Reline or Replace Galvanized Drain & Cold Water Lines

Ditto, 20 years at best for galv pipe. You are not going to be able to reline galv pipe in a building - and even if you could it would cost more than repiping. I would suggest a PEX style piping system, as PEX handles freezing well (but no the metal fittings), easier to manipulate, but cannot stand UV rays. I would choose PEX over CPVC or Cu. Depending on the building and code (which may require cast iron for the main stack), I would pick ABS for all the drain lines. However, keep in mind that the ABS piping will be noisier.

Depending on who the client want to rent to, CI pipe will be quieter and that might be perceived as an advantage.

Copper would be the "green" choice over plastic due to ease of recycling.

I am not a big fan of plastic, except it is cheaper. And any idiot can glue it together. (like me! )

I've seen re-lining companies at trade shows, but do not have experience with actually using the system. The process is to blow high velocity compressed air through the piping to remove scale and corrosion, then spray a plastic material to apply a thick coating on the pipe interior. Sound promising, but to do it right would be time consuming. I have no idea how the economics would work out compared to replacing piping. I got the impression it was being marketed towards higher end, "historic" structures that did not want the expense of refurbishing old finishes.

You did long with your pipes. When you cut some pipes out for inspection, you will see that relining will be hopeless. Besides pinholes, you will find almost rust clogged pipes. Replacement is the way to go.

Galvanised lines on potable water supply is not permitted in the UK. Copper and many plastics are the way to go. Check local building regulations for the requirements for new installations, and follow them.

Relining piping is a money gamble... Your flow will be reduced

I will also bet that there is little difference in the cost to replace piping vs. reline. Its all about the labor costs...

Have you gotten cost estimates ?

Replace the piping....... ummmmm.....many bridges and buildings don't last 60 years !

Without knowing more specifics it is difficult to give an accurate answer as to the best way to replace the piping. Extraordinary circumstances aside, replacement is almost a certainty as far as choices go. I have had extensive experience with galvanized piping and none of it was was good once age was a factor. Plumbing accessibility, code requirements, availability of materials and expertise will play a role in making your choice. I would advocate finding 2 or more highly recommended plumbing contractors who could come and give you their opinions and estimates. I would ask that you be honest with them, let them know you are taking estimates, and don't share their price estimates with their competition. It only creates awkward and imbalanced bidding.

Good luck.

In case of my building which is 70 years old we replaced all the water and drainage lines 10 years back. Water lines have been replaced by heavy duty class "C" galvanised pipes of reputed make whereas drainage lines have been replaced by PVC pipe lines. We got good scrap valued of old cast iron drainage lines. PVC pipes seem to be good replacement.

electrolyisis between two unlike metals may be the problem. exp; zinc from the galanizes pipes connected to copper pipes.