You are in a strong position as you have the offer. How well you do will depend on their budget, politics, attitude as much as how good you are. I would sugge

You are in a strong position as you have the offer. How well you do will depend on their budget, politics, attitude as much as how good you are. I would suggest framing everything in terms of business benefit and the benefit to the person making the decision. Pretend you are them and imagine what they are looking for. The refactoring stuff looks good. So you could frame that as "You are getting a lot of bugs reported every day, which must be frustrating. I bet you'd rather be making progress the more important things in your job. I will fix those bugs quickly. I can deal directly with the support department if required. At the same time I will work to make the codebase better to reduce the bugs in the future by at least an order of magnitude. I will do this by ..." Now this guy will personally want to fight to get you in at the higher salary because you are going to make his life easier. Another way to look at this is how the employer thinks about roles. If you can sell yourself as a senior developer or architect or team leader rather than just a developer, then show comparable market rates, this will help. But you need to figure out what is making them tick.