Postal codes of Brazil and another countries are oriented to distribution logistics, and improvement of reliability in writing the postal addresses; but without much concern for the spatial correlation.
Old codes, with 5 digits, have good spatial correlations (national scale), but new codes, with 8 digits (street scale), no. It is a situation similar to the USA-zipcode.
So, the problem is: "What are the geometric principles and GIS techniques that I can use to generate a postcode coverage of my city at street scales?"
Important-1: the POSTCODE codes are defined only into a cadastral database, there are no geographical association. Each POSTCODE is associated to an street name, range of house numbers, and number parity.
Important-2: brasilian house-numbers have only 70% to 90% (depends of city) of spatial correlation, that is, 10% to 30% can not be interpolated, because they are "numerological" or not checked with the correct street metrics.