Cost-Plus vs Fixed-Price vs Open-Book: Builder Contracts Explained

There are really only three ways a builder can charge you, and everything else is a variation on them. Fixed price gives you one number for a defined scope of work. Cost-plus charges you what the labour and materials actually cost, plus an agreed margin. Open-book is cost-plus with the workings visible: itemised from the start, costs you can see, a stated margin, and reporting against the budget as the job runs. Each transfers risk differently, and each depends completely on one thing people rarely examine: how complete the drawings are.

Notice the pattern: the differences are less about price and more about who carries uncertainty and who can see it.

Because it is priced from drawings, and drawings for a one-off home never capture everything. A fixed price protects you for the scope that was defined. Anything outside it, the ground turning out different, a detail the plans did not resolve, an item nobody specified, is a variation, charged on top, and entirely legitimate under the contract you signed. The certainty is real but far narrower than the words suggest, and the narrower it is, the more it costs you in extras later.

There is a second cost that is easy to miss. A builder pricing a fixed contract is carrying risk, so they must price for it: allowances get padded, contingency gets buried in the number, and you pay a premium for the transfer. On a well-documented conventional build that can be a fair trade. On a one-off, you are often paying the premium and receiving the variations anyway. We go deeper in open-book versus fixed-price contracts.

Structurally, nothing. Paying what things actually cost, plus an agreed margin, is the most honest arrangement there is, and it means you are never paying a risk premium for uncertainty that did not eventuate. The problem is visibility. If you cannot see the supplier invoices, the hours and the allowances, you are being asked to trust a set of numbers you have no way to check, on the largest purchase of your life. That is a lot to ask of anyone, and it is exactly why cost-plus has a reputation problem.

The reputational damage is unfair to the structure and fair to the practice. The fix is not to abandon actual-cost pricing and go back to a padded fixed number. The fix is to open the books.

Open-book keeps the honesty of cost-plus and removes the trust problem by showing you everything. Before the build, you get an itemised, line-by-line proposal, so you can see scope, allowances and the margin stated plainly rather than hidden. During the build, you see the costs as they land and get a monthly report on exactly where the project sits against the budget. Nothing is discovered at the end, because nothing was concealed at the start.

The practical effect is control. When a cost moves, and on a one-off home something always does, you hear about it while there are still choices: adjust a selection, change a detail, use some contingency deliberately. That is a completely different experience from opening a final invoice and finding out. It is also why we run a monthly budget meeting on every project, and why how a builder prices your plans matters as much as the contract you sign.

Match the contract to the completeness of the documentation, not to which word sounds safest. If your plans are fully documented and the project is conventional, a fixed price is a legitimate and well-understood choice, and the risk transfer is worth what it costs. If you are building a one-off architectural home, where the drawings cannot possibly anticipate everything and the site has its own opinions, open-book prices the real thing rather than a guess, and keeps you in control of the budget the entire way.

Whatever you choose, hold a 10 percent contingency. No contract type removes the surprises a one-off build produces; contracts only decide who carries them and whether you can see them coming. If your builder tells you their contract makes a contingency unnecessary, that is the moment to be most careful.

Fixed price buys certainty for a defined scope and charges a premium for it, and on a one-off home the scope is never as defined as the drawings imply. Cost-plus is structurally honest but needs visibility to be trustworthy. Open-book gives you both: actual costs, a stated margin, itemised pricing and monthly reporting, so cost is something you steer rather than something you discover.

We build open book because it is the only version we would want on our own home. If you want to see exactly what that looks like line by line for a build in Nelson Tasman, arrange a consultation and we will show you a real proposal and a real monthly report.