Lots of engineers "consult" part time while having other 40 hr per week engineering jobs. For design, plan review, etc. the PE license becomes mandatory. For other things not related to design then maybe not so much. I've met a lot of utility engineers (gas and electric) that work for a large company and then moonlight doing small design jobs for electricians, contractors, etc. who need an occasional set of engineering plans. I've met several engineers in similar positions that moonlight teach training classes on codes, standards, etc for other companies than the one they work for. I've hired some of these types and they bill 4000-7000 per day doing the specialty technical training classes. They have very little overhead so most of the money they bill goes to their bottom line. Find a niche where you have contacts/experience, throw the shingle out and see what happens.