PEB for an extension

Yes, it's totally crazy, but it's the law. It was my understanding that it only applies for new buildings, or major renovations, so there is scope for exemptions. You should question your architect about whether it is really needed. The urbanisme at the commune may also be able to tell you if the PEB assessment is required for what you are doing. When I did a major extension on my house, I needed one. The guy who turned up was a complete loser. Did a report in half an hour using a template in Word, and charged around what you're being asked for. Even my architect agreed that it was a total waste of time, effort and money.
There are regulations covering this, so it's effectively a tax on building works, and employment scheme for architects and civil engineers. Just another crazy Belgian thing, but we live with it.