Founder secondaries and the tax you actually pay

Selling part of your stake before an exit sounds straightforward until you open a spreadsheet and try to figure out what you actually owe. The tax treatment of founder secondaries sits at an intersection of capital gains rules, employment law, residency tests and obscure reliefs that most founders only hear about after the wire hits their account. Whether you're a US-based founder selling $2M of vested shares to a late-stage investor, or a UK founder offloading a chunk during a Series C, the difference between planning this correctly and winging it can easily be six figures. This piece breaks down the real tax mechanics of a founder secondary sale: not the theory, but the traps, reliefs and timing decisions that determine what you actually pay.

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Why a secondary is not just a smaller primary

A primary round creates new shares and the company receives the cash. A secondary sale transfers existing shares from a founder's personal holdings to a buyer, and the founder pockets the proceeds. That distinction matters enormously for tax purposes because the IRS, HMRC and most other tax authorities treat the character of the income differently depending on who received the shares, when and under what conditions.

If you received your shares through a standard incorporation (say, you co-founded the company and were issued shares at par value), the cost basis is typically near zero. A $3M secondary sale on shares you acquired for $300 means nearly the entire amount is gain. But if your shares were granted as part of a compensation arrangement, or if you exercised options to get them, the analysis shifts. The type of equity instrument, the vesting schedule and whether you filed an 83(b) election (in the US) all feed into the calculation.

Founders often assume a secondary is just a "mini exit." It isn't. The buyer's identity, the structure of the transaction and whether the company facilitates the sale all influence whether you're looking at capital gains rates or ordinary income rates. The same structural thinking that drives ESOP design across multiple jurisdictions applies here: the instrument determines the tax outcome.

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Capital gain or compensation: the line that doubles your tax bill

This is where most founders lose money they didn't need to lose. The core question is simple: does the IRS or HMRC view your secondary proceeds as a return on investment, or as disguised compensation?

In the US, if you hold Qualified Small Business Stock (QSBS) or shares acquired via an 83(b) election more than a year ago, you're generally in long-term capital gains territory: 20% federal plus the 3.8% net investment income tax. But if you exercised incentive stock options (ISOs) and sold within the required holding periods, or if the shares were subject to a vesting schedule without an 83(b) election, the IRS may recharacterise part or all of the gain as ordinary income taxed at up to 37%.

The UK has its own version of this problem. If HMRC determines that your shares were "employment-related securities" and you didn't pay market value for them, the difference between what you paid and the market value at vesting is taxed as employment income (up to 45% plus NICs). Only the growth after that point qualifies for capital gains treatment. The HMRC Capital Gains Manual sets out the full framework for how gains are characterised on share disposals, and the employment-related securities rules are covered in detail there.

The practical takeaway: get your share history documented before the secondary round, not during it. You need a clear paper trail showing acquisition date, cost basis, any elections filed and the vesting timeline.

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The residence question: where the gain actually lands

Your tax residency determines which country gets to tax your secondary sale, and founders who have moved between countries face the most complex situations.

A UK founder who relocated to Dubai in 2024 might assume they're free of HMRC's reach. Not necessarily. The UK Statutory Residence Test has detailed tie-breaker rules, and if you spent 90 or more days in the UK during the tax year of the sale, or if you maintain a home there, HMRC may still claim you as resident. The gain lands in the UK, not the UAE.

US citizens face an even blunter reality: the US taxes worldwide income regardless of where you live. A US founder sitting in Singapore still owes the IRS on the secondary sale. The foreign tax credit can offset double taxation, but only if the other jurisdiction actually taxes the gain.

For international founders selling shares in a US C-corp, the analysis depends on whether the company qualifies under specific treaty provisions and whether the shares constitute a "US real property interest" under FIRPTA. Most tech company shares don't trigger FIRPTA, but if the company holds significant US real estate, the withholding obligations change dramatically.

Plan your residency position at least 12 months before the secondary. Moving countries the same quarter you sell is a red flag for every tax authority involved. If you're working through a restructuring before the sale, the sequencing of entity changes and personal residency moves needs to be co-ordinated, not treated as separate decisions.

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QSBS, BADR and the reliefs founders forget

The single most valuable tax relief for US founders doing a secondary is the Section 1202 QSBS exclusion. If your shares qualify, you can exclude up to $10M (or 10x your cost basis, whichever is greater) of gain from federal tax entirely. That's a 0% rate on what could be millions of dollars.

Qualification requires the stock to be in a domestic C-corp with gross assets under $50M at the time of issuance, acquired at original issuance and held for at least five years. Many founders trip on the holding period: if your company is only three years old and you're doing a secondary, QSBS doesn't apply yet. You might want to wait.

In the UK, Business Asset Disposal Relief (BADR, formerly Entrepreneurs' Relief) reduces the capital gains rate to 10% on the first £1M of qualifying gains. The founder must hold at least 5% of the company's shares and voting rights and must have been an officer or employee for at least two years before the sale. The 5% threshold is critical: if dilution from funding rounds has dropped you below 5%, BADR is gone. If you're based in the UK, confirming your BADR eligibility before agreeing any sale terms is non-negotiable.

