Roof pitch 12 degrees - waterproof design not planned

Hello!
Here's the situation: The construction contract is signed, the building permit has been granted, now comes the first change...
I received an email today from my general contractor / GC. The roofer has now informed him that the roof cannot be done as planned. The plan was a shed roof with a 12-degree pitch on the east side. This is also stated in the building application, in the drawings, etc. Technically not a big problem, right? A watertight underroof is possible, isn't it? The GC now says that this will cost more.
Can the GC demand more money after the offer was made and the contract signed? So he offered me a technically incorrect roof, didn't he? I will talk to him again to see if we can somehow come to an agreement. But changing everything again now and making the roof steeper is out of the question for me.

Question to the forum:
A shed roof with a 12-degree pitch and watertight construction is technically not a problem, right? It is stated like this in the drawings, building application, and the offer is based on that.

I cannot tell from my offer that it was made as a rainproof or watertight underroof. But according to the recognized rules of technology, this should have been clear from the start, since the pitch of 12 degrees is in the drawings, right?

Can someone estimate how high the additional costs (e.g. materials and labor) for the watertight execution could be? The roof is a shed roof with external insulation, size 5.88 x 12.86 meters.

I'm not a big fan of such surprises and that's why I'm a bit angry with the GC right now...

Good morning,
so I think the general contractor had access to the building regulations of the municipality or city. The general contractor makes you an offer that complies with the building regulations and the feasibility of the project. After signing, the offer turns into a contract that both parties must adhere to. So I would say, tough luck for the general contractor. The general contractor only has to comply with what is stated in the contract. Verbal promises are ineffective! If the watertight underlay is not mentioned in the contract, it is not part of the contract!

such nonsense! so I can just let the roof flood because the contract doesn't say it has to be watertight? that can't be true!

Hello,

Waterproof underlay is possible, right?

What, please, is meant by that?

Rhenish greetings