Promissory Estoppel: Key Elements And How It Works

If you run a small business or startup, you’ll probably rely on promises all the time.

A supplier says they’ll hold stock for you until next month. A landlord says you can pay rent a little late while you fit out a new premises. An investor says they’ll put money in once you hit a milestone, and you start spending based on that.

Most of the time, these promises are followed through. But when they’re not, the question becomes: what can you do if you’ve acted based on that promise, even if you don’t have a signed contract?

This is where promissory estoppel can matter. In plain English, promissory estoppel is a legal principle that can stop someone from going back on a promise if you’ve relied on it and it would be unfair for them to “walk it back”. Importantly, it doesn’t always mean a court will order the promise to be carried out exactly as stated - remedies are discretionary and often focus on preventing the harm caused by your reliance.

In this guide, we’ll break down the key elements of promissory estoppel (sometimes searched as “promissory estoppel elements”) in a practical way - with examples that make sense for founders, operators, and growing teams in Australia.

What Is Promissory Estoppel (And Why Should Your Business Care)?

Promissory estoppel is not a “free pass” to enforce every informal promise. It’s more like a safety net courts can use in certain situations to prevent unfair outcomes.

You’ll often see it come up when:

• there’s a contract in place, but one party makes a promise that changes how the contract will be applied (for example, “we won’t enforce that deadline”); or
• there’s no formal contract, but one party makes a clear promise and the other party reasonably relies on it (for example, “we’ll renew your lease” or “we’ll keep you as an exclusive distributor”).

For small businesses, promissory estoppel is especially relevant because day-to-day operations often involve:

• fast-moving negotiations
• relationships built on trust and ongoing performance
• emails, calls, and messages instead of signed paperwork

That said, promissory estoppel is usually more complex (and harder to “prove”) than a straightforward breach of contract claim. The better strategy is still to document the deal properly from the start - for example, with a tailored Contract Drafting process that fits how your business actually operates.

The Elements Of Promissory Estoppel (The Core Checklist)

While the exact wording can vary depending on the case and the court, the core elements of promissory estoppel generally include:

• a clear promise or assurance
• reliance on that promise (you acted on it)
• detriment or harm (you’d be worse off if the promise is withdrawn)
• unconscionability (it would be unfair for the other party to back out)
• and often: an existing legal relationship or expectation (depending on the scenario)

In Australian cases, courts also commonly talk about an assumption you adopt based on the other party’s words or conduct, that the other party induces or encourages you to adopt, and whether it would be unconscionable for them to depart from it.

Let’s walk through each element in a business-friendly way.

Element 1: A Clear Promise Or Assurance

The first requirement is that the other party made a promise (or gave an assurance) that was clear enough to be relied on.

This doesn’t always need to be a formal promise like “we guarantee X”. It can be a clear representation that a party won’t enforce strict legal rights, or will act in a certain way.

What “Clear” Usually Looks Like In Practice

Courts will look at what was actually communicated. A promise is more likely to be “clear” if it includes:

• specific terms (what will happen, when, and under what conditions)
• certainty (not vague or aspirational language)
• consistency across communications (emails, meetings, messages)

Example: Your commercial landlord emails: “Don’t worry about the rental increase clause for this year - we’ll keep rent at the current rate while you stabilise the new store.”

That’s very different from: “We’ll see what we can do about rent.” The second statement may be too vague to support promissory estoppel.

Common Business Trap: “Subject To Contract”

If negotiations are clearly “subject to contract”, that can be a warning sign. It often means neither party intends to be legally bound until a formal agreement is signed.

This doesn’t automatically rule out promissory estoppel, but it can make things harder. If your deal matters to your cashflow, staffing, or deliverables, it’s usually worth getting the terms documented early (even as a short-form agreement) rather than relying on goodwill.

Element 2: Reliance (You Acted On The Promise)

The next element is that you relied on the promise - meaning you changed your position because you believed the promise would be kept.

Reliance can show up in many ways for small businesses, including:

• spending money (for example, ordering inventory, hiring contractors, starting a fit-out)
• turning down other opportunities (for example, refusing other offers because you expected this deal to proceed)
• changing your internal plans (for example, scaling your team based on a promised contract extension)

Reliance Has To Be Reasonable

Courts generally consider whether it was reasonable for you to rely on the promise in the circumstances.

