Our garden wall stands on the neighbor's property - is a notarized contract necessary?

Greetings!
About 30 years ago, together with the southern neighbor, we built a natural stone wall between the two properties. Since there was already a foundation on his side, he allowed us to build on it, and afterwards our property was filled up.

The neighbor is almost 90 years old and he would give us approximately 20 m2 as a gift, since his house will, upon his death, go to a larger community of heirs and will probably be sold.
How can we find a legally sound solution?
Do we need a surveyor and a notarial deed, or is a contract between us neighbors with witnesses sufficient?

Property matters always with a notary. Real estate cannot be transferred otherwise.

Land matters always with a notary. You cannot transfer land otherwise.

Thank you, so we will take care of that with the notary. And what about surveying? Does that also have to be done or is the notary enough?

Friends of ours received a part of the property from their son. It had to be newly surveyed. Costs approximately €3,500. Contracts with strangers should definitely be notarized.

The neighbor is almost 90 years old and he would give us the approximately 20 m2 as a gift, since his house will go to a larger community of heirs after his death anyway

Be careful with gifts. I would officially buy the 20 m2 at the standard land value.

Definitely in writing with a notary! There is a gentleman on this street who has lived in the house for 15 years and had a written agreement with the owner that he may live there forever.

The older gentleman has passed away and the community of heirs is now suing the resident out. He has already been told in court that the written agreement has ZERO effect.