A landscaping company hires twelve extra crew members every April and lets most of them go by October. A real estate developer staffs up a project team for an eighteen-month build and winds it down at closeout. A professional services firm brings on contractors for one client engagement, then does not need to run payroll for them again until the next one lands. None of these businesses has a steady, predictable headcount, and yet most payroll setups, in-house or outsourced, are still built as if every business does.
Ad hoc payroll services let a business run payroll only when it actually needs to, for a seasonal crew, a single project, or a short-term hire, instead of paying for a full-time payroll department or a fixed monthly outsourcing contract year round. It is growing in the US because contingent and project-based work keeps growing too: the Staffing Industry Analysts put the US contingent workforce at 21 percent in 2025, headed toward 26 percent by 2030.
What Ad Hoc Payroll Actually Means
Worth being precise, since the term gets used loosely. Ad hoc payroll means running payroll for a specific stretch of need, a season, a project, a short-term team, rather than maintaining it as a constant, year-round function. It differs from standard payroll outsourcing, which usually assumes a fairly steady headcount and a recurring monthly fee whether or not volume changes. It also differs from hiring an in-house payroll clerk, who costs the same in July as in December no matter how many people are actually on the payroll that month.
The appeal is simple. A business pays for the payroll cycles it runs, not a fixed cost sized for its busiest month.
Why This Is Growing in the US Right Now
The short version: fewer businesses have a headcount that sits still. ADP Research Institute’s November 2025 analysis of anonymized payroll data from 1.1 million US employers and 24 to 26 million workers found that in any given month, roughly 10 percent of jobs were gig-based, and across all of 2024, about 27 percent of jobs held involved some form of contingent work. Independent contractor employment grew 50 percent between 2021 and 2024, from an average of 300,000 to 450,000 workers a month.
The Staffing Industry Analysts’ 2025 survey, generally the most methodologically conservative of the available benchmarks, puts the current US contingent workforce at 21 percent, projected to reach 23 percent by 2027 and 26 percent by 2030. Deloitte’s Global Human Capital Trends research found 41 percent of companies already expect to increase their use of contingent workers, and the US Bureau of Labor Statistics reports that 35 percent of contingent workers are employed in professional services specifically, the same sector where project-based staffing is most common. The US Government Accountability Office puts contingent work even higher, at 30 to 40 percent of the labor market under a broader definition, a reminder that the exact figure depends heavily on how contingent work gets counted, though every credible source agrees on the direction: up.
None of this is a temporary blip. It is a structural shift in how US businesses staff up, and payroll built for a static headcount has not kept pace with it.
The Compliance Complexity Nobody Budgets For
Here is the part that catches most businesses off guard. Hiring even a handful of workers in a new state for a single season or project usually still triggers real obligations there, state income tax withholding registration, unemployment insurance account setup, and sometimes local filings, regardless of how short the engagement is. A business that only needed a crew in a state for four months can end up carrying that registration and filing burden indefinitely if nobody closes it out properly.
This is exactly the kind of task that a fixed in-house payroll process is not built to absorb smoothly, and it is where an ad hoc provider earns its fee. The registration and compliance work should scale with the actual engagement, opened when the work starts and wounds down cleanly when it ends, rather than becoming a permanent administrative tail on a project that finished months ago.
Where Ad Hoc Payroll Makes the Most Sense
A few situations come up repeatedly among businesses that move to an ad hoc model.
- Seasonal staffing, retail during the holidays, landscaping and construction in the warmer months, hospitality during peak travel season.
- Project-based real estate and construction teams that scale up for a build and wind down at closeout, rather than staying at peak headcount year round.
- Professional services and agencies bringing on contractors for a specific client engagement.
- Businesses testing outsourced payroll for the first time, without committing to a long-term retained contract before they know it is the right fit.
- Multi-entity or multi-state operators who need payroll support in a new state for a limited engagement, without standing up a permanent process there.
The common thread is not company size. It is variability. A steady 40-person company with the same headcount every month has less to gain here than a 15-person company that swings to 45 for four months a year.
What It Costs, Compared to the Alternatives
A dedicated in-house payroll clerk carries a national salary range of $43,250 to $55,750 according to Robert Half’s 2026 Salary Guide, before benefits, payroll software licensing, and the training time it takes to get someone fully up to speed. That cost does not flex down in the months payroll volume is lighter.
