I haven't tried this specifically BUT...
If you're developing in .Net, start SOLIDWORKS, set breakpoints in your code, use the "Debug" menu to "Attach to Process..." and select the SLDWORKS.exe process, then run your project. I'm not sure how it will behave though. I use this method when debugging SOLIDWORKS PDM add-ins.
Give it a try and see if it walks through your code.
Hope this helps,
Thanks to all for the input! I will give your suggestions a try and see which way works best. At the moment the most effective way seems to me to develop in a stand alone environment and transfer the code to the addin once it works...