Vesting is one of those concepts founders hear constantly from investors and other founders, yet the UK tax implications are rarely explained correctly. US startup content dominates search results and imports US-specific tax framing that does not apply to a UK company. This guide explains vesting mechanics, then connects them to the UK rules that actually govern your situation: restricted securities, section 431, EMI and the employment-related securities framework.
What vesting is
Vesting is the schedule over which a founder or employee earns the right to keep shares or exercise options. Until shares vest, the holder does not have full, unconditional ownership: if they leave before the vesting schedule is complete, they forfeit the unvested portion. The schedule creates a retention alignment mechanism between the company, its investors and the people building it. Vesting itself is a contractual arrangement; the tax treatment depends entirely on the legal instrument involved (restricted securities, EMI options, unapproved options), not on the vesting schedule itself.
Vesting, cliffs and schedules
Three terms describe how a vesting arrangement works in practice.
The cliff is the minimum period that must pass before any shares vest. The most common cliff is one year. Nothing vests in the first eleven months; at the twelve-month point, the first tranche (typically 25% of the total) vests in one go. If someone leaves before the cliff, they leave with nothing.
The schedule is the drip after the cliff. After the one-year cliff, the remaining 75% is typically released monthly or quarterly over the following three years, so the full grant takes four years to vest completely.
Accelerated vesting is a provision, common in investor documents, that some or all unvested equity vests immediately on a specific trigger, usually an exit or acquisition. Single-trigger acceleration vests on the exit event alone; double-trigger acceleration requires both the exit and loss of role. Which applies to your shares is a negotiated, document-level question.
The table below shows a typical four-year schedule with a one-year cliff on a grant of 100 shares. This is illustrative only and carries no tax figures.
| Month | Shares vesting | Cumulative shares vested | Note |
|---|---|---|---|
| 1 to 11 | 0 | 0 | Cliff period, nothing vests |
| 12 (cliff) | 25 | 25 | 25% vests at the cliff |
| 13 to 48 | ~2.08 per month | Up to 100 at month 48 | Remaining 75% vests monthly |
Founder share vesting (reverse vesting)
For founders, vesting usually works in reverse: the shares are issued at the outset, but the company holds a contractual right to buy back the unvested portion at a nominal price if a founder leaves early. This is called reverse vesting or a repurchase right. The effect is identical to forward vesting; the mechanical difference is that the shares exist from day one but are subject to forfeiture rather than being granted over time.
These shares are restricted securities under the Employment-Related Securities rules. Because a leaver provision (the buyback right) is a restriction that affects the value of the shares, the general ERS rules apply. This matters because when a restriction is later lifted (the buyback right lapses as shares vest), HMRC can argue that income tax arises on the increase in value between the restricted and unrestricted market value at that point.
The fix is a section 431 joint election made within 14 days of acquiring the shares. By electing up front to be taxed on the full unrestricted market value at acquisition (typically very low for day-one founder shares), the individual ensures that all subsequent growth is treated as a capital gain rather than employment income when restrictions lift. Missing the 14-day window is one of the most common and costly funded-startup mistakes. The full mechanics are in the section 431 elections guide; this guide does not repeat them.
EMI option vesting
EMI options vest over a schedule set in the option agreement, which is a document agreed between the company and the option holder at grant. The vesting schedule itself is a contractual matter; the EMI framework governs the qualifying conditions and the tax treatment on exercise, not on vesting.
The key point for founders to understand is that the tax advantage in an EMI scheme arises at exercise, not at vesting. When an option vests, the holder acquires the right to exercise (buy shares at the agreed exercise price); they do not acquire shares and do not trigger a tax charge at that point. When they later exercise, HMRC charges income tax only on any growth above the agreed market value at grant, which under a properly valued EMI scheme is typically nil. Any gain between exercise and eventual sale is then taxed as a capital gain, with BADR at 18% potentially available. The valuation mechanics at grant are in the EMI option valuation guide.
For an EMI option to remain qualifying, the option holder must meet the working-time requirement throughout the vesting period: at least 25 hours a week working for the company, or at least 75% of their total working time if that is lower (gov.uk EMI guidance). A change in working pattern that drops below this threshold can be a disqualifying event.