Other reliefs worth checking:

• Investors' Relief (UK): 10% rate on up to £10M of gains for qualifying shares held three or more years
• EIS deferral relief (UK): if you reinvest gains into EIS-qualifying companies
• State-level exclusions (US): some states conform to QSBS, others don't. California, notably, does not honour the federal QSBS exclusion

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Section 1045 rollover for US-connected sellers

Most founders have heard of QSBS. Far fewer know about Section 1045, which allows you to defer gain from the sale of QSBS by reinvesting the proceeds into replacement QSBS within 60 days. The IRS guidance on Section 1045 applies only to partnerships and individual taxpayers, so the mechanics differ depending on how your equity is held.

Here's why this matters for secondaries: if your shares qualify as QSBS but you haven't hit the five-year holding period, you can't use the Section 1202 exclusion. But if you've held for at least six months, Section 1045 lets you roll the gain into new qualifying stock and restart the clock. You defer the tax now and potentially exclude it later under Section 1202 when the replacement stock hits five years.

The 60-day reinvestment window is strict. You need to have the replacement investment identified and funded within that period. The replacement stock must also be QSBS in a qualifying C-corp, so you can't just park the money in a public company.

This strategy works best for founders who are serial entrepreneurs or active angel investors. Sell secondary shares in Company A, reinvest into a qualifying stake in Company B and defer the entire gain. If Company B's stock eventually qualifies for the full Section 1202 exclusion, you may never pay federal tax on the original gain. It's a material consideration in exit planning that too few founders build into their timeline.

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Earn-outs, escrow and the timing trap

Not every secondary sale pays out in a single lump sum. Increasingly, buyers structure deals with earn-outs tied to company milestones, or place a portion of the purchase price in escrow pending representations and warranties.

The tax timing question here is real. Under the instalment sale method (US), you recognise gain proportionally as you receive payments. But if the IRS determines the earn-out has a readily ascertainable fair market value at closing, you may owe tax on the full amount upfront, even though you haven't received the cash yet.

UK founders face similar issues. HMRC's approach to deferred and contingent consideration can result in a tax bill based on the estimated value of future payments at the time of sale. If the earn-out ultimately pays less than estimated, you can claim a loss, but you've already fronted the tax. The HMRC Capital Gains Manual covers the treatment of contingent consideration and the rules around ascertainable versus unascertainable deferred proceeds.

Practical steps to manage earn-out tax timing:

• Structure the earn-out so it genuinely lacks ascertainable value at closing (consult your tax adviser on the specific language)
• Negotiate escrow release schedules that align with tax payment deadlines
• Set aside at least 30% of the initial payment for taxes, even if you expect the earn-out to reduce your effective rate
• Document everything: HMRC and the IRS both scrutinise earn-out arrangements closely

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Relocation before the secondary: the residency window

If you're considering relocating before a secondary sale, the tax arithmetic can be compelling, but the execution window is narrower than it looks. A UK founder who breaks residence cleanly under the Statutory Residence Test takes their shares out of HMRC's capital gains net. A subsequent sale while genuinely non-resident is taxed where you now live, which for many founders means the UAE at zero.

The problem is the UK's temporary non-residence rules. If you return to the UK within five complete tax years, gains realised while you were away are taxed in the year you come back. The secondary proceeds follow you home. The same logic applies when founders relocate from European jurisdictions with exit tax regimes: the cost of leaving before you sell can itself exceed the tax saving on the secondary.

The decision to relocate is also not just a tax question. Substance requirements in your new jurisdiction, the impact on your company's corporate tax residency and the operational demands on a founder during a fundraising round all interact. Relocating a UK business to the UAE raises different questions from simply relocating yourself, and conflating the two is a common source of expensive mistakes.

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A secondary checklist eighteen months before the round

The biggest mistake founders make with secondary sales is treating tax planning as an afterthought. By the time the term sheet arrives, most of your options are locked in. Start eighteen months out.

• Confirm your share class, acquisition date, cost basis and any elections (83(b), Section 431 in the UK) on file
• Run a QSBS qualification analysis if you're US-connected: check gross asset limits, holding period and state conformity
• Verify your tax residency under the Statutory Residence Test (UK) or substantial presence test (US) for the year you plan to sell
• Model the difference between capital gains and ordinary income treatment on your expected sale amount
• If you're below the five-year QSBS threshold, evaluate whether a Section 1045 rollover makes sense
• Check your BADR eligibility if UK-based: specifically your shareholding percentage after recent dilution
• Get a 409A valuation or independent appraisal to support your cost basis and sale price
• Draft the secondary purchase agreement with earn-out and escrow tax provisions reviewed by your own counsel, not just the company's lawyers

The tax you actually pay on a founder secondary depends almost entirely on decisions made months or years before the transaction. Residency, share structure, elections and relief eligibility are all things you can influence with enough lead time. Founders who treat a secondary like a quick liquidity event and skip the planning routinely pay 20 to 30 percentage points more than those who don't. Talk to a cross-border tax adviser who has handled secondary transactions specifically: not your company's corporate counsel, not a generalist accountant. The stakes are too high for generic advice. Cosmos models the position for founders across the UK, US, UAE and beyond, and can run the residency, relief and structuring analysis before the round opens. Start with the tax product page or tell us your situation directly.