[joecaption]

Please go back and add your location to your profile. Without knowing where you are on the planet it's hard to make the right suggestions.
Just go to quick links to edit.
Have you been to your local zoning dept. and health dept. to see if you can even get a permit for this yet? Adding another bedroom is going to going to be the biggest issue.

Adding insulation under the slab will make the floors warmer.

 

[joecaption]

Hire building the foundation and rough plumbing out.
Done wrong the rest of the building is useless.
Make sure the sides of the foundation raise the side walls at least 6" above finished grade to prevent water damage to the sheathing and siding.
Google "stem wall construction".

 

[joecaption]

Take a look at some of the dozens of post on here in the past month about frozen pipes.
Places that have never had them freeze before are now having trouble.
Just make since to me to prevent all that by running all the plumbing through inside not outside walls when coming up with your plans.
Making sure your shower or tub control valve has an accessible panel behind it for service is also a good idea.
Any walls where large drains and supply's are run are a whole lot easier to work around if you use 2 X 6 walls, not 2 X 4's.