Lump Sum or Cost Plus? Choosing the Right Construction Contract in Saudi Arabia

Two pricing models, two very different ways of carrying risk. Here is how each behaves when drawings change, prices move or the programme slips.

The contract you sign decides who pays when reality differs from the drawings. A lump sum shifts that risk to the contractor and prices it into the number you accept. Cost plus keeps the risk with you and pays the contractor a fee for managing it. Neither is generous or predatory by nature; they simply suit different levels of certainty about what you are building.

Lump sum at a glance

One fixed price for a defined scope. The contractor absorbs quantity variations inside that scope and carries the cost of ordinary inefficiency, so the price includes a contingency you do not see itemised.

Cost plus at a glance

The contractor is reimbursed actual cost plus an agreed fee or percentage. You see supplier invoices, and savings stay with you, but so does exposure when quantities or prices move.

Price certainty

Lump sum: You know the number on day one, which is the entire point. The trade-off is that unused contingency stays with the contractor.

Cost plus: The final figure is only known at the end. Budgets need a working ceiling and a rule for what happens when it is approached.

Handling of changes

Lump sum: Every change becomes a variation order priced outside the original competition, which is where lump sum projects usually leak money.

Cost plus: Changes flow through as cost, which is simpler, but nothing forces the discipline of asking whether the change is worth it.

Supervision effort

Lump sum: Lighter on procurement, heavier on specification. You must police quality against the written spec rather than against price.

Cost plus: Heavier throughout. Someone competent must review purchase decisions, rates and labour allocation continuously.

Quality risk

Lump sum: Because savings belong to the contractor, substitution pressure is real. Named products and approval procedures are the defence.

Cost plus: Little incentive to cut corners on materials, but also little incentive to buy efficiently unless the fee is fixed rather than a percentage.

Programme behaviour

Lump sum: Delay hurts the contractor directly, so the schedule tends to be defended, sometimes at the expense of finishing quality.

Cost plus: Delay costs you preliminaries and site overheads, so time control needs its own contractual teeth.

When lump sum wins

• Drawings, specifications and finishes are complete and approved before tendering.
• You need a number you can take to a lender or a board and defend.
• You have limited time to supervise procurement decisions week by week.
• The site is well understood and soil, access and utilities hold no surprises.

When cost plus wins

• Design is still evolving and you want to start enabling works early.
• The scope involves renovation or unknown existing conditions.
• You have the capacity to review invoices and approve purchases properly.
• You want transparency on material choices rather than a sealed price.

How to decide

Choose lump sum when the design is finished and you value a defensible number more than visibility. Choose cost plus when the design is genuinely incomplete or the works are exploratory, and only if you have someone able to review costs every week.

Before deciding, look at three documents: the completeness of the drawing register, the bill of quantities and the list of provisional sums. A tender full of provisional sums is a cost plus arrangement wearing a lump sum label.

If you are preparing to build, Domira can review your drawings, scope and contractor offers before you commit, and supervise delivery against the programme and specification you agreed.