Andy DIY

I have a conservatory that was built before we moved in so it's probably 20 years old and I believe it did have planning permission. It's about 6 mtr x 2.8mtr, the left wall is single brick but built touching the garage wall so I suppose is double apart from 1 or 2 courses that go higher than the garage , the right side is double brick, both walls are 2050mm high with a angled glass bit on top giving the plastic roof a fall from 500mm to 100mm. The front has a dwarf wall with windows and double doors into the garden. This front wall finishes at the end of the setback original garage. I would like to build up about 200mm and put a light weight roof on with ceiling on the inside making it look more like an extension. I have dug down and have 500mm footing along the front, not sure about the side walls (would they have needed to be 1000mm deep?). I think I would like to put a RSJ across the front to support the minimal extra weight and help raise the front to ceiling height. Also the rear would need to be higher to increase the roof angle a bit as it is quite shallow. Just looking for thoughts on building control or if I can do it under maintenance, will the right/left footings need to be 1000mm deep to use the RSJ? Any advice would be good.