You should put your freelance work in your resume
Your resume is there to highlight your skills, your experience, and your commitment to a job. Any freelance work you have helps show each of these attributes, showing a greater level of skill, further experience, and shows a passion for your field which will separate you from the rest of the applicants. As long as this experience did not conflict with your past employment, and was acceptable by them, there is nothing wrong in showing greater interest in your field.
You should inquire what the company policies are on holding a second job and freelancing
Different companies approach freelancing in different ways, and likely have a policy covering such behavior, such as non-compete clauses, conflict of interest, etc. You should inform yourself of such policies before applying or accepting a position, as regardless of what you tell them of your past freelance work, it will impact any future such work that you might do.
When might freelance work be a negative?
Freelance work would be a negative to a company only when it would interfere with the company's operations. Some possibilities would be:
- If you were to work on your freelance projects during work hours. You should absolutely never do this - there are legal, ethical, and more legal problems associated with such practices. If you have been disciplined, sued, or fired for such practices, you should stop doing that, and reconsider how to schedule your career and freelance work separately.
- If your freelance work would impede your availability to work. Quite simply, if your new job would require core hours, and potential extra hours 'on call,' and your freelancing would cause a conflict of availability in your new job duties, perhaps the two may have issues coexisting with each other. Even if the schedules do not actively conflict, but your extra activities cause problems in your sleep or home life that then impedes your work function, you might have a conflict.
- If your freelance work provides a conflict of interest. This is where things get to be harder. If your freelance work provides a conflict of interest - perhaps similar client space, or industry, you may have issues. Example - if a potential client were to approach a freelance web developer who also happens to work at a design company, there could be a potential conflict of interest over whether this client should be a personal client, or a client forwarded to the company. In short, you should have sufficient forms of ensuring there is no competition of markets at stake.
In each of these situations, the problem does not go away by simply hiding it from your resume - in fact, the problems remain but become much worse as you have hidden it from the company. In each of these cases you should re-evaluate if the company you are applying for is actually a good fit, and should address any potential issues in the interview.
Eg: While my last company did not have a conflict of interest with my Freelance work, I realize that should I accept a position here, I would have to make these certain changes in the scope of what contracts I accept on a freelance basis.