Software developers like to think they are special. They aren't. Software, from a planning perspective, is not much different than physical construction.

When it comes to extending an existing application it really comes down to how well the base application was planned to begin with. In most cases the base application developers have no idea, because they either outsourced the planning to some external artifact or simply pushed through it one line at a time and never looked back. Either way the person writing the extension will be more concerned with the corresponding service data and accessibility than conformance to the base application code if it is not well documented and not well tested in a test automation scheme.