Type M vs Type L Copper Pipe in Residential

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I'm currently in the process of a flip in Chicago, IL and I've had an inspector tell me my plumbing was incorrect and needed to be revisited. After bringing a more reputable plumber in he mentioned that all my rigid copper piping had red lettering (type M - lighter duty) and Chicago required blue lettering (type L - medium grade) throughout. This is likely to carry some significant costs (ripping all the copper out, buying new + labor. Has anyone heard this before? I haven't seen this in other states, so I assume it's a Chicago thing.

It is absolutely a Chicago thing. We do not follow national building codes -- Chicago has their own set of building codes. The plumbing union in Chicago is strong, and the plumbing inspectors are tough. All copper, (no pvc, no pex), and they are particular about the copper used as you have discovered. We had a plumbing inspection a few weeks back and the inspector told us they are doing a pilot test program for pvc but do not have high hopes for adoption in the city. The suburbs are a whole other story but in the city limits - only copper. If your plumber and/or your GC is licensed in the City of Chicago they should know that, its a well known fact. 

Just for the record we own a sewer and water company and we are a licensed GC in the city of Chicago. My suggestion would be to find a new plumber (and a GC) in the city. It is an entity unto itself and you really do need someone familiar with the quirks! Sorry you had to find out the hard way and good luck.