Courtyard entrance / terrace poorly executed

At the moment, in my opinion, you do not need a lawyer yet. At the moment, he still has to prove to you that he has carried out his work professionally and according to the recognized rules of technology (DIN). I would demand this from him in writing and also write a written notice of defects with a deadline and then a second deadline. Only after that would I involve a lawyer or simply not pay the money. You will have set enough deadlines by then.

What is the alternative?

Insist on a rectification and do not negotiate a discount.

Learn something from this: Procuring the materials yourself and letting the craftsman only earn the labor cost almost always goes wrong. And if he charges you outrageously high prices for the work... then just persistently refuse payment as described above. Stay firm with reminders. You provide flawless bills if he delivers flawless work.

You have the money and thus hold the stronger bargaining position. Demand a second correction with a deadline. If he still doesn't do it properly, get a quote to have the area fixed and deduct these costs from his invoice. (ggf. diesen Schritt mit Rechtsbeistand gehen)

To avoid formal errors: Now attorney. That's my stance. He will proceed like this....Invoice. Reminder, reminder, then he hands the claim over to a collection agency. Then it gets rough. Bet? Karsten