How about if the floor boards were labelled in some permanent way?
I tend to agree with Pierre that an access panel looks like an access panel, whereas floorboards are pretty much just floorboards. One option, I suppose, would be to build a trap door into the floor.
I also agree with 'Zapped' - the last thing I want to have happen is someone in the future trying to guess where things are. [I've done a heck of a lot of hard work figuring out what's been done to this house in the past: wiring the garbage disposal into an ungrounded light socket? Yikes!]
Any other opinions or suggestions?
