Permits:

Most cities require that homeowners obtain a building permit before making modifications to their residence. Which modifications require a permit vary by city. Also, some cities are more vigilant than others in enforcing permit laws.

In order for the homeowner to receive a permit, the homeowners are required to file plans and pay fees to the city. In addition, the improvements are given a value. If they increase the value of the property, this may result in an increase in property taxes. Inspections are often required, and this means having to schedule and then wait for inspectors to approve the work to be done. This process can be time consuming and inconvenient in the short run.

Consequences of doing improvements without permit:

If a permit is needed and you fail to get one, the city may discover this at some time in the future and getting a permit retroactively can frequently be significantly more expensive and much more problematic than having obtained the permit before work commenced. If works are not done in accordance with city procedures or if the inspector is unable to determine if the work has been done properly, the homeowner could be required to open walls, tear up floors, so that the inspection may take place.

As mentioned, this can be very costly and time demanding to you.  Please save yourself the trouble and get the permit.