A friend of mine has a valid Work Permit that allows him to work at the location of his business.
He would like to open a second business. This will be a different ztd. Company.

Construction for the Premises are soon to begin, he asked if the new address can be added as another workplace? He told me he heard that there have been some changes here about 1 or two Years ago. Asking at Workpermit office he has been told he can't register new place as it is obviously not ready, that would mean he cannot supervise the construction?

As far I know you can use the existing work permit as he also owns the second company. Similar to something that you would be allowed to help in a wife's business if your work permit was being a teacher or something.

Under changes a few years back, he can work a second job under his existing work permit as long as the second job is not a Thai-only job. No need to list it. 

Your work permit must list the type of work you do, not necessarily the location. A work permit as teacher for example does not allow you to help in your wife's business, unless your wife's business is teaching.

 

If your friend's work permit includes construction supervision he should be good to go.