How to heat an addition on a slab with no basement?

Enough electrical outlets in the room on different circuits? If so, they sell amazing space heaters. How about a pellet stove? They come in old fashioned, traditional to ultra modern styles to fit any decor. Take a look at this . . .

Is the floor finished? What kind of heat in the rest of the home? Oil, electric, or natural gas?

I have two DeLonghi oil-filled heaters that look like portable radiators that come on four wheels. One is used in my TV/sitting room on the first floor for comfort, and one is used in the basement utility room so pipes won't freeze during the winter...live in Maryland. They are available at HD and Lowes and cost varies according to size. The do draw minimal electric but would rather pay higher electric rates then replace frozen pipes. Set mine on low/#3 or #4, and it does the job well. Keep rooms shut off that you don't use often, and close closet and cabinet doors when not in use. It really saves energy.

If you have forced air in the rest of the house, run ducts in atic.

Those underfloor heating mats work really nicely if you need to put a floor down over the slab.

Mitsubishi Mini Split is an excellent choice. We had this system installed 3yrs ago and are 100% satisfied. It also is air conditioning in summer. Uses very little

electric. Info can be found on the internet. It only took 3hrs for professional installation.