Can You Shake on It?
Imagine this: you’ve signed a year-long lease for an office. Three months in, you call the landlord. Over the phone, you work out a new payment schedule—some of the rent gets deferred to the end of the term. You both agree. Have you just modified the contract? Under Polish law, the answer is: it depends. And not necessarily on what you intended, or what made business sense, but on what you wrote down when you first signed, and what form you specified for the agreement.
The Starting Point: Poland’s Form-Identity Principle
Polish civil law addresses this in Article 77, Section 1 of the Civil Code, which states plainly:
“Any supplement to or modification of a contract requires the observance of the form prescribed by statute or by the parties for its conclusion.”
This is the so-called principle of form identity—a change to the contract should take the same form as the contract itself.
Simple enough, right? Except the devil lives in a single word: “prescribed.” Because prescribing a form isn’t the same as using one. And it’s around this distinction that one of the subtler debates in Polish contract law revolves.
When Form Is Absolutely Mandatory
Start with the uncontroversial cases—situations where there’s no ambiguity whatsoever. The modification must take a specific form, or it’s simply void:
1. Contracts requiring qualified form by statute
If you’re selling real estate, you must do it by notarial deed (Article 158 of the Civil Code). Any modification to such a contract—price, timeline, conditions—also requires a notarial deed. No room for oral agreements. Why? Because the form requirement applies not just to the original transaction but to everything that modifies it. The logic is straightforward: if the legislature decided that transferring ownership of real property demands the highest degree of formality, that same formality must govern any changes to the deal.
2. Contracts with explicit form-change clauses (pactum de forma)
Imagine finding this in a franchise agreement: “All modifications to this Agreement shall be made in writing under penalty of invalidity.” Such a clause—called a pactum de forma in legal parlance—is fully enforceable under Polish law. Article 76 of the Civil Code gives parties the authority to specify the form not just for the main contract but for any future legal acts related to it.
In practice, this means that if you and your counterparty agree over the phone to change delivery terms, but the contract contains such a clause, your understanding has no legal effect. Doesn’t matter that both sides were willing. Doesn’t matter that it made business sense. No written form = void modification.
Which raises a question that troubles many practitioners: can you waive the clause itself… orally? That question leads straight to a fundamental difference between Polish and English approaches to contract law.
A Comparative Digression: England’s 2018 Revolution
To understand how modern and rational the Polish solution is, it helps to hop across the Channel to London, circa 2018. The U.K. Supreme Court decides Rock Advertising Ltd v MWB Business Exchange Centres Ltd—a case that upended more than a century of common-law tradition.
The traditional doctrine: you can change anything
Throughout the twentieth century, the Anglo-American world embraced the view expressed in 1919 by the renowned Judge Benjamin Cardozo: “Those who make a contract may unmake it. The clause which forbids a change may be changed like any other. The prohibition of oral waiver may itself be waived… You may bar the door against it, and the windowsills may still be open.”
The philosophy was simple: since the law doesn’t require any particular form for contracts, parties can’t impose restrictions on themselves. And if they do, the first oral modification automatically signals implied consent to abandon the no-oral-modification clause. Logical? In theory. Practical? Absolutely not.
The Rock Advertising case: when theory meets business reality
Rock Advertising rented office space from MWB. The contract contained Clause 7.6: “All variations to this Licence must be agreed, set out in writing and signed on behalf of both parties.” When Rock fell behind on rent, its director worked out a new payment schedule over the phone with an MWB employee. MWB later refused to honor the arrangement and evicted Rock from the premises.
The trial court upheld the clause—the oral modification was invalid. The Court of Appeal reversed, applying Cardozo’s traditional doctrine: if the parties orally agreed to something, they implicitly agreed to disregard the written-form requirement.
