Contract Variations vs Side Letters vs Addendums

In my 10+ years as a solicitor, I have seen many disputes arise over informal contract changes. Whether it’s an 'agreed' price increase over coffee or an extra task promised in a WhatsApp message, if you don't vary your contract correctly, you risk the change being unenforceable (or, worse, breaching the original deal).

Many small businesses believe that an email confirmation is enough to formally vary an agreement. However, most UK contracts contain a variation clause which provides that a variation is only effective if made in writing and signed by both parties. This means that unless you follow the specific variation procedure laid out in the contract, your email might be legally worthless when it comes time to collect payment for that extra work.

Let's break down these three key methods for updating or clarifying your contracts, making it super easy to know which one to reach for!

Commercial life changes fast, and sometimes your agreements need to keep up

🔨 1. Contract Variations: The Renovation

A contract variation (sometimes called an amendment) is the most formal way to change the actual terms and conditions within the main body of your existing contract.

What it is:

A formal, written agreement between all parties to change, delete, or replace specific clauses in the original contract.

When to use it:

• The fundamental deal is changing: You're modifying core terms like the price, the deadline, the scope of work, or payment schedule.

  
  

• You need iron-clad certainty: The change is significant, high-value, or likely to be scrutinised (e.g., changing the entire delivery timeline for a project).

  
  

• The original contract requires it: Most well-drafted contracts will have a variation clause, which typically dictates that any change must be in writing and signed by all parties.

Actionable Guidance:

The document should clearly reference the specific clauses being changed. For example: "Clause 4.1 (Price) shall be deleted in its entirety and replaced with the following:...". Crucially, a Deed of Variation or an Agreement to Vary a Contract must be signed by all parties to be legally binding.

📝 2. Contract Addendums: The Extension

An addendum is used to introduce new terms, conditions, or documents to the contract without changing the existing clauses. It's like adding a new chapter at the end of a book.

What it is:

A document that adds something new to the contract, often a detailed list or a specification that wasn't ready when the original agreement was signed. It becomes a formal, binding part of the overall contract.

When to use it:

• To include specifications or details: You've signed the main service agreement, but now you need to attach the detailed "Scope of Work" document.

  
  

• To formalize a new obligation: You need to add a completely new task or requirement that wasn't covered before (e.g., adding a new service to an existing maintenance contract).

  
  

• To attach missing documents: The original contract referenced an Exhibit or Schedule that didn't exist at the time of signing.

Actionable Guidance:

An addendum must clearly specify which provisions in the main contract it relates to. Again, an addendum must be signed by all parties to be legally binding and often includes a line like: "This Addendum shall be incorporated into and form part of the Agreement dated [Date]."

✉️ 3. Side Letters: The Redecoration

The side letter is the trickiest of the trio and should be used with caution. It is a separate, signed agreement that clarifies or modifies certain points of the main contract, without formally amending the main contract itself.

What it is:

A side letter typically clarifies an ambiguous clause in the main contract or records an informal understanding about how a specific clause will be interpreted or sets out a specific and temporary exception to a clause.

When to use it:

• Tactical or temporary adjustments: You want to temporarily waive a strict deadline for one delivery without formally changing the entire delivery clause in the master agreement.

  
  

• Sensitive issues: You need to record an understanding that might be commercially sensitive and you don't want it to be visible to third parties who may need to see the main contract.

Actionable Guidance:

Side letters should be explicit that they are intended to be legally binding and that they are intended to override specific parts of the main agreement only in the manner described. If you use one, always consult a legal professional to ensure it doesn't inadvertently amend other parts of your main agreement.

Some "Best Practice" Examples

You want to...	Use a...	The risk of getting it wrong
Change the price	Variation	The change is void; you're stuck with the old rate
Add a new service	Addendum	"Scope creep" - where you provide the new service for free under your original contract
Waive a late payment fee once	Side letter	You accidentally waive all future late payment fees

Don’t let an undocumented change become a legal headache

When project scopes creep or deadlines shift, a verbal agreement isn't enough protection. We specialize in drafting and negotiating precise contract variations and side letters for UK freelancers and SMEs. Use our calculator to get an instant fixed-fee quote and let us draft your next deal to ensure your bottom line stays protected!

Clause Two is a legal consultancy, not a law firm. We focus on the provision of non-reserved commercial contract review, drafting, and negotiation services. You can learn more about us on our short FAQ page.