The tax angle: why the instrument matters more than the schedule
Vesting schedules look similar across instruments. The tax treatment diverges sharply depending on the legal form of the equity. The table below maps each instrument to when a tax charge can arise and where to read the detailed rules.
| Instrument | When a tax charge can arise | HP reference | Where to read more |
|---|---|---|---|
| Founder shares (restricted securities) | At acquisition if UMV exceeds price paid; again when a restriction lifts (unless a s.431 election was made). CGT on sale. | HMRC ERS manual | Section 431 elections |
| EMI options | On exercise (income tax only if exercise price is below AMV at grant, which a properly valued grant avoids). CGT on sale, potentially at BADR 18%. | gov.uk EMI, gov.uk BADR | What is an EMI scheme, EMI valuation |
| Unapproved options | On exercise (income tax and NIC on spread between exercise price and market value on exercise day). CGT on any further gain to sale. | HMRC ERS manual | Growth shares and unapproved options |
| Growth shares | At acquisition if acquired above a hurdle at undervalue; income tax and NIC on value received. CGT on growth above the hurdle to sale. | HMRC ERS manual | Growth shares explained |
The ERS rules governing unapproved options and growth shares are set out in the HMRC Employment-Related Securities Manual. Every instrument must also be registered and reported on the annual ERS return by 6 July following the tax year end.
Leavers: good leaver, bad leaver and what happens to unvested equity
Leaver treatment is a contractual matter set out in the shareholders' agreement or option plan. There is no single statutory definition of good leaver or bad leaver; the definitions are negotiated and vary by company, investor and round. The common split is as follows.
- Good leaver: departure for reasons outside the individual's control (ill health, redundancy, or another reason the board designates as good-leaver treatment). A good leaver typically receives fair value for unvested shares or retains a pro-rata portion, depending on the agreement.
- Bad leaver: resignation (except in very specific investor-approved circumstances), dismissal for cause, or breach of restrictive covenants. A bad leaver typically forfeits unvested shares at nil or nominal value and may also lose vested shares above the amount paid, depending on the specific provisions.
For EMI options specifically, certain leaver events can be disqualifying events under the EMI rules. If an option holder falls below the working-time requirement, or the company stops being a qualifying company, the options cease to be EMI-qualifying from that date. The income-tax and BADR treatment on exercise then depends on when the exercise happens relative to the disqualifying event. The full list of disqualifying events is in the EMI disqualifying events guide.
The US comparison: 83(b), ISOs and NSOs do not apply to UK companies
Most vesting content on the internet is written for US companies and US founders. Three US concepts appear constantly and have no direct application to a UK company.
- 83(b) elections are a US Internal Revenue Code provision. The UK equivalent is the section 431 election under ITEPA 2003. The purpose is similar (elect to be taxed up front on full value so future growth is capital), but the deadline (14 days in the UK), mechanics and legal basis are entirely different. If you have read US content about vesting, substitute every reference to 83(b) with section 431 and check the UK rules apply.
- ISOs (Incentive Stock Options) are a US tax-advantaged option type. The closest UK equivalent is EMI. EMI has its own qualifying rules, limits and HMRC approval process; the two regimes are not interchangeable.
- NSOs (Non-Qualified Stock Options) are the US equivalent of unapproved options. The UK treatment for unapproved options is income tax and NIC on exercise under the ERS rules, which differs materially from the US NSO treatment.
Do not apply US tax logic to a UK company's vesting arrangements. The structure of the equity and the applicable UK rules are what matter.
Setting vesting up correctly
Vesting schedules are agreed in the founding documents (shareholders' agreement, articles of association) and, for options, in the option agreement and plan rules. Getting these documents right at the outset, including the leaver definitions, the cliff and schedule, the section 431 election process, and the ERS registration and reporting obligations, is substantially cheaper than correcting them after a funding round or a co-founder departure.
The option pool basics guide covers how to size and structure the pool before a round. For founders planning an EMI scheme, the EMI scheme setup service covers grant documentation, HMRC valuation and ERS registration. For founder share structuring, the share schemes service covers the section 431 election, the shareholders' agreement and the ERS return obligations. The pre-seed founders hub and funded startups hub map out when each of these steps typically needs to happen.