There are some differences between municipalities, but usually there is a Electrical Safety Authority (ESA) permit/inspection and a municipality building permit/inspection that also includes plumbing.
Not getting an ESA permit/inspection may result in fines for the electrician and potential trouble with your insurance coverage. You may need to remove drywall etc. to get an inspection done later.
Not getting a building permit may result in trouble with the municipality, i.e. an order to remove drywall to allow for an inspection of the insulation. For notable changes to the structure such as cutting the foundation wall for a larger windows or a walk-out, they'll ask for an architectural drawing approved by a professional engineer and good luck getting something like this after the fact. Moving support beams, etc will also fall into that category.
If you're going to rent it out, not following rules such as fire separation, smoke detectors, egress windows, could even result in criminal charges to yourself should something bad happen in the future (fire/death). On future resale, a savvy buyer could catch the lack of building or electrical permits and could use this as a price negotiation point or even walk-away. The original contractors don't care, because they have your money for what you paid them to do and it is not up to them to ensure you followed the bylaws and regulations.
If you're just adding stud walls, drywall, fixtures to an existing plumbing rough-in, then I'd recommend an ESA permit/inspection at a minimum. If you're doing more than that (cutting the foundation walls, breaking the floor for new drains, etc) then best to follow the formal route to get approved plans, permits, and inspections.
