Cost Plus Contracts: Legal Essentials In New Zealand

If you run a small business in New Zealand (especially in the trades, construction, manufacturing, or project-based services), you’ve probably had this moment: a client wants a price, but the project scope isn’t fully locked in yet.

That’s where cost plus contracts can be a great fit. They can give you flexibility, protect your margins when costs move, and help you start work sooner instead of waiting weeks for every detail to be finalised.

But cost plus contracts can also create disputes if they’re not drafted properly. The “plus” part (your fee), what counts as a “cost”, and how you prove your costs are all common flashpoints.

Below, we break down how cost plus contracts work in NZ, the legal issues to watch for, and the key clauses you’ll want in place so you’re protected from day one.

What Is A Cost Plus Contract (And When Does It Make Sense)?

A cost plus contract is a contract where your client agrees to pay:

• Your actual project costs (for example materials, labour, subcontractors, hire equipment), plus
• An agreed fee (either a fixed amount, a percentage margin, or a formula) for your profit/overheads.

In simple terms, instead of a single fixed price, the final price changes depending on what the work actually costs.

Common Situations Where Cost Plus Contracts Are Useful

Cost plus arrangements are common when pricing is hard to lock down upfront, such as:

• Renovations where hidden issues might be discovered once work starts
• Commercial fit-outs where design and scope evolve during the build
• Repairs and maintenance where the extent of damage is unclear
• Supply and install projects where lead times and costs fluctuate
• Projects needing urgent commencement before the final specifications are confirmed

They can also be useful where your client wants transparency and is willing to pay for actual costs, rather than paying a premium “risk buffer” that you might otherwise build into a fixed price.

Cost Plus vs Fixed Price (Why Clients Sometimes Push Back)

Clients often like fixed price contracts because they feel certain about the budget. Cost plus can feel open-ended if you don’t put boundaries in place.

From a legal and relationship perspective, the goal is to make your cost plus contract feel transparent and controlled-so the client understands how you charge, and you’re not stuck arguing about invoices later.

How Do You Structure “Costs” And “Plus” So It’s Enforceable?

The biggest mistakes we see with cost plus contracts happen when the parties agree on the concept (“we’ll do it cost plus”) but don’t properly define what that means in writing.

If your contract is vague, disputes can quickly turn into “he said / she said” territory-which is where time and money get burned.

What Counts As “Costs”?

Your contract should clearly define what is included as a reimbursable cost. Depending on your business, this might include:

• Materials (and whether delivery/freight is included)
• Direct labour (and whether it’s charged at hourly rates, day rates, or actual payroll cost)
• Subcontractors and specialist consultants
• Equipment hire, vehicles, fuel, site utilities
• Consent fees, inspections, compliance documentation
• Waste disposal and site clean-up

You also need to decide (and document) whether you can charge for “indirect” costs like:

• Project management time
• Administration time
• Travel time
• Tooling and consumables
• Insurance allocations

There’s no single right answer. What matters is that the contract makes it clear, so your client can’t later argue that these costs should have been included in your margin.

What Does The “Plus” Part Look Like?

The “plus” is usually one of the following:

• Percentage margin (e.g. 15% of costs)
• Fixed fee (e.g. $25,000 project management fee)
• Hybrid (e.g. fixed fee for labour management, plus a percentage on materials)

If you use a percentage margin, be careful about whether it applies to:

• all costs, or only some categories (e.g. materials and subcontractors)
• GST-inclusive or GST-exclusive amounts
• costs you incur internally (your own staff) vs external invoices

It’s worth getting this drafted cleanly. A small ambiguity around “does the margin apply to X?” can turn into a major payment dispute when the project gets expensive.

Record-Keeping And Proof Of Costs

Cost plus contracts rely heavily on documentation. Your agreement should set expectations around what evidence you’ll provide, such as:

• supplier invoices and receipts
• timesheets and labour records
• subcontractor invoices
• variation approvals and supporting quotes

Many contracts also include audit-style rights (for example, a client can review your records on reasonable notice). If you include this, make sure it’s practical and protects confidential information (like supplier pricing you don’t want shared).

Key Clauses To Include In A Cost Plus Contract

A good cost plus contract doesn’t just explain pricing-it sets the ground rules for scope, changes, timing, risk, and what happens if things go wrong.

Here are clauses we commonly recommend for NZ businesses using cost plus contracts.

1. Scope Of Work And Deliverables

Even in cost plus projects, you still need a written scope. Without it, the client may assume certain work is included, and you may assume it’s extra.

Where the scope can’t be fully defined upfront, you can use:

• a clear description of what’s known now
• assumptions (e.g. site access, working hours, availability of services)
• a process for agreeing any additional items

If you provide services generally (rather than a one-off project), it might make sense to document the relationship using a Service Agreement and attach scopes of work as schedules.

