Request for advice

Flat rate offer is also possible according to HOAI, everything is a matter of negotiation.

I think this is only a secondary issue and one should not assume any ill intent at first. I would simply ask why the architect awarded the service elsewhere.

He meant I wanted to do it myself, which is nonsense, I never intended that and was not asked either, even if that were the case it does not explain why he outsourced it.

If you wanted to do it in EL according to his opinion, it is even more inexplicable that he forgave it. Then just tell him that the heating installers should take their stuff out again, as it was agreed to do it in EL. Afterwards, you can accept the cheaper offer.

Unfortunately, it is no longer possible, the house has already been occupied, the screed work was done in February

Flat-rate offer is also possible under [HOAI], everything is a matter of negotiation.

The minimum rate of the HOAI is basically the lower limit for calculating a flat-rate fee! Rhenish regards