Can the neighbor drill into my (private) wall?

Questioner

In 2014, our house was extended with a conservatory with 2 stone walls on either side. These are both on our land so they are our own walls. The neighbours on one side did not want to go along with the extension at the time and even tried to stop it, without success. I did accommodate them by having both walls slope backwards so that the light would not be too disruptive for them. They also had my permission to hang small decorative items on the wall in mutual consultation. Now, halfway through last year, they themselves also had an extension built (open to the rear) with a roof running STRAIGHT to the rear (so horizontally). They also want the inside of their extension to be plastered completely white. Because it concerns our private wall, they have now started making a frame for (plaster?) boards and this has been drilled/screwed to the wall during our absence and without any announcement or communication. In my opinion, this is not entirely in line with the regulations here in the Netherlands, is it? Please respond.

Lawyer

No, that is not allowed, so you must order your neighbors to remove it.