Standard outsourced payroll typically runs on a per-employee-per-month model, commonly $4 to $20 per employee plus a base fee, so a steady 25-employee business might land around $450 to $800 a month. That is real savings over an in-house hire for a business with consistent headcount, but it still assumes a fairly constant employee count each month.
Ad hoc payroll changes the math again by billing to actual activity, the specific pay runs and headcount in a given cycle, rather than a flat monthly fee sized for peak season. For a business with real seasonal or project swings, that is often the difference between payroll being a fixed cost and payroll actually tracking the business.
Run the numbers on a real example. A construction firm that staffs up from 8 to 30 people for a six month build, then drops back down, would be paying a standard outsourced provider’s base fee and per-employee rate at the 30-person level for months it does not need it, or carrying an in-house clerk’s full salary through the slow season either way. Billed to actual headcount each cycle instead, the same firm pays close to nothing in its quiet months and the full rate only while the crew is actually on payroll.
What a Good Ad Hoc Payroll Partner Should Offer
Flexibility only helps if the provider behind it can actually deliver on it. A few things worth confirming before signing: whether they can start and stop service quickly without a long lock-in period, whether they handle multi-state tax registration and withholding for a team that might only exist in a given state for a few months, and whether pricing is transparent per cycle rather than bundled into a package built for year-round use. It is also worth asking directly whether the same provider can move a business from ad hoc support into a full retained engagement later, since switching systems entirely once headcount stabilizes is its own avoidable cost.
Worth asking, too, what happens at the end of an engagement. A provider that quietly leaves a state registration open after the project wraps is handing the business a compliance loose end it does not know it has, right up until a notice arrives asking why a filing is late for a team that left months ago.
How Unison Direct Approaches Ad Hoc Payroll
Our Payroll Services cover federal, state, and multi-state payroll administration, built to flex with a business rather than assume a fixed headcount. Engagements start ad hoc, scoped to a season, a project, or a specific need, and move into a full retained arrangement when the business is ready, without changing providers or re-onboarding from scratch. The team works onshore for client contact and compliance, backed by offshore delivery capacity to keep cost proportional to actual volume. Businesses that need payroll handled alongside the rest of finance can also draw on Unison Direct’s finance and accounts outsourcing, FP&A, and Virtual CFO Service, the same team that put together our recent guide on what to ask before hiring an outsourced CFO.
The Bottom Line
A payroll setup built for a headcount that never moves is a poor fit for a growing number of US businesses, and the contingent workforce data backs that up in every direction it has been measured. Ad hoc payroll is not a lesser version of outsourcing. It is the version built for a business whose staffing actually changes with the season, the project, or the client roster, which describes more companies every year, not fewer.
The businesses that get the most value from it are rarely the ones with the biggest headcount. They are the ones whose headcount actually moves, and who have been quietly overpaying, or under-covering, a static payroll setup to handle a workforce that was never static to begin with.
Book a free consultation with Unison Direct to see what an ad hoc payroll setup would look like for your business, and how it could scale up when you are ready.
Frequently Asked Questions
Ad hoc payroll is payroll support you use as needed, for a seasonal surge, a single project, or a short-term team, rather than a year-round in-house payroll department or a long-term retained contract. You pay for the payroll cycles you run.
It is a form of it, but not the same as a standard ongoing outsourced payroll contract. Standard outsourcing usually assumes a steady headcount and a recurring monthly fee. Ad hoc payroll is built for headcount that moves, project crews, seasonal staff, or a business testing outsourcing before committing to a full engagement.
A dedicated in-house payroll clerk carries a national salary range of $43,250 to $55,750 according to Robert Half’s 2026 Salary Guide, before benefits and software costs. Outsourced payroll typically runs $4 to $20 per employee per month plus a base fee, so a 25-employee business might pay roughly $450 to $800 a month, only for the cycles it actually needs under an ad hoc arrangement.
Contingent and project-based work has grown steadily. The Staffing Industry Analysts’ 2025 survey puts the US contingent workforce at 21 percent, projected to reach 26 percent by 2030, and Deloitte’s Global Human Capital Trends research found 41 percent of companies expect to increase their use of contingent workers. That shift makes rigid, headcount-based payroll setups a worse fit for more businesses every year.
With the right provider, yes. A good ad hoc payroll partner should be able to move a business from occasional, project-based support into a full retained payroll engagement without switching systems or providers, once headcount and volume justify it.