2. Variations And Change Control

Cost plus doesn’t mean “anything goes”. You should still have a tight variation process so you can manage cost blowouts and client expectations.

Your variation clause should cover:

• how changes are requested and approved (email? signed variation? portal?)
• who has authority to approve changes on the client side
• how you price changes (still cost plus? different rates?)
• time impacts (extensions of time)

This is especially important if you’re working under a broader commercial arrangement or multiple stakeholders.

3. Payment Terms And Invoicing Rules

One of the fastest ways for a cost plus contract to go off the rails is unclear invoicing. Your contract should spell out:

• invoice frequency (weekly, fortnightly, monthly, milestone-based)
• what supporting documentation is provided with invoices
• when payment is due (e.g. 7 days, 14 days)
• interest on late payments (if applicable)
• disputed invoice process (what happens if the client queries an item)

If you’re supplying goods as well as services, it’s also common to use broader Terms of Trade to cover credit, delivery, title/risk, and enforcement options.

4. Caps, Estimates, And Budget Guardrails

Many clients will agree to cost plus only if there is a budget ceiling, or at least a requirement to notify them if costs are tracking over an estimate.

Common approaches include:

• Estimate only (not a cap, but used for planning)
• Cost cap (you can’t exceed it without approval)
• Client approval thresholds (e.g. you must get approval if costs exceed estimate by 10%)

These guardrails can reduce disputes and help the client feel comfortable, while still protecting you from absorbing unexpected costs.

5. Timeframes, Delays, And Dependencies

Cost plus projects often shift as scope evolves. Your contract should explain:

• expected timeframes (even if indicative)
• dependencies (client approvals, access to site, materials lead times)
• what happens if delays are outside your control
• whether you can suspend work for non-payment or lack of instructions

If you don’t document this, clients may assume delays are your responsibility-even where they’re caused by design changes, supply issues, or late decisions.

6. Liability, Insurance, And Risk Allocation

Even with friendly clients, things can go wrong: defective materials, subcontractor issues, or accidental damage on site.

Your contract should deal with risk allocation and include (where appropriate):

• insurance obligations (public liability, contract works, professional indemnity)
• limits on liability (where legally enforceable)
• who bears the risk for client-supplied materials or instructions
• exclusions (e.g. consequential loss)

If you’re not sure what a clause actually achieves in practice, it’s usually better to get it reviewed than assume a template will protect you. A tailored Contract Review can often identify hidden risks before you sign.

7. Termination And Exit Rights

Cost plus jobs sometimes end early-maybe the client’s funding changes, the scope reduces, or the relationship breaks down.

Your termination clause should clearly cover:

• termination for convenience (and the notice required, if any)
• termination for breach (including non-payment)
• what happens to materials ordered, subcontractor commitments, and work in progress
• final invoicing and handover obligations

This is about avoiding the nightmare scenario where you’ve incurred costs but the client refuses to pay because “the job didn’t finish”.

What NZ Laws Affect Cost Plus Contracts?

Cost plus contracts don’t exist in a vacuum. Several key NZ laws can affect how your pricing, statements, and contract terms play out.

Here are the big ones to keep on your radar.

Construction Contracts Act 2002 (Payment Claims And Disputes In Construction)

If your work is in the building and construction space, the Construction Contracts Act 2002 is often central to how payment disputes play out in practice. Among other things, it sets up a statutory process for making and responding to payment claims, and it allows disputes to be quickly determined through adjudication.

Whether (and how) the Act applies can depend on the type of work and how your contract documents payments. If you’re relying on cost plus invoicing, it’s worth making sure your contract and your payment claim process line up with the Act’s requirements.

Fair Trading Act 1986 (Pricing And Representations)

The Fair Trading Act 1986 is a major one for any business communicating pricing to customers. Even if you’re using cost plus, you need to be careful about:

• advertising or quoting in a way that could be misleading
• giving “estimates” that are presented like fixed prices
• failing to disclose key assumptions behind your estimate

A practical tip: if you provide an estimate, label it clearly as an estimate, explain what it’s based on, and include the circumstances where it could change (for example, material price rises, unknown site conditions, scope changes).

Consumer Guarantees Act 1993 (If You’re Dealing With Consumers)

If your client is a consumer (rather than a business client), the Consumer Guarantees Act 1993 can apply. This affects guarantees around services being carried out with reasonable care and skill, completed within a reasonable time, and fit for purpose (depending on what was agreed).

Whether a customer counts as a “consumer” can be fact-specific. If you do both residential and commercial work, it’s worth getting advice so you don’t accidentally rely on terms that won’t hold up.

Contract And Commercial Law Act 2017 (General Contract Rules)

The Contract and Commercial Law Act 2017 consolidates key rules that affect commercial contracts in NZ. You don’t need to memorise it, but it’s helpful to understand the bigger picture:

• your written terms matter
• clear variation and payment rules reduce disputes
• if a term is ambiguous, a court may interpret it against the party who drafted or proposed it (depending on the circumstances)

This is another reason it’s smart to use a properly drafted agreement rather than informal emails and assumptions.

