Lump Sum

A lump sum contract is the most widely used form of construction agreement. The contractor agrees to complete the full scope of work for a single stated price, regardless of what the work actually costs to deliver. It is also called a fixed-price contract or a stipulated sum contract. The CIOB Code of Estimating Practice defines it as a fixed price contract where contractors undertake to be responsible for executing the complete contract work for a stated total sum of money.

If the contractor's actual costs come in below the lump sum, they keep the difference. If costs exceed it, they absorb the loss. This structure places price risk firmly on the contractor's side, which makes it straightforward for clients to budget - they know the number before work starts.

When Lump Sum Pricing Is Appropriate

Lump sum pricing works best when the scope is well defined before the contract is signed. The more complete the drawings, specification, and schedule of works provided to tenderers, the more accurately a contractor can price the job - and the less risk that the lump sum becomes a flashpoint for dispute once construction is underway. Where scope is vague or incomplete, contractors build in higher contingency allowances to protect themselves, which inflates the tender price without any corresponding benefit to the client.

It tends to be less appropriate where the work is complex, the design is not yet resolved, or where an immediate start is needed before documents are ready. In those situations, a cost-plus or target cost contract is often a better fit.

What Can Change the Lump Sum

Despite the name, a lump sum is not always a truly fixed price. Several contract mechanisms allow the sum to change after the original agreement is signed.

Variations are changes in the nature or extent of the works instructed by the client or their representative. Most standard-form contracts include provision for the contract administrator to issue variation instructions, and each variation is valued and added or deducted from the contract sum.

Provisional sums are budget allowances included in the original price for work that could not be properly defined at tender stage. Once that work is instructed and priced, the provisional sum is replaced by the actual cost.

Fluctuation clauses appear in longer-duration contracts to allow for material or labor cost changes over the course of the project. Shorter projects typically use a firm price without fluctuations.

For contractors, the practical discipline on a lump sum project is tracking actual costs against the agreed price throughout the job - not just at the end. If material costs or labor hours begin to overrun, identifying this mid-project allows the contractor to assess whether a justified variation exists, tighten site management, or at minimum understand the likely final margin position before submitting the final account.