I didn't quite understand the thing with the gutter, but it doesn't matter. The problem with a drainage system is that it clogs up. If you then have a problem, it's your fault. By then, the developer is also out of warranty. Therefore, I would insist on two things: 1. a professional house sealing according to DIN. This can mean that the developer has to tear down the veneer and insulation again. That is expensive, and that's why you absolutely need an expert on your side. If the developer is already talking about pressing water, he is digging his own hole here. And since you already have water damage, it is definitely not properly sealed. Sealing of floor-to-ceiling windows needs to be checked separately 2. structural measures on the outside facilities to keep the water away. Drainage is one option for this, but it transfers the responsibility for the drainage to you forever
We also have poorly permeable soil and the house is a little below the original ground level at the northeast corner. With today's knowledge, I wouldn't do it that way anymore, but be that as it may. In any case, we make sure to keep water away from the house as much as possible and that everything is well sealed