According to The Real Tom Tom @ www.Love-Calculators.com:
As for the window resizing, I don't think my little bourgh would
require a permit for that. A little secret I've learned, most permits
might be based on some 'safety' issue, but many towns use it as a mean
to keep track of improvements and raise individual taxes. Oh my I
said it, the secret is out.

I used to think it was a combination of safety and taxes. Not as
much anymore.

Watch "Holmes on Homes" sometime[+].

Permits/inspections are primarily a method by which you the homeowner
doesn't get screwed by sloppy/incompetent/unethical contractors.

In most areas, anything more complicated than simple redecorating
needs permits (unless there's specific exemptions for it).

If the contractor says "you don't need a permit to add a new room"
for example, you'd better trust him a lot, because if he screws
up, you're screwed too.

Holmes says "if he says a permit isn't necessary, _run_!".

Holmes periodically has examples where the contractor
said "no permit is needed" (or lie and say "one's on the
way"), they botch the job thoroughly, and when you start
trying to get the contractor to fix it, they report you to
the municipality for not getting a permit.

The definition of chutzpah...

Think of a permit as an insurance policy. It doesn't ALWAYS
protect you, but, especially if you're not familiar with
proper building techniques, it has a very good chance of
catching something wrong before it becomes a major disaster
that costs more to fix than the original job cost in the first
place.

Unethical contractors will obviously prefer you didn't get
a permit, figuring that they won't get caught for poor
workmanship.

It isn't even necessarily unethical ones. Contractors have
their strengths and weaknesses. One that can build a good
house may suck at foundations.

There are times where I understand the work to be done well
enough (especially if I'm doing it myself) that I can
identify a good job, and I don't think I need a permit. But
if it's something I'm not competent enough to inspect it
properly myself, I'll insist on a permit.

[+] Mike Holmes is a Toronto-based contractor who has a TV
show on HGTV and a few other stations. His show is all
about the disasters contractors make, and their consequences.
A fascinating show. We've had our own "Holmes moments"
while renovating our bathroom (plumber practically severed two
adjacent joists to put a drain in, the tub shutoff valves were
hidden behind a "permanent" plywood wall. I'm having to
sister lumber in to reinforce the joists, and the shutoff valves
will be behind a door). The irony being that the plumber
who did the plumbing was building the house for himself...

[Holmes has been responsible for several contractors losing
their licenses and even one or two arrests for out-and-out
fraud.]
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.