The developer does not build according to the plan

The plans according to which the house is currently being built apparently date from April 2014. We signed the purchase contract with the notary in November 2014. In this contract, the patio door as well as the smaller window in the basement were included. This plan was also presented to us in this form by the seller at that time. As for the window changes, these modification requests were communicated to the seller by us on January 9th, and he forwarded them to the architectural firm. It took an eternity there until they became active, and after several inquiries on my part, the plans were then completed and, according to the architects’ statement, forwarded to the seller after my approval.

What does incorporated in the contract mean? In the text or in a technical drawing (of whatever type and quality)?

I would impose a construction halt until the problem is resolved. Everything else will just turn into a mess.

If it is indeed a developer, you can decree whatever you want. You are not the builder and therefore have no say. Sounds stupid, but that's how it is.

I would try to bring about a clarification as soon as possible.

What a blessing that there are purchase contracts, I have now spoken with the construction management and the matter will now be resolved according to my wishes

Is the purchase agreement a contract for work and services?

It would be nice if you could answer the questions: are YOU now the builder or is it the developer?
see here:

If it is actually a developer, you can dictate whatever you want. You are not the builder and have no say in it. Sounds stupid, but that's how it is.
...

If you are the builder yourself, the signatures on the amended plans count.