Hi Glenn,
I do think although where the prices are fixed as lump sum contract, the builder can still cut corners by providing low quality or low workmanship. The word 'quality' is very subjective. Hence as Project Architects administrating the contract, we have to act impartial and always ask for high quality, if not the payments are not approved. I have done lots of Inspection of the Works for my own projects at work, and a lot of times they just try to cut corners or give you defective work, or bill you for incomplete work.
Eg, when allocated a $9000 for landscaping, the builder gave the client tiny little withered new plants. I told them the client doesn't even want them if they were for free and asked him to replant new plants.
Another time, a builder billed the client for roof frames that is only 70% complete,he was billing 100%, just so he can quickly get some cash to cover himself somewhere else.
Eg if an architect designed your home and administers the contract, the architect will represent you to do site inspections, making sure the building is built according to the documented drawings and specifications. Even if work is done, we still have to make a judgment on "quality". A lot of times architects have arguments with builders about what quality is acceptable. (not pointing fingers to all builders here. There are a lot of good builders who provide very good quality as well, sometimes give you more. We're talking about those dodgy builders here). So if its not acceptable quality or incomplete works, the architect as Contract Administrator just don't approve the progress payment and tell the Client to withhold payment or only pay a % of it if it is incomplete. So the architect is on the Client's side, representing the client as an impartial 'judge'.