After struggling with the excel based plot planner for a while, I decided to try a new approach and started an image based plot planner. It is a multi-layer image in .pdn (Paint.NET) format, with one layer per building (and later one layer per plot size as well).
It currently does not have all the buildings and only the 60x60 plot size, but most shops, silos and a few others are in there already and I thought maybe someone may find it useful.
You can find the .pdn and a small README with some tips at http://files.ranta.info/istaria/
I am also looking for screenshots of the "planning" tab of other plots with either buildings on it that I am still missing and/or of other plot sizes. You can find my email at the above page, either in the footer of the listing or in the README.