Understanding Pennsylvania’s Common Interest Privilege
Most people know that conversations with their attorney are generally confidential. That protection is called the attorney-client privilege. However, what happens when two different people or companies, each with their own lawyers, need to work together on the same legal problem? Examples include:
- Plaintiff falls on a sidewalk that’s constructed by Defendant A and owned by Defendant B. Plaintiff sues both for her injuries. Defendants then coordinate to show how the Plaintiff was comparatively negligent and ignored posted warnings, or
- Plaintiff hires a general contractor to build a garage; the contractor subcontracts out the electrical work. The deadline for completion comes and goes; Plaintiff sues contractor and electrical subcontractor for breach of contract. Each Defendant blames the plaintiff, claiming Plaintiff failed to pay for them for the final phase work as agreed.
Can the Defendants keep secret their coordination and sharing of documents?
In some situations, Pennsylvania law recognizes what is known as the common interest privilege. Although lawyers often refer to it as a separate privilege, it is not really a new privilege at all. Instead, it is an extension of the attorney-client privilege. It allows certain confidential legal communications to remain protected even though they are shared with another party.
Because this doctrine is an exception to the usual rules, Pennsylvania courts apply it carefully and narrowly.
Why This Matters
The common interest privilege matters to both plaintiffs and defendants because it can determine what evidence must be produced during discovery. Plaintiffs often want to know what separately represented defendants have been saying to one another about the facts of the case, their potential liability, or their legal strategy. If the common interest privilege does not apply, those communications may have to be disclosed and could become valuable evidence.
Defendants, on the other hand, need to understand that not every conversation with another defendant, contractor, consultant, or business partner is protected simply because they share an interest in the litigation. Unless the requirements of the common interest doctrine are satisfied, communications that one party assumed were confidential may later have to be turned over to the opposing side.
Knowing where the privilege begins—and where it ends—can therefore have a significant impact on both litigation strategy and the outcome of a case.
The General Rule
Normally, attorney-client privilege is lost if confidential legal advice is shared with someone outside the attorney-client relationship. In other words, bringing an unrelated third person into the conversation usually waives the privilege.
The common interest privilege creates a limited exception. It allows parties who share the same legal interest to exchange privileged communications without automatically losing the protection.
Still, the privilege does not apply simply because two parties have similar goals. The requirements are much stricter.
Common Interest: The Four Requirements
A party claiming the common interest privilege has the burden of proving that all of the following elements exist.
1. There Must Be an Agreement
First, the parties must have an agreement to pursue a shared legal interest.
A written agreement is often helpful. However, Pennsylvania courts have indicated that a written contract is not always required. Even so, the party claiming the privilege must prove that an actual agreement existed. Mere assumptions or beliefs are not enough.
2. The Parties Must Share a Common Legal Interest
Next, the parties must share a common legal interest.
This is one of the most important requirements. A shared business relationship, financial interest, or commercial objective is not enough. Instead, the parties must be working together toward the same legal objective, such as defending against the same claims or coordinating a common litigation strategy.
Simply cooperating on a project does not create the privilege.
3. The Communications Must Further That Legal Interest
The communications themselves must also relate to the shared legal strategy.
For example, exchanging legal advice to defend the same lawsuit may qualify. On the other hand, discussions about ordinary business operations, project management, or commercial negotiations generally will not.
4. The Communications Must Remain Confidential
Finally, the communications must remain confidential.
If the information is later shared with people outside the common interest group, the privilege may be waived.
Attorney Involvement Matters
Pennsylvania courts have also emphasized the importance of attorney participation.
Recent decisions strongly suggest that at least one attorney must participate in the communication for the privilege to apply. Some courts have gone even further and concluded that both parties involved in the exchange should be represented by counsel.
The reason is straightforward. The doctrine exists to protect coordinated legal advice, not ordinary business discussions.
The Party Claiming the Privilege Must Prove It
Another important point is that the burden of proof rests entirely on the party asserting the privilege.
That party must establish every required element. Courts will not simply assume that a common legal interest existed because the parties worked together or had a long business relationship.
The Privilege Does Not Last Forever
The common interest privilege also has limits in time.
It protects communications only while the shared legal interest exists. Once that common legal objective ends—for example, because a settlement resolves the issue or the parties’ interests diverge—later communications may no longer be protected.
Likewise, the doctrine generally does not apply if the parties become adverse to one another in the litigation.
