First response was; "You'd never get an Occupancy Permit".

I realize you'd need a sink, toilet.... electric service.
Much else ?
Hell... we do with a helluva lot less out camping.

What is this "Occupancy Permit" ?
Could a township require certain minimal living amenities ?

A town *CAN* require all kinds of bizzare things.

If you're regulated at all, you will
generally, need hot and cold running water,
a source of heat, insulated walls, a stove of some
sort, a shower or bath, a toilet, provision for
natural and/or artificial light and ventilation,
an electrical system that meet certain specifications*,
and frequently some minimum square footage, which
will vary by location but is unlikely to be less than 700sqft.

The most expensive and time consuming issue is probably
getting a legal septic systems put in, which requires permits,
and is the mechanism by which many municipalities regulate
their growth.

Other than that, there's no particular reason that you
can't just live in a giant studio apartment...

* The electrical code (NEC) wants you
to have a least 1 outlet within 6 feet of every point
along the wall, A light source of some kind that's
operated by a switch that you can reach from the
main door, 2:20-amp circuts with GFCI protected
outlets on the kitchen countertops, with no space more than
2' from an outlet. 1:20 amp circut with GFCI protected
outlets serving the bathroom, and, if you have them,
separate dedicated circuts for each of: the refridgerator,
the furnace, and (I think) the well pump.

Bedrooms (anywhere that people are going to sleep
regularly), need to have a second "means of egress"
which is usually a window big enough and low enough
to crawl out, and for places more modern than
1999, AFCI protected circuts for anyplace people
are going to sleep.

I dunno why you'd have a "garage" erected to live in
when it's not particularly harder or more expensive
to build a house than a garage, though. (well, except
that garages don't typically have plumbing).

--Goedjn