I know of much smaller projects where the owners left it to the builders to carry out the work without professional supervision ..... and they got ripped off with a significantly higher bill to pay. And of course we hear of large public projects where the final cost is many times the original estimate because of clever clauses put into the contract by shrewd operators. The hideous modern Scottish Parliament building started out being a £40 million contract and ended up costing .....wait for it ...... £400 million! And there are tens of millions spent annually to correct faults. Will someone please blow it up!
The correct procedure is to employ a team of architects to project manage the job and sign off the work in progress at the end of each stage and approve a stage payment. This is the only efficient method of working..... once people of integrity are appointed.