I live oop norf and am always amazed when people say how much their extension costs I assume down south.
My builder quoted me for my single storey extension and was for first fix plastered shell. ie there are wires coming out the walls where sockets and lights would go and plumbing pipes sticking out the floor but it has been plastered out.
Then he invoiced me a day rate for his labour plus a skilled labourer and on other days a less skilled labourer. He phoned the builders' merchant and placed the order, handed the phone to me and I paid for the bricks, sand, cement etc. So the money didn't pass through the builder.
My extension came in at £1400 psqm. It basically came out to the same cost as what I was quoted for the whole job but I paid the builder and sub-contractors separately plus the builders' merchant. What ramps up the cost is then putting in a new kitchen and en-suite. So the extension was one cost the kitchen for mine was another altogether.
If I tell you that I needed a structural engineer, talked to them on the phone about what the issue was, sent them a photo of the exposed wall so they could confirm the thermalite block and brick outer wall construction of the original house, they drew up plans to embed a steel vertically into the wall to support the new horizontal steel so I don't have a nub of wall half way down my kitchen. Emailed over the plans with the full measurements/requirements on and charged us £160. That was it. Obviously building regs looked at this and signed it off.
Re saving money, if you cut corners you just regret it afterwards. There is a difference, I think, between getting a bargain and compromising on what you want.
Sourcing stuff cheaper is great but it eats into your time. I don't work so had a lot of time to dedicate to researching.