Wii U GamePad needs to be within 8 meters of console

You need to be within 24 feet of the console for best performance.

In an interview with 1up, Nintendo product manager Bill Trinen revealed that the Wii U GamePad has an effective range of about 8 meters (24 feet). We knew there was going to be a limit of course. I think 24 feet is pretty generous, but if you live in a good-sized house and plan on playing games directly on the GamePad while someone else watches TV, it could be a problem if there isn't a comfortable room nearby.

1up: "Is there a limit on the distance that you can take the [Wii U] GamePad away from the system?"

Trinen: “There is. And it’s… Of course, when we’re generally giving limits on things like that, and I think the same thing can be said for the Nintendo 3DS battery life… We tend to be more conservative, because we’re looking at it from a performance perspective. What, really, is the range where you’re going to get the best performance out of that? And you’re going to get the best performance within a range of about… They had a very precise number at one point. In meters it’s like eight meters. In feet it’s about 24 feet. That’s for best performance.”

Source: 1up