How to deal with walls?

Running cable through finished walls or ceilings is a nuisance. Once you’ve determined that a circuit can be extended or a new one added, you should plan the circuit’s route and figure out the least-disruptive route for the cable. An existing circuit can be extended from an electrical box provided that …

• The box is sized to accommodate the additional cable.
• The box isn’t at the end of a switch loop (an end-of-the-run switch).
• The box isn’t a switch-controlled receptacle (unless you want the added device to be controlled by the switch as well).

Your best and easiest route is through an unfinished basement or attic, If you’re drilling into a crawl space, place a flashlight over the top of your drilled hole and turn it on. This makes it a lot easier to find the hole when you’re crawling under the house. If you have several holes to drill, insert a piece of scrap neutral wire into the hole; you’ll be able to see its white insulation more easily and mark each hole. It isn’t always easy to determine the location of a box when you’re up in an attic or down in a basement. From an approximate location in an attic, you can drill a very small hole on the outer edge of the wall’s top plate through the ceiling below and then poke a section of wire hanger or scrap wire through. From the room below, you can locate the necessary drilling location, go back to the attic, and adjust your coordinates. From the basement, you can drill a pilot hole from above (if you’re adding a receptacle) through the sole plate or bottom plate of the wall using a long, narrow twist bit. Then drill the larger hole from below. If the floor is only covered with subflooring material such as plywood, you can use your drill or drive a 16d finish nail through a location near the base of the wall where your new box is going in, looking for the nail down below. If you don’t object, you can do the same through carpet; just be sure to place the nail near the very edge of the baseboard and use a nail set to pound the head below the surface of the carpet
What if you have wood flooring? If you cannot accurately determine where to drill from the basement, drill the smallest hole possible (with a drill bit about four inches long) near the edge of the baseboard. Push a piece of wire through the hole so you can find the location in the basement. A piece of scrap conductor from your NMB cable will work just fine. At the end of the job, fill in the hole with a putty stick in a color matching the floor stain.