If your supplier is one person, the work stops when that person does, and in most engagements the contract says nothing about what happens next. Three things are worth agreeing at the point of engagement: who is allowed to step in, what they would need in order to step in, and what the contract says about substitution, work in progress and payment for it. There is a second reason to get this right, and it is jurisdictional. In the UK, a genuine right of substitution is one of the factors pointing away from employment for tax purposes. In California, the business-to-business exemption in Labor Code section 2776 expressly contemplates a provider whose own employees perform the work under the provider's name. Neither is a global rule. Neither is a reason to write a clause that does not match how the work is really done.
The principles below are general. The two legal points are jurisdiction-specific: one is UK, reflecting GOV.UK guidance on off-payroll working, and one is California, reflecting California Labor Code § 2776. Both are current at August 2026. This is general information, not legal, tax or financial advice; rules vary by country and change, so take professional advice on your own arrangement.
What actually stops when one person stops
The work itself is the visible loss, and usually the smallest one. What stops with a single supplier is everything your organisation has quietly outsourced to one head: why a decision was taken and an alternative rejected two months ago, which of your stakeholders cares about which part of the output, and the half-finished work whose real state exists nowhere but on their machine.
Then there is access. Administrative rights tied to a personal email address, multi-factor authentication tied to a personal phone, a repository registered in their name: each is a door that opens for exactly one person. A competent replacement willing to start on Monday still cannot, if they cannot get in and cannot tell what is finished.
Continuity is therefore mostly a documentation and access problem rather than a heroics problem. It requires a few decisions taken while everyone is well, and the discipline to keep the answers current.
If you engage independent suppliers regularly, this is the part of the relationship the client side of the IFA is concerned with.
What to agree before you need it
Agree the mechanism, not the intention. "We would sort something out" is not a plan, because the moment it is needed is the moment nobody has time to design one. The table below sets out what is worth settling in the contract or statement of work, and who each item protects.
| What to agree | Why it matters | Who it protects |
|---|---|---|
| A named right to provide a competent substitute, and how approval works | Cover becomes possible without you losing your say over who does the work | Both parties |
| Handover documentation kept current as a deliverable, not written in a crisis | A replacement inherits the context instead of reconstructing it | The client, and the substitute |
| Access that survives one person: accounts on your domain, recovery not tied to a personal device | An absence stays an absence instead of becoming a lockout | The client |
| Ownership of work product, data and source files, in writing | Work can only be continued if it is yours to hand over | The client |
| What happens to work in progress, and what is payable for it | The part-delivered position is settled in advance, not argued in a crisis | Both parties |
The approval question is where most substitution clauses come apart. A client that genuinely accepts a substitute will usually want to see credentials first, and that is reasonable: the qualifications, vetting or references a replacement must hold can be written down at the start. What makes the clause workable is agreeing those grounds and a right to refuse on them, rather than an unrestricted veto. A right you can decline for any reason at all is not much of a right, and in one of the jurisdictions below that distinction carries weight beyond continuity. Our companion piece on the right of substitution and how it is usually drafted goes further into the clause.
None of this is exotic procurement. It is ordinary supplier management applied to a supplier who happens to be one person, which is what the client side of the IFA is built around.
The clause has a second effect — and it differs by country
A substitution clause is a continuity mechanism first. In two jurisdictions it is also evidence about the nature of the relationship, and the two points do not travel to each other or anywhere else.
In the UK, the off-payroll working rules apply where a worker provides services through their own intermediary, usually a personal service company, and "would have been an employee if they were providing their services directly to that client" (GOV.UK, last updated 26 February 2026). For public sector clients and medium or large private sector clients, "the client will be responsible for determining the employment status of the worker", and should produce a status determination statement setting out the reasons; where the client is a small private sector business, that decision sits with the worker's intermediary instead (same source). HMRC's Check Employment Status for Tax (CEST) tool helps make the determination. Within that assessment, a genuine right to send a competent substitute, which the client cannot unreasonably refuse, points away from employment, because personal service is a hallmark of employment. A clause on its own is not enough: it must reflect how the arrangement actually works.
In California, Labor Code section 2776 sets out a business-to-business exemption, and where its twelve conditions are met, status "shall be governed by Borello" rather than the ABC test (California Labor Code § 2776). It does not by itself make anyone an independent contractor. One condition is that the provider is "providing services directly to the contracting business rather than to customers of the contracting business" — and the section expressly notes that this "does not apply if the business service provider's employees are solely performing the services under the contract under the name of the business service provider and the business service provider regularly contracts with other businesses" (same source). Others require the provider to be free to contract with other businesses and maintain a clientele without restrictions from the hiring entity, to provide its own tools, vehicles and equipment, and to have a written contract specifying the payment amount and the due date of payment. Section 2776 was "Added by Stats. 2020, Ch. 38, Sec. 2. (AB 2257) Effective September 4, 2020." Our post on who is exempt from California's ABC test covers the rest.
Read together, the two points say the same practical thing from opposite ends. A supplier who can genuinely be replaced usually looks like a business in other ways too: other clients, own tools, own rates, own risk. GOV.UK's description of self-employment in the UK is that "A person is self-employed if they run their business for themselves and take responsibility for its success or failure" (GOV.UK). The paperwork follows that reality; it does not create it. A substitution clause in an engagement where you would never accept anyone else buys you nothing in either jurisdiction. Both points are country-specific and both move, so take advice in the jurisdiction concerned.
If you are the one-person supplier
The objection a procurement or delivery lead has to a one-person supplier is rarely capability. It is cover. Answering the continuity question before it is asked — who could step in, on what terms, with what documentation already in their hands — is a commercial advantage on larger engagements, and far easier in a proposal than in an emergency. Our piece on continuity planning for one-person firms covers building that bench.
IFA provides members with access to a substitution network, a contract pack built on B2B standards, and dispute and late-payment escalation support. Substitution-network access and the contract pack sit at the Professional tier; priority substitution and teaming access at the Certified tier. Those are frameworks, templates, support and access, and nothing more: membership does not change anyone's employment or tax status and does not guarantee any outcome, because status depends on how the work is actually done. IFA is a new, self-regulatory body running its own standard, not an accredited one.
Frequently asked questions
What happens if my contractor cannot finish the work?
Whatever your contract says, which in most engagements is nothing. Without an agreed substitution route, current handover documentation and access that is not tied to one person, the work pauses until they return or until you re-procure and rebuild the context. Deciding in advance who may step in, and making sure they could get in, turns a project failure into a delay.
Should a contract with a one-person supplier include a substitution clause?
It is worth considering wherever cover genuinely matters to you, and worth drafting so it could actually be used: a right to provide a competent substitute, the criteria you would apply to that person, and a right to refuse on those grounds. A clause you would never permit anyone to use is not continuity, and in the UK and California it is not evidence of anything either.
Does letting a contractor send a substitute change their employment status?
In the UK it is one factor among several, not a switch: a genuine right to send a competent substitute, which the client cannot unreasonably refuse, points away from employment for tax purposes, but only where it reflects how the arrangement really works. In California, section 2776's business-to-business exemption expressly contemplates the provider's own employees performing the work under the provider's name. Neither point applies outside its own jurisdiction, and neither settles status on its own.
What should be in a handover pack?
Enough for a competent stranger to continue rather than restart: the state of every live piece of work, where the files and source material live, the decisions taken and the options rejected, who your stakeholders are and what each needs, and how access to the systems is granted. The test is not whether it is comprehensive; it is whether it is current.
The week you need to know who can step in is the worst possible week to start finding out.