Building acceptance of new construction despite missing heat pump. Significant defects?

Now the developer is simultaneously demanding payment of installment F) with the letter regarding the acceptance date. Without this, acceptance would not be carried out. Installment F) is due: "after completion of the tiling work in the sanitary area and the plastering work and after readiness for occupancy and concurrently against handover of possession with 13.3%".

If I understand correctly, these 13.3% refer to the last tranche (?)
What he means by that the "acceptance" would not be carried out without this payment must presumably be that he is making the handover of the keys conditional on this. Only you could refuse the acceptance and not he – of course only with good reason, which in my opinion is undoubtedly given here (simply because the work is essentially incomplete). The heat pump is presumably completely missing; it is not just a matter of delivering the little folder with its operating instructions or that it would have a different housing color than ordered.

I have to honestly say that I am still unsure how I should best behave now. I actually also want to avoid hiring a lawyer,

You are supposed to go to a lawyer precisely because of the uncertainty. You don’t have to engage one right away; an initial consultation is a conversation after which you usually pay around three hundred euros with an invoice or quite often only fifty (cash, no receipt, coffee fund) at the law firm’s desk. Take along the contract, the letter regarding the incompleteness at the planned acceptance date, and what you have specifically written based on my suggestion. "Nothing to be afraid of," to quote the hero of the stones once again ;-)

I would do exactly as 11ant suggested:

He would have either cut himself or dealt with lemons with me. I would gratefully acknowledge receipt of his notification about the circumstance and its consequence of the nullity of the acceptance date, and set him a deadline of 14 days after receipt,
1. to requalify his message regarding the execution and duration of the provisional arrangement;
2. to declare his choice whether he now initially wishes to carry out a partial acceptance;
3. to bindingly state which extension he requests; and
4. to submit two proposals for a mutually agreed date for the partial or final acceptance.

A final acceptance can only take place once all services have been rendered: Without (definitive) heating, this is not given. I would only make the payment of the remaining amount once everything is finished.

If I understand correctly, these 13.3% are the last installment (?) What he means by saying that the "acceptance" would not take place without this payment must be that he ties the handover of keys to this condition. You could only refuse the acceptance, not him – of course only with good reason, which in my opinion is undoubtedly given here (simply because the work is significantly incomplete). The heat pump is presumably completely missing; it is not just a matter of delivering the small booklet with its operating instructions, or that it would have a different casing color than ordered.

You are of course absolutely right. The letter states that without payment there will be no handover of keys.

It is not the last installment, but the second to last. However, the last installment G) is only 3.5% of the purchase price and as has already been mentioned here, the second to last installment F) is contractually linked to the readiness for occupancy. This is not the case because the heat pump is missing.

You are supposed to go to a lawyer regarding the uncertainty. You don't have to retain him immediately, an initial consultation is a conversation after which you usually pay anything from three hundred euros with an invoice or often only fifty (cash without receipt, "coffee fund") at the law firm’s desk. You should bring the contract, the letter regarding the incompleteness at the planned acceptance date, and what you have specifically written based on my suggestion. "Nothing to be afraid of", to quote the hero of stones again ;-)

Yes, I will do that. Thank you for the tips and the late reply. I will proceed exactly as you said.

Thanks again :)

If I understand correctly, these 13.3% refer to the last installment (?)

No, according to the Real Estate Agent and Commercial Builder Ordinance, the last rate of 3.5% after full completion is still missing. In this respect, acceptance (acceptance with defect report) could take place. In my opinion, it depends on the temporary arrangement. For an average house costing €500K, the OP would still have a retention of €17.5K. So it depends on what the defects are "worth."

Rate F) contractually linked to readiness for occupancy. This does not exist because the heat pump is missing.

Wrong. Readiness for occupancy means that proper use of the apartment/house must be possible. It must therefore be reasonable for you to move into the house. For this, a heat pump does not have to be present if the developer – as stated – provides a provisional alternative. That is, a replacement heating system. However, you could hold them responsible, for example, for covering higher heating costs.

P.S. As others have already written, first find out what the developer actually intends to do. And that clearly before the acceptance. Come back here with the information and/or to a lawyer.