Imo cost plus offers zero protection. I looked at builders who offered both kinds of contracts. I chose a hybrid contract with about 80% fixed and about 20% variable. We came in at about 3% over, which I understand is not too bad.

When we discussed this with our architect he said a lot of people who are building prestige properties do the cost plus because they are constantly changing and upgrading send adding. Since we had a tight spec sheet and virtually all of our selections made, he recommended fixed.

You can only be confident that a "Cost of the Work" contract will reduce costs if the GC is not doing too much of the work with his own forces and he bids each sub competitively and you (or your agents) are actively involved in the bidding/selecting process and there is a Guaranteed Maximum Price (perhaps with a Shared Savings to motivate the GC to look for savings).

If none of that is the case, the primary advantage of a Cost of the Work contract is to be able to start quickly with incomplete design documents and an approximate cost estimate/budget and/or a high Guaranteed Maximum Price if there even is one. This approach is for owners with high standards and a flexible budget.

Turnkey is too complicated to discuss without more information. Perhaps you mean a typical Lump Sum contract.

What if you have a firm budget and have about 10percent of that set back for surprises,have a pretty good spec sheet,not full blown architect spec sheet but a pretty good list,not many allowances needed.

The only dependable "cost plus" contract (if cost control is important) is a cost plus with a guaranteed max. With this type of contract an owner has assurance that the maximum construction cost will not exceed the guaranteed max, as long as there are no approved change orders. Cost plus contracts often have a provision to split any savings below the guaranteed max with the builder, commonly on a 50%-50% basis. This provides an incentive to the contractor to stay below the guaranteed max sum.

To achieve a guaranteed max, the contractor will include a sizable (undesignated) reserve to protect him/her from surprises. The same thing happens with lump sum construction contracts. Construction contracts are all about risk sharing and who bears the risk. Cost-plus, like time and materials, puts all the risk on owners.

Every builder has their own preferred contracts which favor them. Thus, for custom homes, owners have to identify what protects their owner's interests and negotiate the type of contract. For tract homes and for builder subdivisions, many builders may not negotiate the type of contract.

I have no idea what a "turn key" contract is.

In my experience turnkey means the builder is the owner of a project and responsible for all aspects of it including design and financing until it is turned over to the new owner at completion. I have seen it done with multi-family housing but not a single-family house. I suspect homebuilders might use modified versions of this contract type or make up their own definition of the term.

IMO the primary owner's risk is determined by the degree of control they have over quality and cost. For single-family homes in order to save the cost of a professional representative, most owners retain very little control over the design and construction process thereby increasing their risk. I can't say that is unwise but I would recommend using that savings wisely because it can sometimes come at a high cost.