Side letters are one of those quiet little tools that commercial lawyers use all the time, yet most business owners never come across until there is a practical problem to solve. They sit alongside the main contract and make targeted changes without reopening the whole document. When used sensibly, they keep a deal on track. When used recklessly, they create contradictions and confusion.

I want to explain in plain English what a side letter actually is, why they exist, and the situations where they are the right choice.

What a side letter is

A side letter is a short, separate legal document that adds to, clarifies or varies part of an existing contract. It does not replace the contract. It simply sits next to it and adjusts one piece of it.

The important point is that it is legally binding. It is not an afterthought or a friendly exchange of emails. When drafted properly, it has the same legal weight as the main agreement.

The main contract remains in place. The side letter acts like a small patch or update to deal with a narrow point.

Why businesses use side letters

There are a few common reasons.

Sometimes the parties agree a change after signing but do not want to pull the entire contract apart for the sake of a single point. A side letter deals with that change quickly.

Sometimes one party offers a concession they do not want written into the main contract, either for confidentiality or commercial sensitivity. A side letter allows the concession without altering the full document.

Occasionally, an error or ambiguity is spotted after signing. Rather than rewriting the contract, a side letter can confirm the intended meaning.

And sometimes time is the driving force. The parties need something agreed quickly, and there is no appetite for a full amendment process.

In short:

• It avoids reopening the whole contract

• It keeps certain changes confidential

• It fixes mistakes or unclear drafting

• It allows quick agreement on minor points

When a side letter is appropriate

A side letter works best where the change is limited, practical and non contentious.

Examples include confirming a temporary arrangement, adjusting a timeline, clarifying how a mechanism works, or agreeing on a commitment that sits outside the core terms of the contract.

The key test is this: if the change does not alter the overall risk balance or commercial structure of the deal, a side letter may be appropriate.

If the change affects price, liability, warranties, termination, or risk allocation, you are usually better off formally amending the contract.

Practical risks to watch for

Side letters can cause problems if not handled properly.

One risk is contradiction. If the side letter clashes with the main contract, and the contract says that amendments must follow a specific procedure, the side letter may fail unless it is clearly drafted as an agreed variation.

Another risk is informality. A side letter needs to be properly executed, clear in wording and signed by authorised people. A loose or informal letter may be treated as non-binding.

There is also a risk of creating a messy bundle of documents. If there are too many side letters over time, nobody knows which document overrides what. At that point, a consolidated amendment is the better approach.

Finally, practical visibility matters. People operationally running the contract need to know that a side letter exists. Otherwise it becomes a forgotten promise that nobody implements.

What a good side letter includes

Although every situation is different, most well drafted side letters follow a similar structure.

1. They identify the main contract.
2. They state that the contract continues unchanged except for the points in the letter.
3. They set out the exact change or clarification.
4. They confirm how long the change lasts.
5. They confirm which document prevails in the event of conflict.
6. They are signed correctly.

When not to use a side letter

A side letter is not suitable where the change is fundamental. If you are shifting risk, changing pricing structures, altering termination rights, modifying liability, or making long term commercial changes, then a full amendment is the correct route.

A side letter should also not be used to avoid proper negotiation of a key term. It is a tool for precision adjustments, not shortcuts.

A side letter cannot be used to agree something in the future

One thing that often gets misunderstood is what a side letter can actually do. A side letter must record an agreed and binding obligation now. It cannot be used as a placeholder for something the parties intend to sort out later.

Courts do not enforce agreements to negotiate, agreements to agree, or vague promises that depend on further discussion. If a side letter says something like “the parties will agree revised fees next quarter” or “the parties will negotiate an extension in good faith”, that is unlikely to be legally binding. It creates false comfort and, in reality, settles nothing.

A valid side letter needs:

• a clear commitment,

• precise wording, and

• an obligation that operates immediately or on a defined trigger.

If the parties are not ready to commit or the terms are not settled, then a side letter is the wrong tool. Either the contract needs a formal amendment once things are agreed, or the issue should remain open until a proper agreement is reached.

Final thoughts

Side letters are incredibly useful when used carefully. They give parties the flexibility to fine-tune a deal without tearing the whole thing apart. They suit narrow, practical issues where everyone is aligned and the intention is straightforward.

They can, however, create problems if they contradict the contract, are drafted too loosely, or accumulate over time. The trick is judging whether the issue is small enough to justify a side letter or meaningful enough to require a full amendment.