Fixed-price vs cost-plus contracts: which is right for you?

One of the biggest decisions in any construction project is the contract type. Fixed-price or cost-plus? Each shifts risk in different directions, and choosing wrong can cost you serious money — whether you're the contractor or the client. Here's how to think about it.

The two main contract types in plain language

Fixed-price (lump sum)

The contractor agrees to deliver the project for a single, agreed price. If costs run over, the contractor absorbs the loss. If costs come in under, the contractor keeps the difference.

Cost-plus (cost reimbursable)

The client pays the actual cost of materials and labour, plus an agreed margin (usually 10–25%) for the contractor's profit and overhead. If costs run over, the client absorbs it. If they come in under, the client benefits.

The risk shift — who carries what

This is the core decision. Risk has to live somewhere:

• Fixed-price: Contractor carries pricing risk, material price risk, productivity risk, weather/access risk
• Cost-plus: Client carries all of these — contractor is essentially being paid as a project manager

Fixed-price contracts are cheaper on average for clients but more variable — some projects come in fine, others have nasty disputes when scope creeps. Cost-plus contracts are more expensive on average but provide cost transparency.

When to use fixed-price (lump sum)

Fixed-price works best when:

• The scope is well-defined — complete plans, finishes locked, materials specified
• Site conditions are known — geotech done, no major unknowns underground
• The client has a strict budget — bonded buyers, fixed-fund developers
• Standard construction — new builds, kitchen renovations, bathroom fit-outs
• Time pressure exists — contractor motivated to finish on schedule

Pricing a fixed-price job (contractor perspective)

When you're a contractor pricing fixed-price work, your formula must include:

• Direct costs (materials + labour) — priced at current rates with margin for fluctuation
• Site overheads (site huts, security, water/power, supervision) — typically 5–8% of direct costs
• Head office overhead — 8–12% of direct costs
• Profit margin — 8–15% of direct costs (lower for competitive tenders)
• Contingency — 5–10% for unknowns and risk

Total markup: typically 30–45% on top of pure material + labour cost.

When to use cost-plus (cost reimbursable)

Cost-plus is the right call when:

• Scope is uncertain — heritage restoration, opening up old walls, working with unknown conditions
• Client wants design flexibility — high-end residential where finishes evolve during build
• Speed is critical and design isn't finalised — emergency rebuilds, fire/storm damage
• Specialist or risky work — industrial process construction, mining-related
• Trust relationship exists — contractor and client have worked together before

Cost-plus margin types

Cost-plus contracts come in several flavours:

• Cost plus fixed fee — flat rand amount as profit, regardless of total cost. Best for client cost control.
• Cost plus percentage — typically 10–20% of cost. Aligns contractor incentive with cost (the more they spend, the more they earn — risky for client).
• Cost plus with GMP (Guaranteed Maximum Price) — cost-plus up to a ceiling, then contractor absorbs overruns. Best of both worlds for many projects.
• Target-cost with shared savings — agree a target, share savings/overruns. Aligns interests but admin-heavy.

Hybrid approaches that often work better

Fixed-price with provisional sums

The bulk of work is fixed-price, but specific line items (e.g., kitchen fit-out, landscaping, specialist tile work) are kept as "provisional sums" — placeholder amounts that get adjusted to actual cost. Most JBCC contracts work this way.

Two-stage tendering

Stage 1: Cost-plus during design development and preliminary work to flush out unknowns. Stage 2: Fixed-price once scope is locked. Common on complex commercial projects.

Fixed-price with re-measurement

Rates are fixed but quantities are measured on completion. Used heavily in civil engineering and earthworks where quantities are uncertain but rates are stable.

What about JBCC and other standard contracts?

South African construction uses several standard contract suites:

• JBCC PBA (Principal Building Agreement) — most common for private-sector building. Usually fixed-price with provisional sums.
• NEC4 — flexible framework, can be fixed-price (Option A), cost-plus (Option E), or target-cost (Option C). Used in major infrastructure.
• FIDIC Red/Yellow/Silver Books — international, used on large projects. Red is re-measure, Yellow is design-build, Silver is EPC turnkey.
• GCC (General Conditions of Contract) — public-sector civil engineering standard. Re-measurable.

The contract type isn't just about who pays — it's about who decides. Fixed-price means the contractor decides how to deliver. Cost-plus means the client stays involved in every decision.

Common mistakes (both sides)

From the client side

• Going fixed-price with incomplete plans — every variation becomes an expensive negotiation
• Pushing the contractor below cost — they'll cut corners or claim variations aggressively
• Going cost-plus without admin capacity — you need to scrutinise every invoice, or costs explode

From the contractor side

• Pricing fixed-price too tight to win — losing money on the job
• Not pricing in contingency — every project has unknowns; budget for them
• Cost-plus without proper records — every hour, every invoice, every kilometre must be documented
• Mixing variation and original work — separate ledgers prevent disputes

The decision framework

Use fixed-price if:

• Plans are 95%+ complete
• Site is well-investigated
• You want budget certainty
• The job is straightforward

Use cost-plus if:

• Scope is uncertain or evolving
• Site has significant unknowns
• Speed matters more than budget certainty
• You trust the contractor and have admin capacity

Use hybrid (fixed-price with provisional sums) if you want the best of both — and that's exactly what JBCC encourages.

Quick reference

• Fixed-price typical markup: 30–45% above raw materials + labour
• Cost-plus typical margin: 10–25% (percentage) or fixed fee
• Most common SA residential: JBCC fixed-price with provisional sums
• Most common SA infrastructure: GCC re-measure
• Best for unknowns: Cost-plus with GMP
• Best for budget certainty: Fixed-price with locked scope