"What happens if I work without a permit?"

The simple answer is: A $5,000 civil penalty, a violation from the Department of Buildings (DOB) for working without a permit, a hearing with the Environmental Control Board (ECB), more fines issued by the ECB which could be up to a few thousand dollars and a ton of paperwork to file to correct the violation.  Not so simple, eh?
Any time you disturb the existing building material or add something to a property (e.g. garage), you must obtain a construction permit from the DOB. Otherwise you are just one 311-call-by-a-neighbor from being visiting by a DOB inspector and issued a violation for working without a permit.

When don’t you need a permit?
Anytime you are doing cosmetic work such as following examples:

• Painting 
• Tiling 
• Installing cabinets 

Unfortunately, no matter how big or small your job is you have to go through the entire job filing process with the DOB. That means obtaining signed/sealed plans by an architect, filing all the required forms with the DOB, meeting with the DOB staff to get your job approved, pulling the work permits and finally signing off the job when construction is complete.

Play it safe and get the proper work permits before you start construction.