Square Foot Gardening Apartment

How can I build an office out-house/pool-house/garden?
I have a first floor apartment with a huge courtyard. I am thinking of building another room in the courtyard (I already have two pre-built the outside). The pre-fabricated and are very expensive so I wanted through the construction of self. I'm looking for material welding home and am thinking of building a steel structure and wood paneled walls and roofs of corrugated steel. I live in a climate Tropical-ish then waterproofing and structural stability are important questions (many typhoons). I also am wondering if I can just extend providing an electrical outlet outside that I have to power the whole thing or do I need to take more comprehensive measures to provide electrical power safely to this thing? If someone did something or can point me to good reference material or even plans, would be greatly appreciated! Btw, I'm aiming to be about 500 square meters surface region.
http://www.lowes-home-improvement.com/category/home-improvement/home-repair/
Donna’s Square Foot Garden Part 3