Just a few more square meters?

Yes, of course, everything is massively possible.

Thanks to everyone for the answers! It’s crazy what all that can entail... But first of all, it’s good to know that it’s basically possible.

Yes and no. The grids are still a reality in manufacturing, but they no longer represent the formerly usual planning corset. You can think of it similarly to ceiling beams or rafters, where corresponding cycles can be "shifted" or replacements can be made. That means economically, planning is still done according to the grid, and changes in whole grid increments are easier to implement, but on request, "Made to Measure" is also possible in almost any desired dimensions. The preferred step lengths for enlargement increments are 62.5 cm for timber frame panelers and 1.00 meter for masons.

which does not mean that there is not still a system behind it

But many GUs don’t do everything or not every type house goes into an extension. Usually it works with half a meter, but not in every direction. It depends on the structural engineering. Many GUs cannot keep the low price because then everything has to be new and therefore they set limits. Building a house with a type house has its limits – in every respect.