Verified answer

Nothing to stop you in most places if you submit plans, apply for permits & get inspections for code compliance

.Also keeps your insurance company happy.

Your local bldg inspector can walk you through the process of being your own GC & cite bldg regs.( if he's not hanging out in a bar like the one in my town).

You will still need the local utility co's to do electric and water tie ins & provide meters.

Best regards.

Edit:

Smokey has a good point.

Electricity is something you have to be carefull, maybe to save hundreds you can loose thousands. The wiring of a house is not that simple: type of wire,guage,amps,breakers a lot of thing to handle. In some State the electrical wiring can only be perform and certified by electricians. Its money well invested (its your home right).If i live near texas i will be more than happy to help.

Speak to the building inspector. You can build it yourself, but it better meet code. Have the building inspector check what you are doing along the way. They can be helpful.

at 250 sq. ft. there wouldn't be enough room for the appliances and a bed.

If it isn't done to code, you'll have to rip it out. That'll cost you more.