But the Supreme Court—Britain’s highest court—broke with precedent and restored the trial court’s judgment. Lord Sumption, writing for the majority, conducted a masterful demolition of the traditional doctrine:
First, he argued, the real violation of party autonomy is claiming that parties can’t bind themselves as to the form of modifications if they want to. Autonomy operates at the moment of contracting—afterward, parties are bound by what they agreed to, including procedures for change.
Second, no-oral-modification clauses serve legitimate business purposes: they protect against false claims of oral agreements (particularly in summary proceedings), avoid disputes about what was actually said, and help corporations control who has authority to bind the company.
Third, many modern legal systems—the Vienna Convention on the International Sale of Goods (CISG), the UNIDROIT Principles, German law—have no trouble combining general freedom of form with recognition of form-change clauses.
Fourth—and this is the most elegant argument—when parties fail to comply with a written-form clause, the natural conclusion isn’t that they wanted to waive it, but that they simply forgot about it. Operating managers rarely have the fine print from page ten of a contract signed a year ago at the front of their minds.
Lord Sumption anticipated the risk of injustice, of course: what if one party behaves in a way that causes the other to rely on an oral modification? Here’s where estoppel enters the picture—the Anglo-American doctrine that forbids invoking your own inconsistent conduct to another party’s detriment. (Continental systems have a similar principle: venire contra factum proprium.)
What this means for us
The 2018 decision shows that Polish legal doctrine proved more prescient and far-sighted in this area. Our Article 76, which allows parties to establish form requirements for future acts (including modifications), and Article 77, Section 1, which mandates compliance with that form—these are solutions that the common law only matured into after decades of practical problems. We’re not backward or excessively formalistic. We’re pragmatic.
Back to Poland: The “Accidental Form” Problem
We know now that when a statute requires qualified form, or when parties expressly include a pactum de forma, there’s no debate—modification requires the same form. But what if the contract was made in writing without any legal requirement?
Example: you rent a room to a student for eight months. The law doesn’t require any particular form—you could do it orally (leases under a year). But you—for good order, because it seems proper—wrote it up. For such a contract, the regulations don’t prescribe written form under penalty of invalidity. And you didn’t insert any provisions that could be interpreted as a pactum de forma.
Now, does modifying this lease (say, reducing rent in exchange for minor repairs) have to be in writing?
The old conception: formalism by presumption
Some older doctrine held: since you made the contract in writing, you sort of “implicitly” prescribed written form ad probationem (for evidentiary purposes). And if so, Article 77, Section 1 requires the modification to be in writing too.
The logic went: Article 76, sentence 2 provides that “the conclusion of a contract in writing… shall be deemed to constitute a stipulation of written form in case of doubt.” Since you made the contract in writing, you must have wanted written form, right?
The modern position: prescribe ≠ use
Contemporary doctrine rejects the concept of automatic “presumption” of written form. The argument rests on distinguishing between “prescribing” and “using” a form.
Article 77, Section 1 speaks of form “prescribed” by the parties, not form actually used. To “prescribe” form in the legal sense means to include an express pactum de forma—an agreement in which parties consciously specify formal requirements. Simply signing a document—when the law doesn’t require it—isn’t such an agreement. It’s just… signing a document.
The difference is fundamental. Under the first approach (old conception), signing anything in writing automatically “activates” the written-form requirement for modifications. Under the second (modern) approach, form only matters when consciously stipulated.
Imagine the absurdity: you exchange emails with a neighbor about borrowing his lawn mower. Email constitutes documentary form (Article 77² of the Civil Code). Does this mean every change to your “agreement” (maybe you’ll return the mower a day late?) requires… another email? That would be formal paranoia.
So what about “accidental” written form?
Contemporary doctrine doesn’t say modification can occur in any form. Although Article 77, Section 1 doesn’t apply (because there was no pactum de forma), under general principles (Article 74) and by analogy to Article 77, Section 2, modification of a contract made in writing should require at least documentary form.
This provision states that if a contract was concluded in written, documentary, or electronic form, its termination, rescission, or cancellation requires documentary form (prescribed ad probationem—for evidentiary purposes).
