Price increase - Our construction company wants 21.4% more

That looks contractually a bit complicated from a distance. I consider consulting absolutely essential. I am keeping my fingers crossed for you.

Since some of this seems familiar to me, the GC doesn't start with K..., right?

Do you sample at the company headquarters? And with whom, foot soldiers or the boss? Is the place big or small? Is there an owner? Do you already know them, perhaps?

I would insist on a conversation with the boss first and foremost. Then politely but firmly point out that you cannot accept it as it is.

Can the participation in the sample selection be legally interpreted as consent to the price increase? There must be a (legal?) reason that the price increase was communicated hours before the sample selection.

Since some of this seems familiar to me, the GU doesn't start with K... does it?

Yes. Minimum of 5 words have now been reached.

Since some things seem familiar to me, the general contractor doesn't start with K... or does it?

Can we get in touch by email or can I call you? I would be interested to know what your involvement with the company was and how things turned out for you. I am still interested in an amicable settlement, but that probably makes no sense for the other party.