How do I turn a 12x28 portable building into an apartment for one?

The first thing you need to do is check local codes, Debbie. If it's unfinished, it will need plumbing, electric, and insulation. Once that is done, you can add finishes such as, sheetrock, doors, moldings, cabinets, appliances, and fixtures, both plumbing and lighting.

Of course you will need electricity to include a panel box with breakers You will also need plumbing and insulation which could be a problem. For any type of plumbing you will need not only a water supply, but also drain pipe for sewage. Most localities do not allow electricity or plumbing to be installed in portable buildings by code which means you won’t be able to insure the building.

Hi Debbie! That sounds like it will be a lot of fun bringing it all together as an apartment for one. Check out this information for ideas!

https://thetinylife.com/living-in-a-shed/

https://www.pinterest.com/glendabuckles/from-a-shed-to-a-home/?lp=true

https://housekaboodle.com/converting-sheds-into-livable-space-miniature-homes-and-spaces/

https://tinyhousetalk.com/woman-converts-barn-shed-into-192-sq-ft-tiny-home/

https://www.youtube.com/watch?v=diNEiyaZK-4

Check with county city codes. Some of the tiny houses have to be in a Camp ground type set up. Others in a tiny house community. Maximize your space the best you can. Storage underbeds, etc

Define "portable building?" Check local codes, where I live, they are not legal even if you have land for them.

If ok to go, you have great suggestions here. Could turn out really cute too.

Definitely check the codes about changing it into a dwelling. You will probably need permits and inspections on just about everything since you will be putting in plumbing, electricity, heat and A/C, Windows, insulation, etc. You may be looking at more than you want to get into.

you don't need sewage lines if you use incinerator toilet https://incinolet.com/?_vsrefdom=adwords&gclid=EAIaIQobChMIsrOLzazz3gIVdBh9Ch2qaQ2FEAAYASAAEgKcK_D_BwE or electric source if you do solar TINY HOMES https://www.thespruce.com/super-affordable-tiny-homes-that-will-inspire-3017220