Is the task for the window fitter actually new construction windows or renovation windows? In the second case, it will probably be smaller than the original. Because as little chipping and plastering work as possible should be done. Furthermore, the actual shell construction dimension is not necessarily known. Then they like to play it safe, measure smaller, and then foam generously and plaster over. After such experiences in the basement, I then ordered and installed my windows in the hallway myself. For this, I read the book from ift Rosenheim and tried to gain the best layman's knowledge. On the top and sides, there is a combination compressible tape from illbruck, under the windowsill the associated foam. Sealed rainproof to the front with the foil from the system and then everything is plastered and integrated into the existing insulation. Inside sealed to the windowsill with the special joint sealant and plastered and wallpapered on the sides/top. No unsightly cover strips, but also a lot of work and dirt, since the window opening was rebuilt to shell construction level.