Do not run extension cords through walls.

No, it’s not safe, and it’s almost certainly against electrical code wherever you live.

Extension cords are designed for temporary, visible use. Running one inside a wall or ceiling traps heat, hides damage from pinching or chewing (rodents love insulation), and creates a serious fire hazard. The cord can overheat, the insulation can fail, and you won’t know until it’s too late. Plus, any inspector or insurance adjuster will flag it immediately.

The proper alternative is to install a new outlet, run NM-B cable (Romex) through the wall, and terminate it in a junction box. If that sounds like too much work, use a surface-mounted raceway that’s rated for in-wall use, or call an electrician. It’s not glamorous, but it’s the only safe way to get power where you need it.

Don’t be the person who burns down their house to save forty bucks on an outlet install.