Does reusing my own code from one project to an other count as plagiarism? I am currently a under graduate and will be going into my final year of a Computer science course in October. 

The final year consists of a large programming project. Over the last 6 months I have been working on ideas for the project. To the point where I have a nearly finished application. 

If I was to use large chunks of code that I have written over the last 6 months in the project would this count as Plagiarism? It doesn't feel right been able to just copy and past code to get a head start on the project even though I had written the code in the first place.
Ask your professors. Your senior project professor/adviser/committee probably has specific expectations about the work they expect you to do, and this is going to vary from university to university.

My feeling is that you'd be OK using this code as long as it was developed for this project and it hasn't been submitted for credit in another class. Even if it has, re-using code from a previous class is not necessarily a bad thing if you can show how the new application is substantially different and worthy as an independent credit.

If your only goal is to avoid punishment then note that many schools have "honesty policies" not "plagiarism policies". It's not enough just to avoid plagiarism, but you must be honest in how you conduct your work. Finishing a project in six months and pretending like it takes you another six months to finish might be just as punishable as plagiarism. Finishing a project in six months and telling the professor that you think you've met the requirements and you'd rather focus on finishing your other classes is not dishonest, although your professor might disagree.