The reasoning is elegant: if terminating a contract—a more “radical” act—requires documentary form, then modifying it should require at least the same.
Practical conclusion: If a contract was made in writing without legal requirement or pactum de forma, its modification should occur in documentary form (this can be email, scan, PDF)—but this is only an evidentiary requirement, not constitutive. Theoretically, an oral modification is valid, but you won’t be able to prove it in court (except in relations between entrepreneurs—here Article 74 liberalizes proof rules).
The Polish Paradox: Easier to End Than to Change
It’s worth noting a peculiarity of the Polish Civil Code:
For MODIFICATION of a written contract: the form-identity principle, along with the debate discussed above about prescribe ≠ use (Article 77, Section 1)
For TERMINATION of the same contract: documentary form ad probationem suffices (Article 77, Section 2)—”unless statute or contract prescribes a different form”
This means in practice that it’s easier to end a contract than to modify it. Why did the legislature decide this? Hard to say. Perhaps it concluded that termination or rescission are actions often taken unilaterally in conflict situations, where strict written-form requirements would be difficult to satisfy. Modification, however, is usually an amicable arrangement, so more care can be required.
Either way, if you’re party to a written contract and want to get out of it—just send notice of termination by email (documentary form—”unless statute or contract prescribes a different form”). But if you just want to change the rent amount? Then you need at least the same form as the original contract (unless it was “accidental” written form, and you accept the reasoning that prescribe ≠ use under Article 77, Section 1).
As you can see, all this is remarkably complicated, and for security of transactions, it’s probably best to stick with the wisdom of previous generations: everything in writing, with handwritten signature and initials on every page. Perhaps this isn’t always necessary, but it never hurts, and it helps avoid many practical problems.
A brief digression here shows that by not adhering to the EVERYTHING IN WRITING principle, we expose ourselves to disputes whose outcome is a lottery.
In a judgment of October 23, 2020 (case no. I AGa 151/19), the Court of Appeal in Kraków ruled completely differently from the trial court in a case concerning a lease agreement. The contract contained a classic pactum de forma: “all modifications require written form under penalty of invalidity.” After the first lease period ended, the landlord issued invoices for over five years (sixty-one invoices!) charging rent lower than the contractual amount. In 2017, he suddenly demanded adjustment to the full rate.
The Regional Court applied strict interpretation: the pactum de forma is binding, invoices didn’t modify the contract, and awarded 145,748.50 złoty. The Court of Appeal completely reversed the judgment, finding that through long-standing practice the parties “may depart from observing written form by adopting a specific practice.” It awarded only 98,099.58 złoty—a difference of almost fifty thousand.
Two courts, the same contract, opposite verdicts. This perfectly illustrates that even the most precise provisions don’t guarantee certainty if you allow “practices” inconsistent with the contract—every deviation can be deemed an implied modification of terms.
Contracts Requiring Qualified Form: Rescission and Termination
There’s one more peculiarity in Article 77, Section 3, often overlooked in practice:
“If a contract was concluded in another special form, its termination by mutual consent requires observance of the form prescribed by statute or by the parties for its conclusion; however, rescission or cancellation should be evidenced in writing.”
If a contract was made in qualified form—say, by notarial deed (such as a contract to sell real estate)—then:
Termination by mutual consent requires a notarial deed (Article 77, Section 3, form-identity principle)
Cancellation or rescission requires only… ordinary written form prescribed under evidentiary restrictions (ad probationem)—a simple letter suffices! This rigor applies regardless of whether conclusion in one of the special qualified forms occurred merely “accidentally” or to satisfy a form requirement prescribed by statute or prior agreement about form. In the latter case, it also doesn’t matter under what rigor form was prescribed for the original contract.
Another paradox: you can cancel a contract obligating you to sell an apartment with an ordinary letter (though the sale itself required a notarial deed), but if you want to terminate it by agreement—you have to visit the notary again.
