Googling suggests that the details differ by municipality. For example, Hilversum requires [1] you to submit a proposal for a facade garden. If approved, the municipality will take care of removing the appropriate width of sidewalk tiles and replacing them with soil. The owner is responsible for planting and maintaining the resulting garden. Other municipalities have different regulations.

[1] https://hilversum.nl/en/mijn-wijk/geveltuintje-aanleggen