Simplest structure that meets requirements for a govt backed mortgage

[Winn Cook]

Looking to acquire vacant lot with cash, and to build the smallest and simplest possible structure to meet the requirements for refinancing to a traditional mortgage. Are there specific requirements as far as condition, sqft., floorplan, etc. to get a traditional mortgage for myself or a prospective buyer? Of course there needs to be electricity, water, appliances, a full bathroom, and a bedroom, but how small can it be? 

I can find regulations on what makes a prefab or a double wide financeable, and it usually hinges on the materials being used and being affixed to a foundation, but striking out as far as what defines a 'home'. Any help or information on this will be greatly appreciated. Thanks!