wet wagon
In the attached image, the game tells me I will be building over the Bath. It should also tell me what yields I will lose by doing so. Right now, it's guesswork and memory when you could just show me the loss.
To go a step further, since you highlight all the hexes I can place in and the yields I will gain by placing there, why not also show the yields I will lose by placing there? You already show gains and losses for specialists. Why not for buildings and improvements?
high trench
You can back out from the build screen, open the city details, go to buildings and then locate the building you're going to overbuild and see its yields there.
I do that multiple times because I can't remember all the buildings I had the option to overbuild. It works, but it's a lot of steps and annoys me every time. Showing it while you are in the screen where you place the new building would be much better. Like OP suggested, showing them as negative yields just like with specialists would be good.