When transmitting on 20 meters my rig is causing interference to our home
intercom (in the wall hard-wired unit) and TV (cable). I've checked the
ground, the rig is new (FT-897), as is the antenna (G5RV-Jr). Antenna is
located in attic due to neighborhood restrictions.

I'm guessing the the intercom wires in the walls are acting as a big
antenna, but where would I start putting chokes on? Or would a filter at
transmitter (low/high pass?) be where to start?

Seeing as how you have a license to be on HF, you should be able to figure
that out. Im quite sure that question could be answered by going through
your General theory book......

If you need to ask that question, you have no business on HF