Your girlfriend is the owner of the land and after marriage she remains the sole owner of the land. When you plan to build a house on the land after the marriage (and you will pay for it) you should best before the marriage register a right of superficies (see reference below). After the marriage it becomes much more complicated to obtain registered rights to the house.
Any way, best advice, do not invest all your money in a house built on another person's land (girlfriend or wife at present or not), because your position will always be weaker than that of the land owner and you could always lose your investment or be denied access to the house.
When you are married you can inherit your wife's land but ownershio registration requires government approval which you won't get. In the structure of the [url=http://www.samuiforsale.com/law-texts/law-text-land-code-act-1954.html]land code act[/url] approval can only given to foreigners based on a treaty allowing foreign ownership. When you inherit the land you must dispose of the land because you cannot own it.
Answered 10 years ago
When your wife dies her assets will be divided according to her last will and testament or when there is no Will her assets will be distributed amongst her legal heirs. The distribution according to the law (when there is no will) is governed by section 1635 of the civil and commercial code: 'The surviving spouse is entitled to the inheritance of the deceased in the class and according to the division as hereunder provided:.... continue reading by clicking reference link below)
When you have a right of superficies you are pretty well protected when there is no last will, but not without some additional paperwork and legal advice.
When married in Thailand you are her legal heir according to the law and you could be the sole heir according to her last Will but you cannot own your wife's land if she would predecease you.