Why? Probably the legislature concluded that cancellation is a formative right, often exercised unilaterally in a dispute situation, and excessive formalism could be a barrier to exercising rights. But amicable termination (actus contrarius) is a calm, controlled situation, so full formality can be required.
Practical Guidance: Avoiding the Traps
1. Always check the “final provisions”
In a typical business contract, somewhere at the end, after all the substantive clauses, you’ll find a section called “final provisions” or “general terms.” That’s where provisions like this hide:
“All modifications and supplements to this Agreement require written form under penalty of invalidity”
If such a provision exists, treat it deadly seriously. Every oral understanding—even confirmed by email—will be legally ineffective. The court won’t even examine whether the parties actually agreed to something. Lack of form = end of discussion.
2. Email isn’t “written form,” but “documentary”
This is the biggest field for errors. Under Polish law we now have three forms similar to each other but legally distinct:
Written form (Article 78) — requires handwritten signature (or qualified electronic signature)
Documentary form (Article 77²) — text enabling identification of the person making the declaration suffices (can be email, scan, PDF)
Electronic form (Article 78¹) — requires affixing a qualified electronic signature, trusted signature, or personal signature to the declaration
If the contract requires “written form” and you send confirmation by email with your name in the footer—this does NOT satisfy the requirement. You need a document with handwritten signature or qualified electronic signature.
However, if the contract requires documentary form (or nothing, because it’s “accidental” written form)—email suffices.
3. “Working arrangements” are legal time bombs
A scenario I know from practice: contract signed with pactum de forma—though everyone treats such a provision as verbal ornamentation at the end of the agreement. The project proceeds, problems arise. The project manager calls his counterpart: “Listen, let’s do this: we give you two more weeks for delivery of Module A, you reduce the price for Module B by ten per cent. Okay?” — “Okay, thanks, that saves us.”
Both managers hang up satisfied. Problem solved, right?
No. If the contract contains a written-form clause for modifications (and most professional contracts do), your “arrangement” is legally void. You prescribed written form for contract modifications. When a dispute arises (and usually it does), the other side can say: “Us? Agree to a price reduction? Never. Show it to me in writing.” And you have a problem.
4. Don’t trivialize documentary form
Even if the contract doesn’t expressly require written form ad solemnitatem, maintain at least documentary form. Why?
You’ll avoid evidentiary disputes
You’ll demonstrate professionalism and gravity of arrangements
You’ll protect yourself against later “amnesia” by your counterparty
You’ll have a crystal-clear timeline of changes for project-management purposes
In practice: confirm every modification by email, preferably numbered (“Amendment No. 1 to Agreement dated…”). This takes five minutes and could save millions in a dispute.
5. Between entrepreneurs it’s (slightly) looser
Important note: if both parties to the contract are entrepreneurs acting within their business, evidentiary restrictions are relaxed (Article 74, Section 4). This means that even if form was prescribed only ad probationem, oral arrangements can be proved by all means, including witness testimony.
But careful: this applies only to form ad probationem. If form was prescribed ad solemnitatem (under penalty of invalidity), the restriction operates even between entrepreneurs. An oral modification is void, period.
When Can You Change the Rules of the Game?
A question that comes up often: can you modify the form-change clause itself?
Answer: yes, but only by observing the same form.
If the contract states: “Modifications require written form under penalty of invalidity,” you can’t orally agree: “You know what, from now on we’ll modify this orally.”
Such an arrangement is itself subject to the form clause, so it’s void.
However, you can execute a written amendment: “The Parties mutually waive Clause X.Y regarding form of modifications. From the date of signing this amendment, modifications to the Agreement may be made in any form.”
International Comparison: We’re in Good Company
It’s worth noting finally that the Polish solution—full effectiveness of form-change clauses—is the global standard in modern codifications:
Vienna Convention (CISG), Article 29(2):
“A contract in writing which contains a provision requiring any modification or termination by agreement to be in writing may not be otherwise modified or terminated”
UNIDROIT Principles, Article 2.1.18:
“A contract in writing which contains a clause requiring any modification or termination by agreement to be in a particular form may not be otherwise modified or terminated”
German law: Recognizes effectiveness of such clauses (though with certain exceptions arising from the principle of good faith—Treu und Glauben).
