Hope someone has some experience and can chime in.  And if this this a stupid question, well, this is GD, and I'm also a 13'er, but I have thick skin so be honest.

Anyway, I want to build an addition on the side of my house.  As it so happens the water line runs into the existing foundation on that side.  The water line is (should be) about 5 feet below grade.  I want to do a crawl space addition, and in this area, the footers need to be about 4 feet deep to stay below the frost line.  Any issues with pouring footers over top of the water line?  

Just want to find out if this is even worth considering, before talking to someone to get some plans drawn up.

Footers should be on virgin soil, the waterline there means it was once trenched out and then filled back in. You might have an issue with settling there over time.

Can that be addressed with footer design?  e.g. modifying/upgrading the rebar in that section.

That is a question for an engineer.

It would probably depend on the soil type and how a perc test goes.

I think another issue with building a footer over a water line could be crushing the line too.

I am going to guess that the correct way to do it is going to be digging up the water line and moving it, then any place where the line was dug up will need a step in the footer down to virgin soil.

Keep in mind though that I don't design footers and walls, I just install them to spec.

I would check with the local building inspector's office.

If this was municipal water main, you'd generally want to avoid being below building foundations (mainly due to access / easement).   And anytime there will be a substantial load above the pipe (such as at the location of a railroad crossing or major roadway) the water pipe is placed inside a stronger carrier or casing pipe which carries the structural load.

Another thing to think about is if the water line breaks, then the leaking water might wash out a footing or two.

It will depend on your building official's interpretation.  Some municipalities only require arches for pipes that are within two inches of the bottom of the footing, and they allow you to build right over anything deeper than that.  The trick for you is finding out the actual depth, and once you disturb the soil to locate the service pipe you'll either need to backfill with engineered fill or place the arch anyway to relieve a step down in the footing, or possibly even arch and fill for a level footing.

The best way to get an answer is to place a call to your local building department.  I've always gotten good answers in a timely fashion by raising questions up front.

If for some reason they do require an arch, hire a local engineer to design it.

edit:
 

He's talking about the domestic service entrance, not the water table.

You will be fine, yes.

LOL Speaking of the water table, pore water pressure (from the water table) provides some of the loading bearing strength of a soil.   If somebody comes along and puts a big water well nearby that draws the water table down, it can make nearby foundations crack and settle because the well drawdown zone or cone of influence has pulled the water from the soil beneath the foundations.  This causes the void space in the soil to collapse and settle down, causing the foundation to settle and crack.

Big lawsuits have happened because of this.