My builder had been one for over 20 years so had the same team of plumbers/electricians etc.
He basically project managed it with me being here full time to nosey consult at any given moment.
Even the structural engineer only cost me £160 he didn't do a site inspection, we literally took a photo of the exposed construction so he could see what our house was made of, he emailed us the diagram with the necessary details in for the builder (and for building regs) as we didn't want a pillar of bricks, so we hid a vertical steel in the wall on which to rest the horizontal steel for the original back wall of the house.
All I had to do was be here to let the trades in. It was a simple kitchen extension. I made a ridiculous amount of cups of tea and biscuits but it was worth it.
I'll be honest, it went smoothly, cost me £300 more than expected due to building regs people asking for something, was done in the time frame expected. Builder was incredibly tidy, not cheeky, did not use my hoover or any of that crap. I paid him directly into his bank account for a week's work.
In the end I just wanted my house back but that was just because I was tired of it all. No complaints about the work or people.
You have to imagine yourselves on the other side of it. What is your extension giving you that you haven't got now?
It's a bit like a crap pregnancy - you think of the baby at the end of it.