Job title for a role with high technical and business responsibility but no subordinates?

Let's say a standard-ish hierarchy of roles in the software industry looks something like this:

• software engineer


• senior software engineer


• technical lead


• solutions architect


• enterprise architect


• (Or pick other titles, these are just examples to get started)

I'm trying to describe a role that entails being the only programmer in the organization, working exclusively on a single project for which the role must fulfill every function from understanding the customer business down to writing code, and all the technology choice/architecture levels in between.

On the other hand, nobody reports to this role, and the senior titles in the software profession usually seem to be reserved for people who do (I think?).

To give another point of reference it needs about 10 (+/- 2) years experience in a software career to fulfill.

How should the role be fairly described to a non technical HR that determines pay based on job title?