The short answer
If you're below £20-25M ARR, not actively in M&A, and not running a complex regulated business (FCA, banking, insurance), a fractional CFO is the right answer roughly 90% of the time. The honest test is "do I have 200+ days a year of board-grade financial work?" — and for most UK SMEs the answer is no.
Above £25M ARR, or with PE backing pushing for an exit, or with multi-entity international structure, the calculation flips. Full-time becomes the right shape because the workload genuinely is full-time, and continuity matters more than flexibility.
Full all-in cost comparison — the honest numbers
The line-by-line cost of a full-time UK CFO at scale-up level in 2025/26:
| Cost element | Mid-market scale-up | PE-backed late-stage |
|---|---|---|
| Base salary | £140,000 | £220,000 |
| Employer's NI (~13.8%) | £18,000 | £29,000 |
| Pension (6% employer) | £8,400 | £13,200 |
| Bonus (typically 20-40% of base) | £35,000 | £75,000 |
| Equity / LTIP fair value (annual) | £25,000 | £80,000 |
| Recruiter fee (amortised over 3 yrs) | £8,000 | £15,000 |
| Onboarding ramp opportunity cost | £15,000 | £25,000 |
| True annual cost | £249,000 | £457,000 |
Compare that to senior-tier fractional CFO at 8 days/mo:
| Cost element | Operating tier | Senior/PE tier |
|---|---|---|
| Monthly retainer × 12 | £36,000 (3k/mo) | £120,000 (10k/mo) |
| Onboarding fee (one-off) | £3,000 | £8,000 |
| Recruiter fee | £0 | £0 |
| NI / pension / bonus | £0 | £0 |
| Notice period exposure | 1 month | 1-3 months |
| True annual cost | £39,000 | £128,000 |
Senior-tier fractional at £128k/yr buys you roughly 8 days a month from a former PE-portfolio CFO. Full-time at £249k buys you 220 days a year from someone slightly less experienced — because senior PE-experienced operators usually prefer the fractional model.
Capability — is the work actually the same?
The honest answer is yes for ~80% of CFO work, no for the last 20%:
Identical capability (the 80%): board pack, forecasting, KPI design, cash management, hiring plan modelling, fundraise prep, contract review, audit prep oversight, ERP/system selection, financial controls, vendor management, runway planning. A senior fractional does these as well or better than most full-time hires, because they've done them across 8-15 companies recently rather than one company for 5 years.
Where full-time wins:
- Same-day responsiveness during a crisis — a Friday-night cash crunch, a customer trying to pull a contract, a key hire threatening to quit. Fractional CFOs are usually responsive within 4-24 hours; full-time is in your Slack.
- People management of a finance team of 4+ — at this size the CFO's job becomes 50% line-managing FC, FP&A and senior analyst. Fractional can oversee a 1-2 person team via the FC; beyond that you need full-time.
- Cultural carrier of finance discipline — being in the office, walking up to people, "no we can't sign that contract this week" in person.
- M&A execution under pressure — once an LOI is signed the work is full-time for 8-16 weeks. Fractional can run the prep and the strategy, but execution often needs daily presence.
None of these are reasons to hire full-time at £2M ARR. They become reasons at £25M+, in an active M&A window, or with regulated complexity.
Time-to-value
The biggest under-appreciated argument for fractional is the time-to-value gap:
| Stage | Fractional CFO | Full-time CFO |
|---|---|---|
| Search to offer | 2-3 weeks | 3-6 months |
| Start to offer-accepted | 1-2 weeks | 4-8 weeks (notice period) |
| Onboarded to first board pack | 2-4 weeks | 8-12 weeks |
| Full operational productivity | 4-6 weeks | 4-6 months |
| Total elapsed time | ~6 weeks | ~10 months |
If you have a board meeting in 8 weeks or a fundraise in 12 weeks, you cannot solve that problem with a full-time hire. Fractional is the only realistic shape.
The inflection points — when does fractional stop working?
Realistic inflection points where most UK businesses move from fractional to full-time:
- £25M+ ARR with year-on-year doubling. The board calendar alone consumes 4-6 days/mo at this stage; close discipline another 4-6; FP&A and strategy another 6-8. The maths breaks.
- PE/VC active board push for full-time. Funds increasingly write "full-time CFO" into Series B and beyond term sheets. Pushing back is possible up to a point; eventually it's a fight you'll lose.
- Multi-entity, multi-currency, international group structure. Once you're consolidating across 3+ trading entities in different jurisdictions, the close-and-control workload pushes full-time.
- Active sell-side M&A. An exit process is 8-16 weeks of near-daily CFO work. You can run the lead-up with fractional; execution typically needs full-time.
- Regulated environment (FCA, banking, insurance, payments). Many regulators expect a named, full-time CFO on file. Check your specific regulatory requirements — for FCA-regulated firms above certain thresholds, a part-time arrangement may not satisfy the SMCR senior-manager regime.
The hybrid stack — fractional CFO + full-time FC
The most cost-effective UK SME finance shape at £3M-£15M ARR:
- Bookkeeper — £200-£600/mo, transactions, payments, expenses
- Financial Controller (full-time) — £55-£85k base, runs monthly close, manages bookkeeper, prepares the data the CFO needs
- Fractional CFO — £4-£8k/mo, board pack, strategy, fundraise, hiring plan, contract review
Total annual cost: ~£140-£180k for a full mid-market finance function vs ~£250k for one full-time CFO doing some of this and overseeing the rest. The hybrid shape wins on coverage and cost up until the £20-25M ARR inflection.
For context on the FC vs CFO scope split, see fractional CFO vs management accountant.
Transitioning from fractional to full-time — done well
If you're in the inflection band (£20-30M ARR, active board pressure), the right shape is usually:
- Keep your fractional CFO for 6-9 months while you search for the full-time hire.
- Have the fractional CFO help define the JD, sit on the interview panel, run reference checks. They know the market and they know your gaps.
- Overlap by 2-3 months — fractional and full-time both engaged. Fractional hands over board cadence, FC relationship, contract knowledge.
- Then taper the fractional to advisory (1 day/quarter) for 6 months while the full-time hire settles.
This costs an extra £30-40k vs a hard switch but materially de-risks a hire that, if it goes wrong, costs you £80-120k in severance and re-search time.