ᐅ Conversion of a Multi-Generation House – Uneasy Experience with General Contractor

Hello everyone,
My wife and I, together with her mother, have decided to convert the existing single-family house into a multi-generational home. Unfortunately, the house is not large enough to accommodate two households, and additionally, the roof is not insulated yet. Besides an extension, there is also renovation work needed.

Below, I will summarize our experience with our general contractor (GC), as we are currently very uncertain about how to proceed.
In short: Are we being too naive, or do we simply have the wrong expectations?
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In January 2021, based on a recommendation, we got in touch with a GC who inspected the house with us and discussed our ideas. Afterwards, he gave us an offer for the planning phase, which we started with his wife as the architect.

This was the first time we asked for a rough cost estimate for the entire project. Our GC replied that it was difficult for him to give a price without plans but promised to provide one before submitting the building permit / planning permission application.

Planning
Since the planned extension requires approval, we decided with the architect to create a preliminary design to use as a basis for a building inquiry at our local planning authority.

Our planning authority now generally rejects preliminary building inquiries and only responds to specific applications. So basically, we have to plan and hope... (2 drafts were basically wasted).

The planning phase has been slow because the architect keeps forgetting agreements, and we have to remind her of previous points during meetings. Additionally, the first two plans did not include access to the existing basement, which caused problems in the designs.
Between June and December, we worked with her on four different drafts.

We have come to realize that we probably focused too much on details.
Now, one year later, we have a design we are about 90% satisfied with. In hindsight, it might have been better to move forward earlier with a 70% solution...

Then came the big day for the rough cost estimate. We sat down with our GC and discussed the desired quality levels, etc.

Yesterday, we received an email from the GC stating that his brother (who runs the construction company) cannot prepare the cost calculation without structural engineering and formwork/reinforcement plans. (Cost for these is 2,500 Euros).

Now we are a bit shocked and at a loss:

We have already spent nearly 11,000 Euros on the architect’s plans without knowing whether the entire renovation project is financially feasible for us.
The existing plans have not been checked by a structural engineer, nor is it clear if the planned extension will even be approved in its current form.
Our GC still refuses to provide a cost estimate, citing the lack of structural engineering as the reason.
(Surveyors, energy consultants, and others are also still missing, of course.)

We are currently concerned that this might be getting out of hand and are feeling uneasy about the whole situation.

I would appreciate any advice.

My main questions:
Is all of this normal? Are we simply inexperienced?
Should we seek a second opinion or look for another GC, etc.?

Thank you!