Cost Plus Contracts UK: Construction Guide (2025)
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Construction projects rarely unfold exactly as planned. Scope shifts, ground conditions surprise everyone, and material prices move. For work where the full picture cannot be fixed at the start, a cost plus contract can offer a workable middle ground between rigid lump sum pricing and total open-ended risk.
Under this arrangement, the employer reimburses the contractor for actual costs properly incurred, then pays an additional fee on top for profit and overheads. It is a model often seen on refurbishment jobs, heritage works, early-stage developments, and fast-track projects where design is still evolving.
This guide walks through how cost plus contracts work in practice under English law, where they sit well, where they carry hidden traps, and what to consider before agreeing to one. If you are weighing up pricing models for a build or renovation, understanding the mechanics matters as much as the headline number.
What this document is
A cost plus contract, sometimes called a cost reimbursable contract, is a construction agreement where the employer pays the contractor the genuine cost of carrying out the works, plus an agreed additional amount to cover profit and head office overheads. The reimbursable element typically covers site labour, subcontractor invoices, plant and equipment hire, materials, site running costs, and defined preliminaries.
The 'plus' component can be structured in several ways: a fixed lump sum fee, a percentage of the prime cost, a sliding scale, or a target cost arrangement with pain and gain share provisions. Standard forms such as the JCT Prime Cost Building Contract and NEC4 Option E (Cost Reimbursable Contract) set out accepted frameworks for how this works in the UK market.
Because payment tracks actual expenditure, accurate record keeping, open book accounting, and clear definitions of what counts as a reimbursable cost are central to making the arrangement work fairly for both sides.
How to use this document
- Decide whether the model fits your project. Cost plus works best where scope cannot be fully defined upfront, such as urgent repair works, heritage restoration, or projects where early contractor involvement is needed before the design is complete. If the works are well-defined and competitive tendering is possible, a lump sum contract will usually give better cost certainty.
- Choose a standard form or draft bespoke terms. In the UK, the JCT Prime Cost Building Contract and NEC4 Option E are the main recognised forms for reimbursable work. Using a familiar standard form reduces drafting disputes later. If bespoke terms are used, make sure the definitions of prime cost, disallowed cost, and the fee calculation are watertight.
- Define reimbursable costs precisely. The contract should spell out exactly which categories of expenditure qualify for reimbursement, what evidence the contractor must provide, and what is excluded. Common exclusions include rectification of defects, costs caused by contractor negligence, and head office overhead already covered in the fee.
- Agree the fee structure and any cost ceiling. Decide whether the fee is a fixed sum, a percentage, or tied to a target cost with shared savings and overruns. Many employers insist on a guaranteed maximum price or cost cap to protect against runaway expenditure, which shifts some risk back to the contractor above the agreed ceiling.
- Set up open book reporting and audit rights. Because payment depends on actual cost, the employer needs genuine visibility of invoices, timesheets, and purchase orders. Build in regular cost reports, an audit right, and a clear process for challenging items the employer believes should be disallowed. Without this discipline, the arrangement can drift.
Common questions
Q When is a cost plus contract the right choice?
This model suits projects where scope is genuinely uncertain at the outset, such as refurbishment of older buildings, emergency repair work, or jobs where the contractor is brought in early to help develop the design. It can also work where speed matters more than fixed pricing. For straightforward new-build projects with a complete design, a lump sum contract usually delivers better value and clearer cost control.
Q Who carries the risk under a cost plus contract?
In a pure cost plus arrangement, the employer carries most of the cost risk because they pay whatever the works actually cost. The contractor still carries risk for its own negligence, defective work, and any costs excluded by the contract. Many UK employers address this imbalance by adding a guaranteed maximum price, a target cost with pain share, or tight rules on what counts as a reimbursable cost.
Q How is the contractor's fee usually calculated?
The fee can be a fixed lump sum, a percentage of the prime cost, or a sliding scale that reduces as expenditure grows. A fixed fee gives the contractor less incentive to inflate costs, which is why many employers prefer it. Percentage fees are simpler but can create perverse incentives, since the contractor earns more as costs rise.
Q What is a guaranteed maximum price?
A guaranteed maximum price, often shortened to GMP, is a cap on the total the employer will pay under the contract. Below the cap, costs are reimbursed in the usual way. Above it, the contractor absorbs the overrun. This structure blends the flexibility of cost reimbursement with some of the certainty of a lump sum arrangement and is commonly negotiated on larger UK projects.
Q Does the Construction Act apply to cost plus contracts?
Yes. The Housing Grants, Construction and Regeneration Act 1996, as amended, applies to most construction contracts in the UK regardless of pricing model. This means statutory rights to interim payment, payment notices, the right to suspend for non-payment, and the right to adjudicate all apply. Make sure the contract's payment mechanism complies with the Act.
Q What records does the contractor need to keep?
Detailed records are essential. The contractor should retain timesheets, wage records, subcontractor invoices, purchase orders, delivery notes, plant hire dockets, and any other evidence of expenditure for which reimbursement is claimed. Open book accounting is standard, and the employer typically has the right to audit records during the project and for a period afterwards.
Q Can disputes be resolved through adjudication?
Yes. Any party to a construction contract covered by the Construction Act can refer a dispute to adjudication at any time. Disputes on cost plus contracts often concern whether specific items qualify as reimbursable cost, whether work was within scope, or whether the fee has been correctly calculated. Adjudication gives a binding interim decision within 28 days, subject to later court or arbitration proceedings.
Sources
This guide is based on primary UK law and official guidance.
- LegislationHousing Grants, Construction and Regeneration Act 1996legislation.gov.uk
- LegislationLocal Democracy, Economic Development and Construction Act 2009 (Part 8 amendments)legislation.gov.uk
Legal disclaimer
This article is for general information only. It is a tool to help you find your way — not legal advice, and not a substitute for speaking to a qualified adviser about your situation.