Would anyone have a suggestion on how to generate discrete Lat/Lon taking into account Apartment or Unit #? I know that geocoding is primarily based on street address. However, is there a way to build a geocoder or use a service that would take into account secondary address information? It is a requirement for a project I'm working on.
The reason secondary addresses are very hard to obtain is because there are a lot of them. I would estimate they maybe between few thousands or ten of thousands different types of secondary addresses that exist around the world, as each country has their own standards. Some of these secondary addresses are not known to the general public but they exist somewhere in paper. Here is repository where you can see just USPS ones.
It will be hard to build from scratch new service that takes into account secondary addresses. I wrote an article on this topic - The Impact of Secondary Address Designators on Coordinates where in details you can find how to approach it and the tool that can handle it. In general I would recommend the following tools for USA and Europe and parts of Latin America:
I would summarize in general, use always paid tools when comes down to secondary addresses. The open source/free tools are not there yet.