I personally would not touch the supply line to the underground sprinkler system.  Each head has a flow rate, such as 3 gallons per minute.  Your city supply is checked to see what it's rate is, then each zone is set accordingly with the maximum number of heads your supply can handle before dropping pressure.

Just run a new line out to that area.  It will take you all but a day by yourself to do and you'll be much happier and have more peace of mind in the end.

you could do whatever you want.  AFAIK, youre stll in the USA.

however, if you do splice in a T.  you may have an issue.  If you run that part of the T, your sprinkler system may not have enough pressure to run at the same time.

It's not a problem.  Every valve downstream from the meter has to share the flow and pressure no matter where it's located.  Just don't open too many valves at the same time.

Quoted: The main purpose of this would be to add a small drip irrigation system (separate timer from the lawn sprinklers) for some planted boxes in the backyard and/or some light hose watering (while the lawn sprinklers aren't running).  Running a new supply line would be great, but the backyard is surrounded by either A) a rocky slope or B) a stone walkway.  There's no supply anywhere near the backyard that wouldn't entail me tearing up flagstone.

If that is your only option then go for it.  Like you've stated, along with others, as long as that zone is not being used at the same time as the sprinkler system then it should be OK.

Quoted: Problem:  I have NO hose bib in my fenced backyard.  The only water supply there is for the backyard zone of the sprinkler system.  The electric valve for these heads is located back there too.  Is there any harm in cutting the supply line to the valve (I think it's a 1" pipe), putting a T fitting with a 3/4" outlet on it, and adding a hose bib to that?

You'll have to add a galvanized union in the plan. If you don't you'll be digging up about 10-20' of pipe to make the joint!



ETA So your system will look like this:

^^^^

Unless his existing system is using plastic....if that's the case then he won't need a union, just a T, hose clamp, and more pipe.

You can do it from the mainline (before the valves)
but not after.
If you do it after, it will only run when that zone is running, which is not what you want.
if you do it before, it will just be a manual valve.

NOBODY uses galvanized for sprinklers (not since like 1962), it's either PVC (white), in which case you need a slip joint and a tee)
or Poly (black) in which case you just need to cut back a foot or so and cut in the tee.

The other thought is to hook into the mainline and run the pipe up to the area you want, basically adding a new valve.  Yes, you'll have to dig the pipe in, but that's not really that far (well, once anyway)

Quoted: You can do it from the mainline (before the valves) but not after. If you do it after, it will only run when that zone is running, which is not what you want. if you do it before, it will just be a manual valve. NOBODY uses galvanized for sprinklers (not since like 1962), it's either PVC (white), in which case you need a slip joint and a tee) or Poly (black) in which case you just need to cut back a foot or so and cut in the tee. The other thought is to hook into the mainline and run the pipe up to the area you want, basically adding a new valve.  Yes, you'll have to dig the pipe in, but that's not really that far (well, once anyway)

+1

By adding a new zone, you can operate it off the same timer as the sprinkler system, if you have an available zone.