If you do go ahead in the future you can agree to do what my builder did. I never paid him upfront for anything. He would put in the order in to the builder's merchant, pass the phone to me and I would pay for the materials myself.
At the end of the week the builder would invoice me for the 5 days he had done so 5 X his day rate then bill me for his labourers, one was skilled one was unskilled so slightly different amounts. I would then transfer the money straight into his bank account whilst he was still in my house, he could see the transfer and he would give me a receipt.
I paid all of the subcontactors directly, so plasterer, electrician, window company etc but the original quote for all the work included these costs so I knew in total how much it would cost me in total.
It came in pretty much on budget, building regs chap asked for something and we went halves on the cost so an additonal £300 for me and he was on time, turned up exactly when he said he would and he was incredibly tidy too. And sadly I cannot recommend him as he is no longer a builder after 20+ years of doing it he wanted to do something else.
I did view his work mid-progress at another property so I could see how tidy he was and talked to the customer. They did all the work outside but then had to knock through.
We did a kitchen extension so bear in mind that the build was to first fix, ie bare walls and wires sticking out the wall waiting for sockets and pipes sticking out waiting to be connected. Then we had to pay for the kitchen too. Even if it isn't the kitchen you are exending think about sockets, flooring, window and door locations.