Detached garage on house wall - plaster?

The prefabricated garage is plastered on all sides.

It is the same with us. They just build the wall tightly against it. The joint will be sealed afterward. The roof overhang, it is a bungalow, also protects the joint from above. I rely on the fact that they know what they are doing.

the prefabricated garage is plastered on all sides.

.. or what one calls that

Common for concrete prefabricated garages of the construction type "prefabricated cell" (= garage delivered in one piece by low-loader) is a surface texture that harmonizes with plastered areas. This can be integrated into the wall or a coating – however, not comparable to classic plaster of about finger thickness; the wall is more homogeneous in cross-section.

Common for concrete prefabricated garages (= made from on-site assembled slabs) is delivery in a rough structural state with plaster applied on site (production and layer thickness as with masonry). Concrete prefabricated garages made of individual wall elements (rear side single-piece for single garages, long sides one- or two-piece) are typically designed like their siblings of the construction type "prefabricated cell."

This is it. A prime example of aesthetics, perfect form, and elegance. And roughly plastered. Or sprayed with a kind of plaster.

looks sandy

, can you still find out why your site manager considers plastering the wall to be wrong? Wrong in the sense of not permitted, not in the sense of unnecessary.