Basis for creditable total construction costs

That is also a question that only the architect can answer.

a stupid question, why did the architect only state costs of €50,000 in the building application itself, but sets €240,000 in his HOAI.
Typo?

Nonsense - wanted to do you a favor - low construction costs, low approval fees.
Bad for him, that’s how you get him, because €50k was exactly what was discussed from the start.

Nonsense - just wanted to do you a favor - low construction costs, low approval costs....

That's right. It's only a small renovation.
There is an online calculator on the HOAI site. You can try calculating with it and, in addition to the other points I mentioned, confront him with the fact that for estimated construction costs of €50k, a fee of X is due. You would also be willing to pay that.

He invoiced all 4 phases in full.

In my opinion, all of them still have to be provided in full, I merely quoted the reality.

Fee zone 3 middle rate

I don’t find such a flat rate across all phases practical – but that is also the nature of a flat rate approach, to treat all cases as a standard case and to brush them all with the same broad brush.

As a kindness, I was granted a further 15% discount at the end.

That is customary on the market, to discount the “price recommendations” of the HOAI, which are very generous in many single-family house mandates (actually only applicable to new construction projects), by this factor.

Ps. Yes, I could kick myself right now, I also always believe everyone should be paid appropriately. Unfortunately, I have the feeling that ignorance was mercilessly exploited.

You should have informed yourself about how the default valid HOAI approaches the topic of fee determination, namely basically like a fee schedule, which was also its predecessor. One could almost speak of a “(Imperial) Reich-HOAI” due to how pre-EU internal market liberal the thing was “designed.”

But nobody “exploited” you there; you alone are responsible for your loose attitude towards large-scale transactions. The HOAI is at least publicly accessible, unlike the DIN standards behind a hefty paywall.

Why does one only commission the architect verbally? Complaining afterward helps little. Asking here beforehand is more advisable.

No one is stopping you from concluding a lump-sum contract with the architect.

Why does one only commission the architect orally? Complaining afterwards helps little. Asking here beforehand is more advisable. No one prevents you from concluding a lump-sum contract with the architect.

I suspect that the OP has assumed this in a similar situation. So, dear readers: the HOAI is by default always based on the architect contract; it must be expressly displaced/replaced if necessary. The same applies to the RAG for lawyers. Both fee schedules are related to the fact that contributions for professional liability insurance are included in the tariffs.