Contract Structures for Interiors Projects: Where the Risk Sits

One question, four answers

Every structure is answering: who pays if the work turns out to be more than expected?

Structure	Quantity risk	Cost visibility	Suits
Lump sum	Contractor	Low — one number	A complete, frozen scope
Item rate	Client	High — per item	An evolving design, with measurement
Cost plus	Client	Very high — open book	Genuinely undefinable scope
GMP	Shared, capped	High — open book	Understood but not fully drawn

Lump sum

One price for a defined scope. The contractor absorbs the risk that quantities exceed their estimate, and prices an allowance for that risk which you pay whether or not it materialises.

It works when the scope is genuinely complete. It fails when it is not, because every gap becomes a variation negotiated after you have lost competitive leverage. The apparent certainty of the number at signature is exactly what makes an incomplete lump sum dangerous — it feels safe while it is not.

Item rate

Unit rates are agreed and actual work is measured against them. You carry the risk that quantities are higher than expected, and you gain visibility of what every element costs and the ability to change things without renegotiating.

It demands measurement discipline: someone on the client side checking quantities against a stated method, commonly referenced to IS 1200. Without that, item rate becomes an invoice you cannot challenge — and at that point you have taken the risk without the control it was meant to buy.

Cost plus

The contractor is reimbursed actual cost plus a fee. Complete transparency, no price certainty.

Appropriate where scope truly cannot be defined — emergency work, unknown existing conditions, or starting before design completes. It needs open-book records, an engaged client and, in almost all cases, a cap. Without a cap there is no structural incentive to control cost.

Guaranteed maximum price

Cost plus with a ceiling. You pay actual cost plus fee up to an agreed maximum; above that, the contractor carries it. Savings below are often shared on an agreed split.

It suits projects where the scope is understood in principle but not fully drawn, and it aligns incentives better than most structures. The requirements are real: open-book records, an agreed basis for the cap, clarity on what constitutes a change to the cap, and client-side capacity to review.

Hybrids are normal

Most real fit-out contracts are mixtures:

• Lump sum for the drawn, settled elements
• Schedule of rates for work behind existing finishes, services alterations, and anything depending on late decisions
• Provisional sums for elements not yet specified, converted to firm prices when they are
• Cost plus with a cap for genuinely unknown conditions

What matters is that the contract states clearly which parts are which, and how something moves between categories.

What to settle regardless of structure

• The scope, itemised, with exclusions written down
• Completion criteria — what must be finished, tested and handed over
• How variations are priced, and against what rates
• Documentation deliverables and format
• Retention percentages and release points
• The defects liability period, and whether it spans the relevant season
• Sectional completion, if the floor is handed over in phases

The honest summary

Scope definition matters more than structure. A clear scope makes almost any structure work; a vague scope makes every structure fail, just differently.

If you cannot describe what you are buying, that is the problem to fix first — no contractual mechanism substitutes for knowing what you want.

Standards referenced

Administrative provisions covering contracts and responsibilities in NBC 2016, Part 2; method of measurement for building and civil engineering works in IS 1200. Contract structure, terms and risk allocation for a specific project should be settled with legal advice.