I am trying to get water into a building in my back yard for a sink and washing machine. I have consulted a plumber who wants to start at my meter (In my front yard) tear out some of my driveway and go around my house to the building, making a total of 4 turns. They want $6000.00 for the project. I want to do the project myself. I want to know if there is a reason I cannot just go from the line where my exterior faucet is and use that line.
While there is no reason you can't do it this way there is a reason a licensed plumber says to do it that way and it is called sizing.
Now don't think I am pounding my chest and preaching I am just explaining.
Proper water service sizing will allow for even constant flow and when your house was built the main water entrance was sized for the total number of fixture units. This allows for constant flow and pressure. Most the time service entrance is sized for larger than needed for an added bath and so on.
An example of improper sizing is flushing a toliet and getting scalded in the shower.
Adding a 1/2" line and going to the backyard sink and washer shouldn't be to big of a problem. Just keep in mind that if you start to see low flow or pressure in the house while using the backyard sink or washer this may be your cause.
Legal - more than likely but should check with local code and make sure you bury to proper depth
John Wayne once said "Life is tough and it is even tougher if you are stupid"