It all depends on what type of room addition You want. Is it a bathroom addition, bedroom addition? Typically when there is a bath room addition the cost will be in the ball park of $7500.00 for a basic bathroom. A Living room addition will Typically be around $20,000.00 for 400 square feet.
Depending on the size, but typically You will need to have the deck engineered for plans review from the county before You begin construction?
Normally a slab for any house or garage structure is a minimum of 4 inches thick with a 12″ x 16 ” footing. For heavy vehicles I would recommend a 6″ slab
Yes, You have to treat the area that is getting the concrete poured to prevent termites from getting into the wood structure of the home. then the inspection comes next.
It depends on the size and what type of project that Your wanting done. The building department has a schedule they use to determine the cost of the project permit.