Cost of knocking through 2 walls & adding roof to side return?

We are in the process of buying our first home and the kitchen is VERY small. Eventually we are looking to knock through to the dining room but I still don't think this will give us enough space (the width of the current kitchen is only 1.7m. There is an existing side return on the property (approx. 1.2m wide x 5m long) that has a sloping plastic roof. There is a step down into this side return and it is where the drainage from the kitchen sits also. So I guess my question(s) are: 1) Is it possible to put a roof on an existing side return? 2) If so, how much could it cost to put a proper sloping roof onto the side return? 3) How much would it then cost to knock through 2 walls of 3.8m w x 2.3m h (assuming both are structural) and levelling the floor in the side return to make it one big room with the kitchen/dining room & side return? Appreciate there's a lot of info there and happy to send further pictures if it helps. We're on a tight budget but want to make the most of the space.