I have a roofed area in my backyard which I intend to close to make it an extension of the house and I want to use it as a workshop/office. The final room will be around 3 by 3 metres. I have attached a photo.

I want to keep it simple and cheap and council-friendly. I want to yank the window off the wall and put it on one of the timber walls. I don’t want it to be like a bedroom, it does not need electricity and no bathroom.
I know there are builders and I have talked to them but they don’t seem to be interested in what I want; they want to make the expenses to be no less than $50,000, which is ridiculous.
Any ideas will be appreciated, thanks.