SEIS offers generous tax reliefs because the investments are genuinely risky. The government is not providing 50% income tax relief out of benevolence — it is compensating investors for backing companies that might fail. Before investing in SEIS, you need to understand what that risk looks like in practice.
The failure rate
The majority of seed-stage companies fail. This is not a fringe outcome — it is the base case. Studies of early-stage company survival consistently show that 50% to 70% of seed-stage companies do not return investors' capital. Some fail quickly. Some run for three or four years before the founders accept the business is not working. Some pivot repeatedly before either finding a model that works or running out of runway.
The SEIS tax reliefs are designed with this failure rate in mind. Loss relief ensures that the effective loss from a complete failure is substantially less than the nominal investment. But it is still a real financial loss.
Illiquidity
SEIS shares are not listed on any exchange. You cannot sell them when you want to. Your capital is locked up until one of the following happens: the company is acquired (an exit), the company floats on a stock exchange (an IPO), the company offers a secondary sale to new investors, or the company fails. None of these events is within your control, and none is guaranteed to happen within any particular timeframe.
A realistic SEIS investment timeline is five to ten years from initial investment to potential liquidity. Some investments take longer. You should not invest capital in SEIS that you might need access to within that timeframe.
Dilution
If the SEIS company progresses — which is the outcome you want — it will raise further rounds of funding. Each round issues new shares to new investors, which dilutes the percentage ownership of existing shareholders including you. A 5% stake at SEIS stage might become 2% or 1% by the time the company reaches Series B or C. This is not necessarily bad — 1% of a £100 million company is more valuable than 5% of a £5 million company — but it is a feature of the asset class that investors sometimes underestimate.
Anti-dilution protection and pro-rata rights (the right to invest in future rounds to maintain your percentage) can partially address dilution, but these terms are more commonly found in institutional investment documents than in SEIS round terms for angel investors.
Tax relief clawback
HMRC can withdraw SEIS tax relief if the company breaches the qualifying rules within three years of the investment. This can happen without any fault on the investor's part — if the company changes its trade, becomes controlled by another company, or issues the wrong class of shares in a subsequent round. If the reliefs are withdrawn, you will receive an assessment for the tax that was previously relieved.
The risk of clawback can be mitigated by working with companies and advisers who understand SEIS rules well, and by ensuring SEIS compliance is built into the company's ongoing governance.
Portfolio construction
Given the high individual company failure rate, SEIS investing requires diversification. A single-company SEIS investment concentrates all the risk in one outcome. A portfolio of ten or more companies across different sectors spreads the risk — some may fail, but the winners need to cover the losses and then some.
Most experienced SEIS investors target a minimum of 10 companies per portfolio, investing smaller amounts in each rather than larger amounts in one or two. This approach accepts the high individual failure rate as a feature of the asset class and relies on the portfolio-level return distribution to generate acceptable overall returns.
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