/tax-on-rental-income-in-thailand-what-should-you-know