Been discussed plenty of times on the forum here. Building regs. Here's my experience :

http://tintabernacle.blogspot.com/2019/10/building-regulations-plans-and-building.html?m=1

At the very beginning, around 3.5years ago, it wasn't clear to me at all what the difference between Planning Permission and Building Regulations are. I didn't understand why you need two different set of plans and what different professionals are involved and why.

It took me around 2 years to find this forum and get all the answers I needed and took me another 1 year of reading to gain the knowledge to tackle a lot of things myself.