Leaky front door Kfw70 house

Hello,
the service will come on August 30th, also because of some other defects. Let's see what they say about the door.
We also have two seals, one on the door and one on the frame. When the door is closed, I can get my little finger in from the outside above the lock between the door and the frame, and it is similar at the lower edge of the door.
Best regards from [Bayern], which is once again rainy today

Send the complaint of defects with a deadline by registered mail (not delivery confirmation!), so that you have proof of delivery. Then I would consider appointing an expert. We often have heavy rain here, which splashes against our front door, although we have a canopy, water has never entered the house and our front door is now 16 years old. Therefore, I expect higher tightness from a modern front door. What does the Blower-Door-Test say??

just by the way, with us the water also splashes heavily against the front door during heavy rain, which then gets really wet. inside it is still absolutely dry. water must not come in through windows or doors as long as the water does not accumulate more than 20cm+ outside (flooding). by the way, that is also what is meant by "not waterproof". with hydrostatic pressure due to flooding, the door cannot hold tight. otherwise, of course the door must be tight, if only because of the blower door test and other energy loss.

Registered mail (not delivery!)

Yes, registered mail with delivery. This is also legally effective if the recipient is not present -- and is cheaper than registered mail with a return receipt, which you probably mean. Proof of delivery can then be found at weh weh weh deutschepost dot de. And then the receipt of the notice of defects with deadline setting can (quite certainly) be proven. That is what matters if it should go to court after all.

Yes, registered mail with delivery into the mailbox. This is legally considered delivered even if the recipient is not present—and it is cheaper than registered mail with return receipt, which is probably what you mean.

That may be, but I prefer to have proof in paper form with a signature! Every lawyer and court wants it that way... I wouldn’t care about a few euros! Recipients come up with the most incredible stories about why they didn’t receive the letter...

How do you ultimately prove what was actually in the letter? I mean, just because you have proof that "a letter" arrived, that doesn't yet prove what was inside...