I think the mirror needs to be a perfect fit for the polished look you’re after, no compromises. If the contractor measured the space and those measurements were not accurate then they need to remove this mirror and replace it with one that fits the space exactly.

Thank you - I would say that it’s part the contractor’s fault but part the wall not being straight. Unfortunately the latter would be difficult to address other than rebuilding the wall.. hence I am trying to find a solution

I think a stainless steel cover strip similar to a tile edging might very well work. Short of taking it off and trying to straighten the wall which might involve replastering that cover strip is a practical idea. Is it a solid wall or a stud wall? That clean line polished look you are after is so hard to achieve. I have renovated 4 houses now and I also struggle with anything that is not perfectly straight. As a structural designer I can spot if something is 1mm out. Another option may be to remove the mirror and try and make up that 2 - 3 cm difference with some sort of filler.

I understand but I would have thought a mirror could be cut to any size?

Thank you Isla - the wall which is not straight is a stud wall but removing it and straightening would be quite a work. Thank you for your idea re the stainless steel cover. As mentioned, the wall not being straight is an issue but I am quite disappointed that the contractor got the measurements so wrong (he said he needed that space to be able to for the mirror - which is not true imho)…

Hi Betty Boo, it’s not a problem if size but shape. It’s not a perfect rectangle as the lower (long) side is about 1 cm longer

I would think how to conceal the edge then. It's too expensive to remove and try and straighten the wall. There are so few contractors that tell the truth... most are full of excuses and ridiculous explanations.

Indeed my thinking - stainless steel strip be it then! Hopefully he won’t mess that up too… thank you Isla

It is all but impossible to get this spot on as walls are usually a bit wonky and to fit the mirror requires a bit of wriggle room. Therefore it is always necessary to finish with a frame. We used a stainless steel strip when we installed our mirrors. Not perfect but looks ok

Given the rounded corners on your basin, how about a surround with similar curved? Would hide the lack of squareness.