Open-Book Costing

Open-book costing is a contract arrangement in which a contractor provides the client with access to their actual cost records throughout the project. Rather than agreeing a single fixed price upfront, the contractor invoices for verified direct costs - labour, materials, plant, and sub-contractor charges - plus an agreed percentage or fixed sum for overhead and profit. The client can review, and in some cases audit, the underlying records at any point. This transparency distinguishes open-book arrangements from conventional lump-sum or fixed-price contracts, where the contractor's actual costs are their own affair.

The approach is most common in construction, facilities management, and complex service contracts where the scope of work cannot be fully defined at tender stage and fixed pricing would either carry a large risk premium or generate a string of variation disputes. Framework agreements and NEC contracts regularly use open-book mechanisms, particularly under Option C (target cost with activity schedule) and Option D (target cost with bill of quantities).

When Open-Book Costing Applies

Three conditions typically drive a move to open-book arrangements. First, genuine scope uncertainty: if the precise quantity of work cannot be determined before it starts, asking a contractor to commit to a fixed price places disproportionate risk on one party. Second, a collaborative client-contractor relationship where both parties benefit from transparency and shared cost control. Third, public sector and regulated procurement: many government frameworks require open-book reporting as a condition of contract, allowing spending to be audited and benchmarked against market rates.

In facilities management and reactive maintenance contracts, open-book is often the only practical basis for pricing. A contractor managing an estate of properties cannot predict the volume or nature of reactive callouts in any given month. Open-book lets them operate at agreed rates, with the client paying for what was actually delivered.

What Open-Book Costing Demands of Contractors

Open-book arrangements put a premium on clean, auditable cost records. A contractor who cannot produce clear, project-level records of their actual labour, materials, plant, and sub-contractor costs is in a weak position when a client's auditor requests documentation. Every cost must be attributable to the specific project and, ideally, to the specific work package within it.

For sub-contractors working under an open-book main contract, the transparency obligation typically flows down the supply chain. Main contractors will often require sub-contractors to share their own supplier invoices and labour timesheets to demonstrate that the costs being passed through are genuine. This makes accurate job costing a precondition for operating on open-book contracts, not an optional management tool.