disclaimer: I'm not a lawyer so I can't give legal advice, etc etc. This is meant to just give you of an idea of what to consider
Ethical:
Only take on a job that you are qualified to do and can accomplish reasonable on schedule and budget (as much as any engineering project can).
Make sure this isn't anything in your contract with your current employer that prevents you from moonlighting.
ensure there are no conflicts of interest between your primary job and your side gig, or between various contracts
Legal:
see above.
check to see if there are any special requirements in your state: for carrying insurance, or structure for setting up an engineering business i.e. setting up as a professional corp or LLPC or LLP or PP etc
ensure you are set up for the necessary record keeping requirements
Business Ethics and Professionalism
There are ways that you could set up a business to limit your personal liability, really cheap ways too. But if you 'screw up' and leave your client hanging, you've destroyed your personal reputation. Besides the being-an-awful-human-being aspect of it all, you won't get future business.
I looked into setting up a side business, but the effort and investment wasn't worth the benefit. But my areas of proficiency don't lend themselves to side-gigs. I'd expect civil, structural, environmental, some mechanical, and some electrical to be in a better position for such work though.