Most agency founders worry about IR35 when they hire contractors. That is the obvious risk. But there is a quieter, more dangerous problem that hardly anyone talks about: HMRC reclassifying your retainer contracts as employment arrangements, for your own staff.
Here is how it works. You sign a long-term retainer with a client. That client expects your senior account manager to be available every Tuesday and Thursday, on-site at their offices. They direct the work. They set the deadlines. They review the output. After twelve months, HMRC looks at that arrangement and says: that person is not self-employed. They are an employee of the client. And you, the agency, are the employer who failed to operate PAYE and National Insurance.
The liability lands on you. Not the client. You owe the tax, the NI, the interest, and the penalties. For a 12-person digital agency billing £800k per year, that could mean a six-figure bill you never saw coming.
This is not a theoretical risk. HMRC has been quietly investigating retainer-based agency arrangements for years, particularly in marketing, PR, and creative agencies where staff work closely with client teams. The test is not what your contract says. It is what actually happens on the ground.
In this guide, I will walk through the specific factors HMRC uses to determine employment status for your agency staff working on retainer deals. Then I will show you how to structure your contracts, your working practices, and your billing to reduce that risk. If you are serious about agency tax compliance UK, this is one of the most overlooked areas you need to get right.
The Three Tests HMRC Applies to Retainer Staff
HMRC uses the same three common law tests for employment status that apply to IR35. But the context is different. With IR35, the question is whether a contractor is really an employee of the end client. With retainer reclassification, the question is whether your agency employee is really an employee of your client, and whether you, as the agency, are acting as a front.
Control
Control is the most important factor. If the client directs how your staff do their work, not just what they produce, that points toward employment.
Example: Your creative agency places a designer on a retainer with a corporate client. The client tells the designer which software to use, what time to start, which internal meetings to attend, and how to format the files. That is control. Your designer is now behaving like an employee of the client.
Compare that to a retainer where the client says: "We need three social media posts per week, two blog posts per month, and a monthly performance report. You decide how to deliver it." That is a genuine business-to-business service. Control stays with your agency.
Substitution and Personal Service
If the retainer contract requires a specific named individual to do the work, and that person cannot send a replacement, HMRC sees that as employment. The client is hiring that person, not buying a service from your agency.
Most agency retainer contracts name the team members. That is fine, clients want to know who they are working with. But the contract must also give your agency the right to substitute that person if necessary. You do not have to exercise it. You just need the right to do so.
If your contract says "Sarah will be the primary contact" without a substitution clause, HMRC can argue Sarah is the client's employee. If it says "Sarah will be the primary contact, but the agency reserves the right to provide a suitably qualified replacement at any time," you have a much stronger case.
Mutuality of Obligation
This is the clunkiest legal phrase in employment status, but it matters. Mutuality of obligation means the client is obliged to offer work, and your staff member is obliged to accept it. In a genuine employment relationship, the employer must provide work and the employee must do it.
On a retainer, you are selling a block of time or a defined set of deliverables. The client pays for that block whether they use it or not. Your staff are not obliged to accept additional work outside the retainer scope. That lack of ongoing obligation helps your case, but only if your contract and working practices reflect it.
If your retainer says "up to 20 hours per month" but the client regularly asks for 30 hours, and your staff always say yes, you have created mutuality of obligation in practice. The contract no longer reflects reality.
Why Retainer Deals Are Particularly Vulnerable
Retainers create a long-term, regular, integrated working relationship. That is exactly the kind of arrangement HMRC looks at when assessing employment status. The longer the retainer runs, the more the working practices drift toward employment.
Consider a PR agency that places a senior account director on a 12-month retainer with a tech client. Month one: the director works from the agency office, sends weekly reports, and attends a fortnightly client call. Month six: the director is in the client's office three days a week, attending their stand-up meetings, using their Slack, and reporting to the client's head of marketing. Month twelve: the director is effectively a member of the client's team, just paid through your agency.
That is the drift. It happens gradually, and most agency founders do not notice it until HMRC sends a letter.
The risk is highest for creative agencies, digital agencies, and PR agencies where staff work closely with client marketing teams. These roles involve ongoing collaboration, shared tools, and regular feedback loops, all factors that look like employment to an HMRC inspector.
How to Structure Retainer Contracts to Reduce the Risk
You do not need to abandon retainers. They are the backbone of predictable revenue for most agencies. But you do need to structure them with employment status in mind. Here is what to include.
1. Scope-of-Work Clauses, Not Time-and-Materials
A retainer that says "20 hours per month of account management services" is weaker than one that says "monthly deliverables: two press releases, one media list, one coverage report, and up to four hours of reactive media relations." The second version describes a service, not a person's time.
HMRC looks at whether you are selling a result or a person. Sell the result. Define the outputs. Let the client understand that they are buying a service from your agency, not renting one of your staff.