You are mistaken there, Merlin. In this case, the HOAI probably overrides the contractual agreement. If the client can excuse themselves with ignorance of the fact, it might perhaps pass, but here it is well documented that the client is practically running headlong into trouble.
Conclusion: If the architect is otherwise okay (cautious, you can also really mess up with architects, happened to me. Just think about why they offer so cheap, during the construction boom?! Below minimum rate... I myself found only few at the middle rate.) proceed. Worst case is that the minimum rate is billed after all and you have to pay. Whether you now definitely waive the 20% discount or maybe later - then rather later perhaps, but I would put that € aside.
Well, whether I like it or not a judge will probably decide in the end. I won't look for really conclusive material now; however, a quick Google search brought me to the following.
In particular, I have never heard of a case where the judgment was able to enforce his claim.
That a client "runs headlong into trouble" will hardly ever be proven.
@ Aloadhio: Minimum fees for architects are currently being investigated for being against EU law.
