I have just recently started my career as a web developer for a medium sized company. As soon as I started I got the task of expanding an existing application (badly coded, developed by multiple programmers over the years, handles the same tasks in different ways, zero structure).

So after I had successfully extended this application with the requested functionality, they gave me the task to fully maintain the application. This was of course not a problem, or so I thought. But then I got to hear I wasn't allowed to improve the existing code and to only focus on bug fixes when a bug gets reported.

From then on I have had three more projects just like the above, that I now also have to maintain. And I got four projects where I was allowed to create the application from scratch, and I have to maintain those as well. 

At this moment I'm slightly beginning to get crazy from the daily mails of users (read managers) for each application I have to maintain. They expect me to handle these mails directly while also working on two other new projects (and there are already five more projects lined up after those). The sad thing is I have yet to receive a bug report on anything that I have coded myself. For that I have only received the occasional let's-do-things-180-degrees-different change requests.

Anyway, is this normal? In my opinion I'm doing the work equivalent of a whole team of developers.

Was I an idiot when I initially expected things to be different?

I guess this post has turned into a big rant, but please tell me that this is not the same for every developer.

P.S. My salary is almost equal if not lower than that of a cashier at a supermarket.

During one of my internships I found I spent a lot of time doing bug fixes. You have to realize that as an entry level employee you aren't going to get the sexiest work, you're going to get the grunt work no one else wants. It's unfortunate, but it's how it is at every job.

Additionally, you have to realize that to a company, having code that works is more important than having code that is clean. From your company's perspective, you changing the existing structure is money wasted on redoing something that is already done and potentally introducing even more errors. Usually these types of companies aren't computer/software companies so no sufficiently high manager has the technical background to know that sometimes you need to do these major overhauls. That said, if your company is run by technically competent people and they understand the value of good code, you may get more leeway, although sometimes you need to choose your battles (the main purpose of a business is still to make money, after all).

That said, you are not unreasonable in wanting to be able to leave your mark on the software and wanting more meaningful work. It is also unfortunate that you have to deal with so many projects at once while fielding requests from so many different managers.

As a programmer, it is a fact of life that you will spend more time maintaining and modifying other people's code than you will writing your own from scratch. If this is a problem for you then perhaps you should stick to developing as a hobby and pursue a different career. If you are OK with maintaining code, but you feel you are not being used effectively or are being overwhelmed, then that is a matter you need to discuss with your manager. If your problems are more serious than that or if you feel like your managers don't know how to effectively manage your skill set then it would be a good idea to consider finding a position at a different company. Given your stated low salary, this is probably your best course of action.
