This all sounds good--and it can be done--but you have some things to consider first. Depending on where you live, you probably will have to have a building permit. For that you will need relatively good drawings. There will be a building code that you will have to meet. One of the issues of that is that you will have to have electric outlets every 6 feet around the walls. I'm sure that there will be plenty of other similar issues.
It sounds good to just go ahead and make the addition but your municipality can and very well might make you tear it all down. Then they probably will give you so many problems that I will be impossible to satisfy them. And to a big extent, I can't blame them. Most of the items in the building code are there because someone was killed. An example is the spacing of the electric outlets. Most lamps, clocks, TV sets, et cetera come with 6 foot long cords. Before this spacing was specified, people were running extension cords all over the place and causing fires. People were killed. Now thay can not make you update existing construction to current standards, but they can make you build new projects or extensions to current standards and you should expect them to do their job. The rules are there for your safety. Bottom line--don't even think about a project like this without checking with your local building inspector to see what he will require. It could save you a lot of time, money and maybe your life.