This is important for founders, because startups often move fast - but “fast” doesn’t always mean “reasonable” if the promise was unclear or obviously conditional.

Example: A supplier says they’ll give you 60-day payment terms “once your account is approved”. If you immediately place a huge order and assume the 60 days applies before approval, your reliance may not be viewed as reasonable.

Tip: Keep Evidence Of Reliance

If a dispute arises, you’ll want to show:

• what the promise was
• when you received it
• what you did as a result
• why you did it (for example, internal notes, emails, board minutes)

This is one reason why good record-keeping (even in early-stage startups) is helpful, especially where major spending or operational decisions are made based on discussions or email assurances.

Element 3: Detriment (You’d Be Worse Off If The Promise Is Withdrawn)

Promissory estoppel usually requires that you would suffer a detriment if the promisor is allowed to go back on the promise.

Detriment does not always mean a direct financial loss (although it often does). It can include:

• wasted expenditure (money spent preparing for a deal that now won’t happen)
• lost opportunities (you passed on alternatives)
• commercial disadvantage (you’re now “locked in” or exposed to risk)

Example: You operate an eCommerce business. A marketing agency promises an exclusive partnership and encourages you to pause working with another agency. You do so, ramp up campaigns with them, and then they withdraw the promise and move to a competitor - leaving you with sunk costs and no backup partner.

Detriment Usually Needs To Be More Than “Disappointment”

Feeling disappointed, frustrated, or inconvenienced is understandable, but courts tend to look for a real change in position that has consequences.

In a business context, a helpful question is: What did we do (or not do) because of the promise - and what is the measurable impact now?

Element 4: Unconscionability (It Would Be Unfair To Back Out)

This is often the most important - and most misunderstood - element.

Promissory estoppel is ultimately about preventing unfairness. Courts look at whether it would be unconscionable (in plain terms: seriously unfair) for the promisor to go back on their promise after you relied on it.

Unconscionability can be influenced by factors like:

• the knowledge of the party making the promise (did they know you were relying on it?)
• the sophistication or bargaining power of each party
• whether the promisor encouraged reliance (or “induced” you to act on an assumption)
• whether the promise was withdrawn suddenly, without warning
• whether the promisor gained an advantage from your reliance

Business Example: Landlord And Fit-Out Works

Let’s imagine you’re negotiating a lease for a new retail store. The landlord tells you (in writing) you can begin fit-out works immediately and confirms you’ll be granted a longer lease term “as agreed” next week.

You spend significant money on fit-out works. Then the landlord changes their mind, refuses to grant the longer term, and demands a much higher rent.

A court may consider whether it’s unfair for the landlord to benefit from your fit-out (improving the premises) while reneging on the promise that induced you to invest.

This is one reason leases, variations, and side agreements should be documented carefully - and reviewed before you spend money. If you’re dealing with premises, a Commercial Lease Review can help you understand what is (and isn’t) binding, and how to avoid relying on “handshake” assurances.

How Promissory Estoppel Plays Out In Common Small Business Scenarios

Knowing the promissory estoppel elements is useful, but what does it look like in real life?

Here are common scenarios where small businesses and startups can accidentally end up relying on promissory estoppel.

1. “We Won’t Enforce That Clause” (Contract Flexibility Promises)

This is one of the most common triggers.

You have a written contract, but during the relationship the other party says:

• “Don’t worry about that deadline.”
• “We’re not going to charge late fees.”
• “We’ll accept a smaller minimum order quantity for now.”

If you rely on that assurance and later they try to enforce the strict term without warning, promissory estoppel may become relevant.

Practically, if terms are changing, it’s usually better to capture that in a written variation (even if it’s short). If you’re dealing with customers, suppliers, or commercial partners regularly, it can help to standardise your contracting process with proper Contract Review support so your documents match what you actually do.

2. Pre-Contract Negotiations (When You “Start Performing” Before Signing)

Startups often do this - especially when you want to move quickly.

You may start:

• building the product
• delivering services
• sharing strategy or IP

…before the final contract is signed.

