The startup tax deadlines and reliefs first-time UK founders miss

The dangerous deadlines are the ones no one warns you about

Every founder knows they have to pay Corporation Tax eventually. The dates that actually catch people out are the quieter ones — the reliefs you have to claim before a window closes, the elections you have to make within days, the filing that isn't tax at all but carries a penalty anyway. Nobody sends you a reminder for most of these, and by the time you discover them the window has often shut, taking real money or a tax advantage with it.

This is the list of the ones first-time UK founders most often miss. None of them is hard once you know it exists — the whole problem is that most founders don't, until it's too late. Tax rules change, so treat this as a "make sure you've looked into these" list and confirm the current detail with an accountant; the point is to know they're on your radar.

Section 431 elections — you have 14 days, and most founders never hear about it

This is the sharpest trap on the list, because the window is tiny and the cost of missing it is invisible until years later. When you or your employees acquire shares that carry restrictions — which founder and employee shares very often do — HMRC can treat future increases in their value as employment income, taxed at income tax rates, when the restrictions lift. That can be a nasty bill on shares you never sold.

A Section 431 election, made jointly by you and the company within 14 days of acquiring the shares, sidesteps this: you elect to be taxed up front on the full unrestricted value (usually little or nothing at incorporation), so any future growth is taxed as capital gains rather than income. It's the UK's rough analogue of the US 83(b) election. The catch is the same: the 14-day deadline is absolute — miss it and you simply cannot make the election. If you've just issued founder shares or are bringing a co-founder onto the cap table, this is a "this week" item, not a "someday" one. Talk to an accountant the moment shares are issued. (Issuing the shares themselves is its own process — see How to issue shares and file an SH01 →.)

EMI options — the tax-advantaged scheme with a notification deadline

When you start hiring, you'll want to give employees equity, and in the UK the Enterprise Management Incentives (EMI) scheme is the gold standard — a genuinely generous, tax-advantaged way to grant share options to employees. But EMI comes with paperwork that founders routinely fumble:

• The grant has to be notified to HMRC within the scheme's deadline. Miss the notification window and the options can lose their EMI tax advantages — turning a tax-efficient reward into a taxable one.
• It's strongly advisable to agree a valuation with HMRC before granting, so everyone knows the option price is defensible.
• The company and the options both have to meet the qualifying conditions — not every company or employee is eligible.

The deadline for notifying EMI grants has changed in recent years, so check the current rule rather than relying on an old "92 days" figure you might read elsewhere. The principle to remember: EMI is worth doing, but it's worth doing properly and on time, because the tax advantage is exactly what you'd be throwing away by being late.

R&D tax relief — money many startups leave on the table

If your startup is doing genuine technical work — resolving real scientific or technological uncertainty, which a lot of software and hardware companies are — you may be able to claim R&D tax relief, reducing your Corporation Tax bill or, for loss-making companies, potentially yielding a cash credit. For an early-stage startup spending its round on building something hard, this can be meaningful money.

Two things founders get wrong. First, they assume they don't qualify — "we're just building a product." But qualifying R&D is broader than people expect; if you're solving problems where the solution wasn't readily deducible by a competent professional, it may count. Second, they leave the claim too late — R&D claims have a time limit tied to your accounting period, and there are additional notification requirements in the current rules for some claimants. The scheme and its rates were significantly reformed recently (the SME and RDEC schemes were merged for accounting periods beginning on or after 1 April 2024, with extra support for R&D-intensive loss-makers), so the exact mechanics depend on your period and circumstances. Get an accountant who does R&D claims to assess it early — this is one of the few places a young startup can get real cash back, and missing the window means it's simply gone.

The confirmation statement — not tax, but it bites anyway

This one isn't a tax at all, which is exactly why founders forget it. Every UK company must file a confirmation statement with Companies House once a year, confirming your directors, shareholders, registered office and people with significant control are up to date. It's quick and cheap to file — but forgetting it is a real problem: persistent failure to file can ultimately lead Companies House to strike the company off the register. Diarise it from your incorporation date. See Companies House filing: what's due and when →.

The "obvious" ones — diarise them anyway

The headline deadlines are easier to find, but for completeness, don't let these slip either:

• Corporation Tax: tell HMRC the company is active within 3 months of starting to trade; the return (CT600) is due 12 months after your accounting period ends, and any tax is payable 9 months and 1 day after period end. See What is Corporation Tax and when do I pay it →.
• Annual accounts: filed with Companies House (generally 9 months after year-end for a private company; your first accounts have a longer initial window from incorporation).
• VAT: watch your rolling 12-month turnover against the registration threshold and register when you cross it — or voluntarily to reclaim VAT on spend. See When do I have to register for VAT →.
• PAYE/RTI: once you pay anyone a salary, report to HMRC on or before each payday.

The full plain-English calendar lives in The UK small business tax calendar →, and how these fit into your first weeks after raising is in First-cheque compliance →.

How to never be the founder who missed one

The pattern behind every missed deadline here is the same: it wasn't written down, and no one was watching for it. Two habits fix it. First, at two moments — when you incorporate and whenever shares or options move — sit down with an accountant and ask specifically about elections, notifications and reliefs, because those are the time-critical ones that don't announce themselves. Second, keep a single compliance calendar with every date on it, tax and non-tax alike, and check it monthly. Software that tracks your filing deadlines for you removes most of the risk; the rest is just remembering that the quiet deadlines are the expensive ones.

None of this is hard. It's only costly when it's a surprise — so make sure it never is.

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Ledgers keeps your filing deadlines visible and your records ready, so the quiet dates don't become expensive surprises — VAT, Corporation Tax reminders, payroll and Companies House dates tracked in one place, with clean books your accountant can use for R&D claims and year-end. Let Ledgers watch the calendar for you → free to try.

This is general information, not tax advice — the rules and time limits here change, so confirm the current position with a qualified accountant for your situation.

Next: Bookkeeping vs accounting vs a fractional CFO →

Frequently asked questions

What tax deadlines do UK startup founders most often miss?

The less-obvious ones: notifying HMRC of EMI share option grants, making Section 431 elections within 14 days of acquiring shares, claiming R&D tax relief within the time limit, and filing the annual confirmation statement at Companies House. The headline dates (Corporation Tax, VAT) are usually diarised; these quieter ones are the ones that slip.

What is a Section 431 election and why does it matter?

When founders or employees acquire shares that carry restrictions, a Section 431 election — made jointly with the company within 14 days of acquiring the shares — means you're taxed on the full unrestricted value up front, so future growth is taxed as capital rather than income. Miss the 14-day window and you can't make it. It's the UK's rough equivalent of a US 83(b) election.

Can I claim R&D tax relief as an early-stage startup?

Possibly, if you're doing genuine technical R&D — resolving scientific or technological uncertainty. The rules and rates changed recently, so check the current scheme and take advice, but many software and hardware startups qualify and leave real money unclaimed simply because no one told them to look. Claims have a time limit, so don't leave it too long.

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