Kitchen surcharge after order placement

Huh? No, if the OP signs the contract like that, it stays at 4.5% (at least according to the information the OP has disclosed here). It is clear that we are living in interesting times. But that's not the point.

That's not true, because the terms and conditions will always override something like that. We've signed all kinds of things, had lawyers look at them, etc., and in the end it always came down to the same thing, namely the payment. They refer to "force majeure" and similar things, and that's it. We have clients who build with us, who also signed a document that nothing more will follow, etc., and in the end paid an additional 50k.

That’s not true, because the terms and conditions will always override something like that.
We have signed all kinds of things that lawyers have looked over, etc., and in the end
it always came down to the same thing, namely payment.
They rely on "force majeure" and the like and that’s it. We have clients building with us who also signed a piece of paper stating nothing more will follow, etc., and in the end still paid an additional 50k.

This is not about the legal side at all... One wants to build a proper relationship - customer / supplier -
I’m also not concerned about any increases that the dealer also has to bear and so on and so forth...

It’s about the way

a) the whole thing was communicated
b) that we hadn’t even negotiated the price yet and now have to pay a penalty if our construction is delayed
c) that the proposed solution after consultation was not satisfactory
d) that you can’t/mustn’t make a fuss over 1-2 months, no matter who bears the costs, as a studio you don’t let a contract like that slip away.

Anyone working in sales themselves will agree with me.

People, the professionals (KFBs) told him very clearly what’s going on and made suggestions. Somehow, he doesn’t seem to have noticed the general price increases and that in the end not every wall is exactly where it is in his plan. His house stairs and driveway are an example.

The warehouses of the kitchen studios have been full to the ceiling with kitchens for months because no house is finished the way the homebuilder imagined. There are no dishwashers, refrigerators are missing, etc. Some complain that furniture fronts are warped due to the long storage, etc.

Anyone still building now apparently doesn’t realize what’s going on.

And precisely because the situation has been so clear for some time, no one is forced to accept the price increase only at the last minute; instead, the situation is made clear from the very beginning. "Force majeure" doesn't apply either when you know that raw material prices and transport costs have risen sharply for a year. Instead, that is simply factored in. Or it is made clear already in the offer phase that the offer price is only valid for the next 30 days.
That is one thing.
The other is what the OP can do now, and I agree with the other discussants here. You don't have much leverage. Your single order is probably not that important to them. There are plenty of other customers. On the other hand, one more month probably doesn't really hurt them either.

By the way, there are also other examples. My stair builder, whom I commissioned about a year ago (i.e., before the extent of the raw material crisis was really clear), came to take measurements recently and sent the 3D visualization for visual approval. It turned out that not all posts had the same groove. He suggested equipping two more posts with grooves to create a more uniform appearance (about 150 EUR additional charge).
My wife and I thought, well, actually we could live without grooves altogether and communicated this. He then called and said he wasn’t going to make any money on the stair project anymore anyway; if we now removed margin-generating items, he would get into real trouble.
That meant for me:
1. He sticks to the price offered over a year ago (where I secretly feared there might still be a hefty adjustment coming)
2. He is open and honest in advance and talks about his problems
3. Naturally, I accepted the extra charge and commissioned the additional grooves
4. I will probably order a fall protection and possibly window sills from him as well.

What does that mean? He will earn more from me, even though he does not pass any price increases on to me later.
Everyone feels better. Classic win-win.

My general contractor, whom I am truly not happy with, has not written anything about a price increase so far. Although of course, anything is still possible there....

Guys, the professionals (KFBs) told him clearly what was going on and made suggestions. Somehow he seems to have not noticed the general price increases and that in the end not every wall stands exactly where it is in his plan. His house stairs and driveway are an example.

The storages of kitchen studios have been filled to the ceiling with kitchens for months, because no house is finished as the homeowner imagined. There are no dishwashers, refrigerators are missing, etc. Some complain that furniture fronts are warped due to the long storage, etc.

Anyone who is still building now apparently does not realize what is going on.

For me, this has nothing to do with the way the surcharge was secretly added, at least without active consultation with the kitchen studio, in the very last offer. The original poster already describes it correctly, it is not a way to proceed if a partnership relationship has been maintained until then.
I stick to my suggestion , would that be something? When do you have a conversation with the studio?

This is not about the legal issue at all... One wants to build a proper relationship - customer / supplier -
I am also not concerned about any increases that the dealer also has to bear and and and ...

It is about the manner,

a) how the whole thing was communicated
b) that we hadn’t even negotiated the price yet and now have to pay a TZ in case our construction is delayed
c) that the solution approach after consultation was not satisfactory
d) that one cannot/must not make a fuss over 1-2 months, no matter who bears the costs, because as a studio you don’t let such a contract slip away.

Anyone who works in sales themselves will agree with me.

I have been in sales for 20 years and would almost claim to have experienced every scenario once (except the current one).
How the whole thing was presented to you and what tone was used, we all cannot understand,
but I can reassure you, million-dollar deals have already been canceled over far less, compared to that a kitchen is nothing.

If I give the "order" in my role that from 01.03. the new prices plus TZ apply, then that also applies to the salesperson on the floor, because we know that the other studios are facing the same problem and will follow suit. It’s roughly like gas stations, which all watch each other and react accordingly.

My proposed solution would be that you take the measurement before the deadline? Isn’t that practical?