If the other side clearly promised certain terms, and you relied on that by performing early, promissory estoppel may be argued if they later deny the deal or try to change key terms.

If you’re sharing sensitive commercial info during negotiations, it’s usually worth putting an NDA in place early, even before the main agreement is finalised.

3. Founder, Investor, Or Advisor Promises

Sometimes the promise isn’t from a customer or supplier - it’s from someone inside (or close to) the business.

Examples include:

• an advisor promising ongoing support in exchange for equity later
• a founder promising shares will be “sorted out” after funding
• an investor promising to invest if you hit a milestone, and you incur costs chasing that milestone

These are high-risk areas because the business stakes are high and the documentation is often light.

If you have co-founders or you’re allocating equity, the best way to reduce misunderstandings (and the need to argue about estoppel later) is to document expectations clearly through a Founders Agreement or a Shareholders Agreement.

Practical Tips To Avoid Promissory Estoppel Disputes In Your Business

Promissory estoppel is useful when things have already gone wrong. But as a business owner, you’ll usually want to avoid being in that position in the first place.

Here are practical habits that help reduce your risk.

Put Key Promises In Writing (Even If It’s Just An Email)

Written promises are easier to prove. If the deal changes, confirm it in writing.

For example:

• “Just confirming you’re happy for us to deliver next Friday instead of this Friday, and there won’t be any penalty.”
• “Thanks for confirming the rent will remain at the current rate until 30 June.”

Be Careful With What Your Team Promises

Promissory estoppel risk isn’t just about what you rely on - it’s also about what your business promises to others.

Train your team (especially sales, partnerships, and customer success) to:

• avoid making commitments outside the contract
• use conditional language when appropriate (“subject to approval”)
• escalate contract changes internally

If you employ staff, it can be helpful to reinforce roles, authority, and escalation pathways in internal policies and employment documentation (for example, a tailored Employment Contract for key commercial hires).

Use Clear “No Reliance” And Variation Clauses Where Appropriate

Many commercial contracts include clauses that say:

• the written contract is the whole agreement (so side promises aren’t relied upon)
• variations must be in writing

These clauses don’t automatically eliminate estoppel risk (courts look at fairness and conduct too), but they can help set expectations and reduce ambiguity.

Document Your Decision-Making When You’re Relying On A Promise

If you’re taking a big step (like spending on a fit-out, hiring, or ordering inventory), it’s worth recording why you’re doing it and what assurance you’re relying on.

This could be as simple as:

• saving the email thread
• capturing a note in your project tracker
• writing a short internal memo or board minute for major decisions

Consider The Legal “Cost Of Speed”

Moving fast is often essential for startups. But speed without clarity can create legal uncertainty later.

If the deal is important to your business (revenue, supply chain, premises, equity, IP), getting the paperwork done early can be the difference between a manageable hiccup and a long dispute.

Key Takeaways

• Promissory estoppel can prevent someone from going back on a promise where you relied on it and it would be unfair for them to withdraw it.
• The key elements of promissory estoppel usually include a clear promise, reasonable reliance, detriment, and unconscionability (serious unfairness) - and in Australia, courts often frame this in terms of an assumption you adopted that the other party induced, and whether it would be unconscionable for them to depart from it.
• Promissory estoppel disputes often come up in real-world business situations like lease negotiations, flexible contract arrangements, supplier terms, and pre-contract performance.
• To reduce risk, confirm key promises in writing, be careful about what your team commits to, and document changes to agreements properly.
• Even where estoppel applies, the outcome is not automatic - courts have discretion and remedies commonly focus on preventing reliance-based harm rather than strictly “enforcing” the promise in full.
• If a promise affects major spending, deliverables, or equity, it’s usually worth getting it documented early rather than relying on informal assurances.

Note: This article provides general information only and does not constitute legal advice. Estoppel is highly fact-specific - if you’re unsure about your situation, it’s best to get advice tailored to your circumstances.

If you’d like a consultation about promissory estoppel risks (or documenting your commercial arrangements properly), you can reach us at 1800 730 617 or team@sprintlaw.com.au for a free, no-obligations chat.