How to Write an Engagement Letter for Extension Season
It’s early September. A client who filed an extension back in March finally sends over the last of their documents. Except now there’s a second entity that wasn’t part of the original conversation, a K-1 that needs to be reconciled, and a question about whether last year’s estimated payments were even calculated correctly.
The work has changed. The scope has changed. The engagement letter has not.
Most firms signed that client’s engagement letter back in January or February. It said something like “services will be performed for the tax year ending December 31, with delivery by the original filing deadline.” That sentence never accounted for September. It was written for April.
This happens at almost every firm that carries extension clients into September or October. The letter on file covers the wrong window of time, and nobody goes back to fix it before the work starts again.
Why Your Original Engagement Letter Doesn't Cover Extension Work
This isn’t a mistake anyone is making on purpose. It’s a structural gap in how most firms build their engagement letter process.
Engagement letters typically get written once a year, usually in December or January, ahead of the main tax season rush. At that point, extension season feels far away. The letter gets scoped and priced around the work the firm expects to do by April 15. Nobody is thinking about the client who will still be on the books in September.
Then April 15 comes and goes. Some clients are done. Some are on extension. For the ones on extension, the firm just keeps working under the same original letter, because there was never a moment built into the process to stop and ask if a new one was needed.
The result is that firms are doing real work, often more complex work than the original return required, without a signed document that actually describes it. If a scope disagreement or a fee dispute comes up later, there’s nothing on file that covers the extension period specifically.
What Belongs in an Extension Season Engagement Letter
An extension engagement letter doesn't need to be a full rewrite of the original one. It works best as a short addendum, tied specifically to the extension period. Four things need to be in it.
Name exactly what’s being done during the extension period. If the original return has grown to include a new entity, additional schedules, or planning work that wasn’t part of the original conversation, say so directly. A client who discovers mid-September that their return now includes a second K-1 should have already seen that in writing, not found out when the invoice arrives.
The original letter likely promised delivery by the standard filing deadline. That date has passed. The addendum should state the new target date, whether that’s tied to September 15 for entity returns or October 15 for individual extensions.
This is the part firms skip most often, and it’s usually where the money is being left on the table. Extension work is frequently more complex than the original return, not less. If a firm doesn’t build the additional fee into the addendum before the work starts, it’s much harder to bill for it after the fact. The addendum should state the additional fee, or at minimum the basis for how it will be calculated, before the work resumes.
The addendum should require a signature before the firm continues. This protects the firm the same way the original engagement letter did. If a client comes back with a materially different situation than what was originally scoped, that’s the moment to pause, not the moment to keep going and sort out the paperwork later.
What an Extension Engagement Letter Addendum Can Look Like
Here’s an example of what the addendum language can look like. This is meant as a starting point, not a finished legal document.
“This addendum supplements the engagement letter dated [original date] between [Firm Name] and [Client Name]. As your return has been extended beyond the original filing deadline, this addendum confirms the following updates to our original agreement: the scope of services now includes [specific additions], the revised delivery target is [new date], and an additional fee of [amount or basis for calculation] applies to the work described above. Please sign below to confirm these terms before work resumes.”
Keep it short. The goal is clarity, not a wall of legal text that a client skims past without reading.
One note on the legal side. An engagement letter is a legal and liability document, and requirements can vary by state and by firm. It’s worth having your addendum language reviewed by your attorney or your professional liability carrier, especially if you’re introducing new fee language for the first time. This post is meant to help you think through what belongs in the letter, not to replace that review.
"Won't This Feel Like Nickel and Diming My Client?"
This is the objection that stops most firms from sending an extension addendum at all. It feels like an awkward conversation to have with a client who’s already stressed about their return being late.
It usually plays out the opposite way. Clients respect a firm that’s clear about what’s being done and why. What actually damages trust is an invoice in October with a bigger number on it than the client expected, and no explanation for where it came from. That’s the moment that generates a phone call asking why the fee is higher than last year.
An addendum sent before the work resumes does the opposite. It tells the client exactly what changed, why it changed, and what it will cost, while there’s still time to have that conversation calmly. It’s the difference between a client who feels informed and a client who feels surprised.
When to Send It
The best time to send an extension addendum is as soon as you know a client’s return has grown beyond what the original letter covered, not in the final week before the deadline.
For firms with a heavy load of business extensions, that often means late July or early August, ahead of the September 15 deadline for partnerships and S-corps. For individual extensions, it means checking in as documents come in through September, ahead of the October 15 deadline.
Building this into your process now, while there’s still a few weeks before the September crunch, means it’s one less thing to think about when the deadline actually hits.
Make This Part of Your Process, Not a September Scramble
The firms that handle this well aren’t doing anything complicated. They’ve simply built the extension addendum into their process the same way they built the original engagement letter into it. It happens automatically, every time a client goes on extension, instead of getting remembered (or forgotten) case by case.
