"Someone told me under code rules I'll need to remove the drywall and use backerboard..."

There are always local caveats to code...however...

Only if it's a wet area...in general, the lower portion of a shower wall or tub surround wall that will receive direct water spray. That's where IRC requires a certain type of tile backer board.

"Decorative tile" in a non-wet area...which this seems to be from your description...you can tile directly on gypsum board.

My apologies if I'm mis-reading your question.

Best, Mongo

Mongo, you read me right. It's 'decorative' in the sense that it's not protecting against water, heat perhaps. Using a modified thinset, I'm assuming it won't mind a little heat and won't let go (I've read about the trick of making the 'circle' in the thinset to create a suction cup) This guy - ok, he works at Home Depot - says there's a code about it. That being said, I've yet to see ANY builder of a new home here in British Columbia use ANYTHING but regular drywall in bathrooms in new construction here. When I'm working on the houses it tends to be just before insulating and around completion, so who knows, they may be using Redgard or Kerdi. Somehow I wonder.

No water spray. Shouldn't be, anyway, directly over a sink. As for expansion and contraction, it'll butt up against a Schluter edge on one side, and a corner on the other, which I'll do with silicone or caulk vs grout.

Thanks for weighing in.