How is it actually... Do I have to tell everything we have done (renovated) or does "as is" apply here?
It always applies "as is". But you must not conceal any defects. On the other hand, it is actually also a "must" to mention all renovations. Don’t you want to get a good price? So you have to promote your house and report what you have already done.
Average price with us is 3100€/sqm I don’t know to what extent the renovations and the age lower the price :/
Average price of what? sqm basement? sqm plot? sqm living area? sqm garage? Renovations increase prices. Open construction sites and defects lower them. Just like age, which usually goes hand in hand with renovations/repairs still to be done. By the way, average means average. There are top properties and also old clunkers. You have to know which average you are talking about.