Can anyone let me know if there are any limitations to the amount of land a Thai can own if she is married to a foreigner. I've seen it mentioned a few times before but cannot find anything definitive.

There is no restriction.

Just to add a little bit extra. If the wife is a civil servant there are restrictions on owning some types of land, farmland for example.

There is no restriction.

Lopburi: If the Thai wife has changed her surname to that of the foreign husband, can she own land?

There is no restriction.

Lopburi: If the Thai wife has changed her surname to that of the foreign husband, can she own land?

Yes, there was a restriction but the rules were changed some years back, there is now no problem. You (as the foreign husband) will be expected to sign a paper stating you have no financial interest in any land she purchases.

Land departments in areas where there are few farangs may not know of the rule changes and will need to be enlightened.

...Land departments in areas where there are few farangs may not know of the rule changes and will need to be enlightened.

And will not have an English translation of the document. Sign it anyway...

TH