A Recent Pennsylvania Superior Court Decision
The Pennsylvania Superior Court recently addressed these principles in Solid Waste Servs., Inc. d/b/a J.P. Mascaro & Sons v. Ember Partners, LP, No. 1674 EDA 2025, J-A08001-26, slip op. (Pa. Super. Ct. June 9, 2026).
The case arose from disputes involving the development of a power plant. During discovery, Birdsboro Power withheld numerous communications that had been shared with its project manager, claiming they were protected by the common interest privilege.
The court disagreed.
First, the court found little evidence that the project manager had actually agreed to participate in a shared legal strategy. The company’s in-house lawyer testified that he believed a common interest existed. However, the court held that this belief alone did not establish the required agreement.
Second, the court concluded that any common legal interest concerning earlier disputes over waste tonnage had ended when the parties executed settlement-related agreements in May 2017. Once that shared legal issue was resolved, the privilege no longer applied to later communications about different legal disputes.
Third, the lawsuit centered on an alleged oral promise made by Birdsboro Power after those agreements were signed. The court found that the project manager did not share the same legal interest regarding that separate issue.
As a result, the Superior Court affirmed the trial court’s order requiring production of the documents. The court explained that simply including a third party in communications because it had been involved in earlier negotiations does not satisfy the common interest doctrine. The party asserting the privilege must prove every required element with evidence, not assumptions.
If any one of those elements is missing, the privilege may not apply. As the Superior Court’s recent decision demonstrates, courts will closely examine the facts before allowing a party to withhold documents from discovery.
The Importance of a Written Joint Defense Agreement
Although Pennsylvania law does not appear to require a written joint defense or common interest agreement, having one can greatly strengthen a later claim that the common interest privilege applies. A well-drafted agreement provides evidence that the parties expressly intended to share confidential legal communications in furtherance of a common legal strategy. It also helps eliminate disputes about whether an agreement actually existed—an issue that proved significant in the Superior Court’s recent decision.
Without a written agreement, the court — and the plaintiff — may need to take a close look at the Defendant’s private communications to evaluate whether the privilege could apply, thus stripping away privacy.
Written Language Needed
A written agreement should clearly state that each party is represented by separate counsel, that the parties share a common legal interest, and that they are expressly relying upon the common interest doctrine as an extension of the attorney-client privilege. It should also explain the nature of the shared legal objective, identify the types of information that may be exchanged, and require that all shared communications remain confidential.
Defining What Constitutes “Adverse”
Equally important, the agreement should define when the parties are considered “adverse.” For example, the agreement may provide that the parties are not adverse merely because they have asserted cross-claims for indemnity or contribution, or because they may disagree about litigation strategy. Instead, it should specify that adversity arises only when one party affirmatively asserts a claim against another, settles in a manner that is inconsistent with the common defense, begins cooperating with the opposing party against another member of the group, or otherwise abandons the shared legal objective.
The agreement should also identify when the common interest is expected to end, such as upon the final resolution of the litigation, the settlement of one party’s claims, or another defined event.
Complex Cases
This is especially important in complex litigation, where separately represented defendants often need to coordinate an efficient defense. They may share confidential witness interviews, legal research, draft pleadings, and expert analyses that would be prohibitively expensive to duplicate.
For example, one defendant may retain an engineering expert who prepares a detailed report evaluating the cause of an accident. The remaining defendants may wish to review that report while deciding whether to retain the same expert, hire a different expert, or rely on entirely different theories at trial. Without the protection of a valid common interest arrangement, sharing that report could expose it to discovery by the plaintiff or result in the waiver of otherwise applicable privileges. A carefully drafted joint defense agreement cannot create the privilege where the legal requirements are absent, but it can provide strong evidence that those requirements were satisfied and help preserve the confidentiality necessary for an effective, coordinated defense.
These provisions not only provide guidance to the parties, but also create a clearer record for the court if a privilege dispute later arises. They help demonstrate that the parties carefully considered the scope and duration of their shared legal interest rather than simply assuming that all communications would remain privileged.
Practical Takeaways
The common interest privilege can be an important tool. It allows parties with the same legal objective to work together without automatically sacrificing attorney-client confidentiality.
However, it is not a broad shield for every communication involving business partners, contractors, consultants, or affiliated companies.
Instead, Pennsylvania courts require proof of a genuine shared legal interest, an agreement to pursue that interest, communications made to advance the shared legal strategy, continued confidentiality, and meaningful attorney involvement. The best practice requires the the agreement be in writing.
Defendants working with each other: be sure to have a written joint defense agreement in place.
Plaintiffs: be ready to request it in discovery, especially if you think the Defendants are coordinating defense strategy.
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