Poland isn’t somehow exceptionally “formalistic” or “backward” here. This is international consensus: parties have the right to specify procedures for changing their contracts, and that right should be respected.
The Psychology of Obligations Law—Who Benefits from Avoiding Written Form?
Psychological analysis of negotiations reveals a significant pattern: a party striving for vagueness in contractual provisions often anticipates the possibility of not fulfilling obligations on their side. This observation finds confirmation in the broader context of research on bad-faith negotiations, strategic ambiguity, and contract psychology.
A party that deliberately introduces ambiguity into a contract creates maneuvering room for the future. Negotiation psychology distinguishes two basic approaches: “constructive ambiguity,” which serves to overcome negotiation impasse, and strategic ambiguity deliberately exploited for one’s own benefit.
When a party negotiates in bad faith, it often engages in the process without genuine intention to honor commitments. Research shows that negotiators acting in bad faith may use vague formulations as a form of insurance against future consequences of their actions or inactions. This tactic allows them to later argue that they technically fulfilled contract terms, even if they didn’t realize its intent.
Parties that intend to respect contractual provisions will therefore have a natural inclination toward choosing at least written form, which by its nature is highly precise. Negotiators who resort to sophistical tricks about being “modern” and doing things “on WhatsApp” (or—even better—”on Signal”) very often want to create space for circumventing inconvenient arrangements in the future.
Summary: Certainty More Valuable Than Convenience
Polish contract law gives parties a powerful tool: the ability to determine with certainty how their legal relationship can be modified. This isn’t a limitation of freedom—it’s the realization of freedom, the freedom to bind oneself to rules that both sides consider reasonable.
Yes, sometimes it’s inconvenient. Sometimes you have to interrupt a phone call and say: “Hold on, I’ll send you an email to confirm.” Sometimes you have to wait for signatures instead of acting on the fly. But that inconvenience is the price for something priceless: legal certainty.
Because in business, it’s not about everything being fast and flexible. It’s about things being predictable. So that when three years from now a dispute arises about what you agreed to, it’s not your word against your counterparty’s word, but a black-and-white document that closes the discussion.
Form is not the enemy of commerce. Form is its guardian.
Practical Checklist:
✓ Always read “final provisions”—that’s where form-change clauses usually hide
✓ If contract requires “written form”—email isn’t enough, handwritten signature needed
✓ If contract requires “documentary form”—email or PDF scan are fine
✓ Confirm all working arrangements in writing (email) immediately
✓ Don’t count on “both sides remembering”—document everything
✓ Number amendments sequentially (Amendment No. 1, 2, 3…)—facilitates change management
✓ When in doubt: choose more formal, not less formal, form
Founder and Managing Partner of Skarbiec Law Firm, recognized by Dziennik Gazeta Prawna as one of the best tax advisory firms in Poland (2023, 2024). Legal advisor with 19 years of experience, serving Forbes-listed entrepreneurs and innovative start-ups. One of the most frequently quoted experts on commercial and tax law in the Polish media, regularly publishing in Rzeczpospolita, Gazeta Wyborcza, and Dziennik Gazeta Prawna. Author of the publication “AI Decoding Satoshi Nakamoto. Artificial Intelligence on the Trail of Bitcoin’s Creator” and co-author of the award-winning book “Bezpieczeństwo współczesnej firmy” (Security of a Modern Company). LinkedIn profile: 18 500 followers, 4 million views per year. Awards: 4-time winner of the European Medal, Golden Statuette of the Polish Business Leader, title of “International Tax Planning Law Firm of the Year in Poland.” He specializes in strategic legal consulting, tax planning, and crisis management for business.
