Actually, no, not stressful. There were tricky bits such as dealing with rainwater drainage. Other than that the builder was great and quiet. Arrived when he said he would etc.
This was a fairly straightforward rear extension. We were just within the larger extension permitted development rights. Roughly 5m wide, 4m deep single storey. This gave us a new sitting room. We then remodelled the kitchen.
DH is a very experienced DIYer and used to be a Part P electrician. He hadn't worked on building sites but he had worked on renovations.
DH did a lot of the labouring work eg he dug out the trenches for the footings. Tricky access meant it would have to be dug out by hand whoever did it.
Every day after the builder had finished DH would sweep up, re-stack the bricks ready for the next day.
We employed the builder to do brick, block and roof. DH did everything else: electrical work, dry walling, windows and doors, kitchen fitting etc.
My advice would be to really educate yourself about the basics and get to know terminology. If it is still there gov.uk website has a really good planning portal.
If possible isolate the building area. We have a nervous dog so DH built a partition where the knocking through was done so the dog couldn't see what was happening. Dog was happy, builders weren't barked at.
We planned everything in stages so that we always had space to sit and a functioning kitchen.