Change requests as a tenant in a new building

Hello,
in November last year I rented a partial area in a large new building for 10 years on a fixed lease. It will be ready for occupancy only next December. I had already specified interior finishing wishes before the shell construction began and was verbally assured that changes would be "unproblematic".
Of course, I would bear the additional costs compared to the basic equipment still being planned at that time.
Now it is shaping up as follows:
The trades are gradually being awarded by the builder. Naturally, "standard brand". If I want to change something, e.g., a higher quality push-button panel in the WC (visually and qualitatively better), I am issued an invoice. There I see the deduction for the planned (not yet ordered) push-button set by the builder (same trade company) and the cost for the better push-button set that I want. I am charged the additional costs AND 15% AGK (general business costs).
I have discussed the changes in the entire sanitary trade directly with the executing contractor myself (my time, my planning).
The builder and later landlord only receives the new offer for invoicing. He has nothing else to do.
The same applies to all other trades as well. If the basic equipment for electrical work amounts to 5,000 euros, and I want installations (as a small example only the dimmers and dimmable lights in the ultrasound room) for 10,000 euros, the builder/landlord simply approves this and collects 15% for it. So I pay 5,000 euros in additional costs and 750 euros AGK.
Remarkable is the air conditioning. This was not planned but was expressly requested from the outset. "No problem" now costs me 34,000 euros AND 15% (= 5,100 euros). After 10 years, I am not allowed to remove and take it with me.
Equally serious is the "acoustic ceiling". I want to have an acoustic ceiling installed in the rooms (since it is a medical practice) instead of the originally planned simple grid ceiling, so that I can better protect privacy. Additional costs 20,000 euros, and again 15% on top.
These AGK, which were not mentioned anywhere before, are they actually permissible?
Does anyone have experience with this?
Thanks very much in advance!

The rental and preparation of rooms is your landlord's business. Why shouldn't he also profit from special requests? Your special requests don't just cost the purchase. For example, does the landlord also assume the maintenance obligation for the air conditioning, just like for everything else? Some special features might also need to be removed after 10 years, which also incurs costs.

It's unfortunate that this wasn't clear to you beforehand. Was there no agreement on this? What does the contract say about special requests? Everything only verbal (considering the sums ...)?

Does the landlord also take on the maintenance obligation for the air conditioning, for example, as for everything else?

Otherwise

after 10 years I am not allowed to remove it and take it with me.

makes little sense. Incidentally, from a depreciation perspective, this is not inappropriate at all.

Thanks first of all for the previous answers! I just want to point out once again that these are not extreme "special features." I am talking about simply better equipment (visually and qualitatively). It is not understandable to me that if I already pay the builder = landlord for the air conditioning including installation, I should also have to pay so-called "general business expenses" (and then 15%) on this increase in value for the rooms (they remain, including the air conditioning, in his possession). And I also cover the ongoing operating costs, the maintenance, and the repairs. For him, this is an immediate plus of 5000 euros and the air conditioning permanently, because I am not allowed to dismantle installations without consent. Something about this feels very wrong to my sense of justice... I am only concerned about the AGK. Their calculation was never mentioned or even contractually agreed upon. The rental contract was even reviewed by a lawyer beforehand. Contractually agreed upon, on the other hand, were so-called possible tenant installations with the option to take care of them oneself. But what good is that to me if all offers obviously have to go to the builder and he then holds out his hand for the additional costs... without having to do anything for it! Is "approval" common today and above all chargeable... and then only "merely" for a tenant???

For him, an immediate plus of 5000 euros and the air conditioning in the long term, because I am not allowed to dismantle installations without consent.

Then I would only prepare pipes and drill holes ;-)

Basically, a processing fee linked to the costs for special requests is not wrong and can be a good deal. There are also special requests with a different price-planning-effort ratio. A mixed calculation fits well here. And given the effort for routine deviations, it also makes sense to activate the surcharge amount as a thought aid for how necessary each special request is.

The lease agreement was even reviewed by a lawyer beforehand.

Well, probably not by one familiar with commercial long-term leases. General contract law is not enough there.

Is "signing off" so common today and above all chargeable... and then only "merely" for a tenant???

Real estate sharks have known about additional services much longer than doctors. Home advantage, I would say.

What kind of relationship is there between the landlord and the construction company? Could it be that he himself has to pay a change surcharge called "Zuschlag allg. Geschäftskosten" and is now passing it on to you?

Best regards, Yvonne