New extension on top of existing extension so it becomes double storey?

No not until you have dug out a couple of test holes of the existing foundations and had them inspected by building control if they are happy with foundation yes you can if building contol fail them you may have to under pin existing foundation

The last answer was correct but if you already have a double extension then I would suspect that that the original foundations will be suitable but like the last answer you may need to see if your original foundation are suitable to take the extra weight but if not then you may try and find out if you can do this in timber but first check with your council. Good luck

Yes u can but it all depends on the foundations so u would have to dig a trail hole to expose the foundation for architect/ building control to see if they are deep enough and if not the then they could ask for concrete pads and steel colums and beams or would have to underpin existing foundations.

Planning permission would be required, investigations into existing foundations would need to be undertaken, it may have to be a lightweight construction, however it is possible if all these issues are addressed.