Yes.

 

You won't be building your own house, unless you have it set up as a separate legal entity to the land.

 

Which almost no foreigner does.

 

So you will be building a house for a Thai.

 

Being a builder is a forbidden occupation.

 

So yes, you would need a WP which you would not be able to get.

 

Sorry. 

1 minute ago, Khun Robert said:

Why you want to talk to the local Immigration Office unless you need advice about your Visa or extension of stay.

Try the ministry of Labor, that is the only office who know all the rules about Work Permits.

 

"Still no hard evidence for cases where farangs have been processed for building or taking care of their own property. Despite the "obvious" wording of the law."

 

That doesn't mean it's never happened, if caught plenty of folks would possibly not publicize what happened, and may have concerns that they are now on an internet watch list (if such a thing exists). 

By the time we've finished debating all this, the OP could have had his foundations done....

 

Crack on pal...