Which dimension the window fitter must take – that is, exactly where to measure – naturally depends on the plane in which the windows are to be installed.
But your question was, when. And that is clear: it can happen now. So measure, produce, and deliver – if necessary, install only in coordination with the ETICS installers.
I had understood your words to mean that all the windows below would have to fit with the cladding, and below most windows only with the ETICS. The drawing, however, shows that all upper windows – not only those that reach down to the "normal" parapet height, as I thought – sit in wall sections that are also affected by the cladding.
But that was only a small misunderstanding without practical effect.
I will clarify the connections again, independent of the exact installation plane: the fact that measurements must be taken at all is related to tolerances between the planned dimension and the reality of the execution. In this sense, one can only measure what is already there. So far, this applies only to the structural wall shell and the cladding wall shell.
But that is sufficient, which simply has to do with the hierarchy: compared to the window, the wall is higher ranked – better to know the final order dimension of the window later than to have to chisel away something again from the stones – whether masonry or cladding stones. Compared to the ETICS, however, the window is higher ranked: the panels are easily cut to a different dimension (and are anyway only adjusted to the exact size during installation, where they form the last panel in their row). Here, the window does not have to conform to the "wall" and can already be there. Whether it should already be installed makes sense to coordinate with those carrying out the work in terms of the sequence.
For the lower windows, everything is fixed with the finished cladding. For the upper ones, I thought they only needed to fit with the ETICS. In that case, I would have produced them – where the actual dimension of the structural wall shell does not contradict it – according to the dimensions of their counterparts on the ground floor. So – with cladding also in the area of the upper windows – the rule “measure individually” applies upstairs as well. That is the only difference arising from our misunderstanding.
Since the entire upper floor height is not ETICS here, I assume an installation on the outer line of the structural wall shell. As soon as you can replace this assumption with case-specific knowledge, the windows on the ground floor can be installed. Upstairs then, when the guys from ETICS say they would like to have them already set.