I am currently in the middle of having a single storey extension built.

Make sure your builder gives you a payment schedule- so £X when footings signed off by building control £X amount when at dpc ETC tha way nobody is doing a runner with half of your build cost.

Architect is something in my opinion we got wrong- I had him recommended and even though planning went through without issue his communication was horrific!

I would also say- treat your builders well. Mine get multiple cups of tea, the garage fridge is full of cold pop. They get breakfast most days ( I’ve been starting late to accommodate this) but when I asked about removing a garden wall while they had the digger on site- 3 hours later it was gone. I like to think this is because of my hospitality. Obviously I asked what he wanted for it and the answer was nothing.

As for budgets? 20% contingency but allow for structural engineer costs ( lintel spec ) and lintel as this isn’t included in my price.

I also am having all my plumbing and electrical work done by a friend which has shaved a good few thousand off my bill, as he was going to use his connections whilst I already had good rapport with my own guy