Why build on property you don't and can't own? You're basically leasing the land you build on.
I built a place in the village ages ago that was already owned in my wife's name on family land she inherited. (And not some McMansion that did not fit in the neighborhood, better than some, but plenty enough better as well around owned by Thais with some money to splurge and show face.) In case something happened to me she had a home, free and clear of any mortgages. If she left me, up to her, and I could easily leave and rent something for myself if I wanted to stay, far away.
Rent is cheap here. Love is grand, but it does not always last, as seen by a 50% marriage failure rate in much of the world. And if the Thais get crazy and do some sort of peasant revolution, communist stuff, or change the laws at least you can up and leave and not lose everything. Many guys seem to put their life savings into building the home of their dreams here, and it can all go to shite easily, and not in their favor.
You can/should only invest what you can afford to lose and still have money left to live on if it all turns to crap. JMHO