That depends on how comfortable you are with doing this and how much knowledge you have about electricity. If is a fairly simple project if you know what you are doing. The hardest part would probably be running the new wires. If you don't know what you are doing then it is probably best to call the electrician.
If you answer some of Dave's questions you may be able to learn how to do this. Even if you don't do it yourself (since you are not comfortable with it) at the very least you can "supervise" the electrician and have questions to ask and make sure the job is done right. Usually when hiring a "professional" if you don't know absolutely nothing about the project you tend to take their word as law and once in a while you will find a "professional" that is not worried about doing the best quality of work. I study up on anything I am hiring a pro to do. Good luck.
What one person thinks is easy another may not
In this instance middle of the circuit outlet is your exisitng outlet
Garages require GFCI, so this could be different
Shut off power 1st - verifiy oit is off
You connect a new wire, & then connect to the new outlet
But codes do vary by area on what is required & the specifics of how to wire
Specifics depend upon your situation & existing wiring & codes
It's easier to extend from the last outlet box on the daisy chain as opposed to branch off from a box not at the end.
Having a third cable come out from a box to extend in a different direction may run you into the problem of box fill, having too many different wires inside that box. Box fill should be discussed in any good do-it-yourself guide book.
You can install a GFCI receptacle unit in the first outlet box so as to protect all of the receptacles new and old.
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