Since you are actually opening a food service business, you will need to obtain a license from your county department of environmental health services or whatever they call it in your area. Here in NY there are variable fees for this license depending on what type of business you operate.
You should also check the zoning rules in your neighborhood. Some municipalities are very strict about running businesses out of your home and what types of businesses are allowed.
If you are preparing food for commercial sale, here are some building requirements that are pretty much universal:
All floors, walls and ceilings in food prep area must be smooth and washable.
All lighting fixtures need to have some kind of approved cover to protect against glass falling into food if a bulb breaks.
You must install sealed coping around the entire kitchen where the floor and wall meet. The coping prevents rodents from finding their way into the kitchen from the outside or through the walls.
You must have a bathroom with a toilet and handwash sink that is separately vented.
If you do not have a dishwasher, you must have a three compartment sink with an indirect drain. This is basically where the drain from the sink is separated from a hole in the floor that drains to the sewerage system and prevents any possible backups into the sink. Instead, it backs up onto the floor.
There must be a separate hand washing sink in the kitchen for people to wash their hands. Depending on your layout, I believe you will also need a hand wash sink in the food service area, though this seems to vary from state to state.
You must also install a separate mop sink and provide an area where cleaning supplies and chemicals are stored. This stuff must be kept separate from any food or food supplies storage.
Regarding these sinks, this means that generally, aside from the bathroom, you will need plumbing for all three, though I think they may share drains as long as they are indirect drains. It's a good idea to install grease traps as it will save you hassles in the long run, especially since it's likely you will be using and disposing of a higher level of fatty foods than the normal home. This stuff accumulates in drains and causes a lot of trouble for local water and sewer utilities-they may have some specific requirements for you too.
All food storage must be kept at least 6 inches from the floor. This is also the case for any utensils you may use to prepare the food.
All large appliances like refrigerators, freezers and stoves will need to be food service/commercial quality. All cooling appliances will need to have an accurate thermometer living inside them.
Make sure your place of business is connected to local water and sewer service, otherwise, you will have to pay for frequent water testing and the testing, maintenance and documentation of an approved septic field.
The fire department will require you to have fire extinguishers and if you have any kind of stove and/or oven, ventilation and some kind of fire suppression system.
You might also be required to take a food safety and sanitation course to maintain your license, though this might also vary from state to state and county to county.
To obtain your permit to operate in a facility that is being converted to a food prep and service operation, you will probably need to submit your plans to the engineering department at the health department. Best to be as specific as possible in your plans and as detailed as you can because they charge you (at least in NY) a new approval fee for each revision.
Contact the health inspector to get information about the local requirements before you start any building plans. Befriend the guy or gal and do plenty of sanitation and food safety research beforehand so that few of the department's requirements will come as a surprise (read, expensive) to you.
Do not forget liability insurance, which you will need to show before your are issued a permit.
Above all, get a license to operate from the local health authority, as it will protect you from many nasty fees and penalties that could affect you and your patrons if you don't.
This is just a start, there are plenty of other requirements.