Shower has become too narrow - Custom-made shower enclosure

Hello,

we are pretty much at the end of our construction with a general contractor and now have the following problem:

The width of the shower niche in the family bathroom is only 117.9 cm after tiling. The rough opening was specified as 124 cm. The shower enclosure we selected for 120 cm wide niches has only a tolerance of 15 mm and, according to our current understanding, will not fit. The tolerance seems to be common because we have not been able to find any alternatives quickly.
The plumbing company, which we informed about the deviation, has now sent us a supplementary offer for a custom-made solution including measurement costs of over €600 net.
We would like to avoid these additional costs and would like to know at which point things went wrong??? Was the rough opening already too tight? Who should have monitored the adherence to the measurements? In our opinion, the site manager (unfortunately quite a slowpoke). Should the plumbing company have given a notice when installing the shower tray that sufficient clearance must be ensured (possibly less tile adhesive, thinner tiles)? Should the tiler have known that this would be critical?
We are currently quite frustrated because towards the end quite a lot is going wrong with the agreements and the promised deadlines that are not being met. This issue is just additional fuel to the fire, which one might otherwise accept in an otherwise smooth process.

Tomorrow we have an on-site appointment with the site manager (actually to relocate a missing boundary point), I would like to calm down beforehand so the meeting can be constructive.

Thank you very much for your assessments and possible solutions!
Nina

I would like to ask how the 3 cm installation height of the wall covering came about. I suspect that an error occurred already with the raw construction measurement.

Well .. rough opening 124 cm - about 3 cm plaster - 2x tile thickness plus adhesive, I also only get about 118 cm. Besides, you never get a rough opening measurement exactly to the millimeter. Who determined the 124 cm? Who coordinated the selection of the shower door?

Have you already bought them? If it’s not a standard door, someone actually comes to measure it and then it is made to fit.

....and if you just ask a glazier or someone from the region who regularly installs custom shower glass etc. We had two of them installed in 2019 in 140x200 and the whole thing cost less than €800 including installation.

We want to avoid these additional costs and would like to know at what point it went wrong??? Was the rough construction measurement already too tight? Who should have monitored compliance with the measurements? In our opinion, the site manager (unfortunately quite a slowpoke). Should the plumbing company have pointed out when installing the shower tray that sufficient distance must be observed (possibly less tile adhesive, thinner tiles)? Should the tiler have known that it would become critical?

So I walked around with a tape measure and took measurements of the rough construction, measurements after plastering, and after tiling. At least everywhere it could have become critical. If something is important to you, you just do it yourself and point it out. But try your luck with the site manager.