Is your employer unwilling to pay you if you work longer hours? If you want more work, ask him/her for it. If you want to work for yourself, do it. Do you think it is fair to use your current employer’s resources for work outside their firm? Example: Do you plan on buying a copy of NFPA, or will you use your current employer’s copy for referencing work that you will be doing on the side? Do you think any of the firm’s drafting details will end up in your work?
Ask your employer if he/she minds you using their resources to do jobs on the side? Also, ask him/her if they mind you quitting and taking some of their clients if your side work proves profitable.
I believe that it is a conflict of interest to try to do work on the side and work full time as an employee doing the same type of work. It is difficult to do without your employers blessing. And, if you ask your employer the question stated above, they would be nuts to say they did not mind if you were a valuable employee.