Health And Safety At Work Act 2015 (If You’re On Sites Or Managing Contractors)

If your cost plus project involves physical work (especially construction and trades), you’ll also need to stay on top of health and safety obligations under the Health and Safety at Work Act 2015.

This isn’t just “paperwork”. It affects how you manage subcontractors, site risks, and who does what. Even where a client is paying your costs, you can’t “contract out” of core safety duties.

Common Disputes In Cost Plus Contracts (And How To Avoid Them)

Most disputes don’t start because anyone is trying to do the wrong thing. They usually happen because expectations weren’t aligned early, and the paperwork didn’t force clarity.

Here are the most common pain points we see with cost plus contracts-and how to reduce the risk.

Dispute 1: “That Cost Shouldn’t Be In There”

This happens when “costs” aren’t clearly defined. For example, the client might accept materials and subcontractors but dispute admin time, project management, or travel.

How to avoid it: list categories of costs, define rates where applicable, and specify what evidence is provided with invoices.

Dispute 2: “I Didn’t Approve That Spend”

Cost plus can lead to uncomfortable conversations if the client expects to approve purchases (or subcontractors) but the contract doesn’t require it.

How to avoid it: include approval thresholds and a variation/approval process for major cost items.

Dispute 3: “Your Margin Is Too High / Not What We Agreed”

This typically happens where margin is described casually (“cost plus 15”) and then applied in a way the client didn’t expect.

How to avoid it: state exactly how the “plus” is calculated, and on what base amounts (GST exclusive? all costs? only certain costs?).

Dispute 4: Cashflow Problems And Late Payments

Cost plus contracts often involve ongoing costs you’re paying upfront. If the client pays late, you can end up funding the project yourself.

How to avoid it: set firm payment terms, require deposits where appropriate, and include suspension rights for non-payment.

Dispute 5: Scope Creep (“Just Add This In”)

Even if you’re charging costs, scope creep can cause delays, coordination issues, and friction-especially when the client expects the same completion date.

How to avoid it: have a variation process and make it clear that scope changes can impact timeframes.

What Other Legal Documents Might You Need Alongside A Cost Plus Contract?

For many small businesses, the cost plus contract is just one piece of the puzzle. Depending on how you operate, you might also need supporting documents to protect your business day-to-day.

If You’re Hiring Staff Or Growing Your Team

If you’re scaling up and bringing on employees to deliver projects, you’ll want a solid Employment Contract in place so pay, hours, duties, IP, and confidentiality expectations are clear from the start.

If You’re Engaging Contractors Or Subcontractors

If you’re using subcontractors, make sure your upstream promises to the client (timing, quality, warranties) match your downstream arrangements with subcontractors. A mismatch is a common way small businesses get squeezed.

It’s often worth using a tailored contractor agreement rather than relying on invoice terms alone, especially where the contractor is customer-facing or doing high-risk work.

If You’re Collecting Customer Or Client Data

If you collect personal information (even something as simple as names, emails, addresses, photos of worksites with identifying details), you should consider having a Privacy Policy that aligns with the Privacy Act 2020.

This is particularly relevant if you use online forms, booking systems, mailing lists, or job management software.

If You’re Operating Through A Company

If you’ve set up a company (or you’re planning to), it’s smart to make sure your internal governance matches how you run the business. Depending on your situation, you might need a Company Constitution, especially if you have multiple shareholders or want specific rules around decision-making.

And if you have co-owners, a written Shareholders Agreement can help prevent disputes about who controls what, what happens if someone wants to exit, and how profits are handled.

Key Takeaways

• Cost plus contracts can be a great fit when scope or costs aren’t fully known upfront, but they need clear drafting to avoid disputes.
• Your contract should define what counts as “costs”, what evidence you’ll provide, and exactly how the “plus” fee is calculated.
• A strong cost plus contract usually includes clauses for scope, variations, payment terms, budget guardrails, delays, liability, and termination.
• Be careful with estimates and pricing communications-your statements can still be regulated by the Fair Trading Act 1986.
• If you deal with consumers, the Consumer Guarantees Act 1993 can affect service standards and what terms you can enforce.
• Don’t rely on informal emails or generic templates-cost plus arrangements are especially prone to ambiguity, and small drafting gaps can become expensive disputes.
• Cost plus contracts often work best alongside other key documents (like Employment Contracts, Terms of Trade, and governance documents if you operate through a company).

Note: This article is general information only and isn’t tax or accounting advice. If you have questions about GST or how to treat costs and margins for tax purposes, it’s a good idea to speak with your accountant.

If you’d like help drafting or reviewing a cost plus contract (or tightening up your broader contract suite), you can reach us at 0800 002 184 or team@sprintlaw.co.nz for a free, no-obligations chat.