Proximity Rules
Some towns have proximity rules, that is, rules that require officials to recuse themselves from any matter dealing with property within a certain distance of property they own or rent. But it is hard to have a set number of feet or yards. A distance appropriate to an urban environment is very different from one appropriate to a rural environment.
There are considerations to balance here. On one side, there is the good of making ethical guidance as clear as possible. On the other side are several bads.