Building a second storey over a one storey extension

[CoolShoeshine]

The original extension is about 30-35 years old so quite modern. If you have done this what do we need to take into consideration?
Will we have to have foundations checked?

You'll need to check the foundations can support a second storey and you'll need to investigate what planning is required as the single storey might have been done under permitted development.

We had a one storey done then built on top, we ensured we had the foundations done to accommodate a second storey when we were ready so yes I imagine building control would want to see that the foundations were sufficient for the second storey

It's likely the ground floor will have needed to be built to support another story. My FIL is a structural engineer and when we built our extension, he recommended we amend the original plans so that a second story could be added later. I'm not sure what the amendments were, but it wouldn't have been possible with our original plans to later add another floor.

Yes foundations check unless you know already, ie have the plans etc. planning...the architect will advise on. The big think ( and linked to foundations is what the structural engineer requires for steels and supports. Ie we had to have the existing wall supporting back of the house and new steels taken down, foundations drilled out and reinstated with reinforced concrete down 5 feet and special pad stones. However, when the new steel went in there was virtually no settlement. The builder did moan about how over engineered it was!