UK Start-Up Founder Tax Guide

Q: What happens if I miss the Self Assessment deadline?

The Self Assessment deadline is 31 January (online filing) for the previous tax year. Miss it and HMRC apply an automatic £100 penalty. Beyond that: £10 per day from 3 months late (capped at £900), 5% of tax due at 6 months late, and another 5% at 12 months late, under Schedule 55 of the Taxes Management Act 1970. If you have a genuine reason (serious illness, bereavement, technical issue with HMRC's systems) you can appeal with evidence; HMRC accepts reasonable excuse appeals in most genuine cases.

Q: Do I need an accountant or can I file Self Assessment myself?

Legally you can file Self Assessment yourself via gov.uk for free, most simple sole-trader returns (single income source, basic expenses) are realistic to self-file. An accountant adds real value when: your trading profit is above £40,000 (extraction-strategy decisions matter), you have multiple income streams (PAYE + self-employment + property + dividends), you've crossed the £90,000 VAT threshold, you're considering incorporation, or you have an HMRC enquiry. Expect to pay £400-£1,500/year for a typical sole-trader accountant; the cost is itself a deductible expense.

Q: How do payments on account work?

When your Self Assessment tax bill exceeds £1,000 for the first time, HMRC requires payments on account toward NEXT year's tax. Half the current bill is due 31 January (alongside the current bill); the other half is due 31 July. So your first January after crossing the threshold can hit with a double-bill: last year's balance + first payment on account. Adjust via Form SA303 if you expect next year's income to drop substantially. Payments on account don't apply if more than 80% of your tax is collected via PAYE.

Q: I'm pre-incorporation, can I claim setup costs back when I do incorporate?

Yes, under pre-trading expenditure rules (ITTOIA 2005 s.57 for revenue + CAA 2001 s.12 for capital). 7-year window before trading commences. Costs treated as incurred on Day 1 of trading. For ex-PAYE founders launching businesses, year-one losses created by pre-trading expenditure can be carried back via s.72 early-years carry-back against 3 preceding PAYE years, generating cash refunds of tax already paid. Often £5,000-£15,000 of recoverable tax for substantial pre-launch investment.

Q: Should I raise via SEIS or EIS first?

SEIS first if the company qualifies (early-stage, <3 years trading, gross assets <£350k, <25 employees, lifetime SEIS raise <£250k). SEIS gives investors 50% income tax relief + 100% CGT exemption after 3 years + 50% permanent CGT exemption on reinvested gains. After SEIS allowance exhausted, transition to EIS (30% income tax relief, CGT deferral, IHT Business Relief). Most start-ups follow SEIS-first-then-EIS pattern.

Q: What's the most-overlooked relief for tech start-ups?

ERIS (Enhanced R&D Intensive Support), for loss-making SMEs with R&D ≥30% of total expenditure. 27p per £1 of qualifying R&D vs 16.2p under the standard Merged Scheme. Pre-revenue biotech, deep-tech, AI research start-ups burn cash on R&D before generating commercial revenue, ERIS gives substantial cash credit (14.5% of enhanced loss surrendered) during the pre-revenue phase. Many founders default to the Merged Scheme without checking ERIS eligibility. For genuine R&D-intensive SMEs, ERIS often delivers £10,000-£50,000+/year more relief than Merged Scheme.

Q: When do I think about EMI share options?

From the point you're recruiting key employees + can't pay competitive market salaries. EMI lets you offer up to £250,000 of share options per employee (£3m company-wide cap pre-April 2026; £6m from April 2026) with extraordinary tax treatment: no income tax at grant or exercise if granted at market value + held appropriate period; CGT only on disposal of shares; BADR-eligible at 14%/18% with NO 5% shareholding requirement (the key EMI advantage). April 2026 EMI expansion: gross assets test rises £30m → £120m (4x); employee headcount <250 → <500; company-wide cap £3m → £6m.

UK start-up founders face a distinct tax landscape across the company lifecycle: pre-incorporation expenditure relief; early-stage funding via SEIS / EIS for investors + Employment Allowance + R&D Tax Credits / ERIS for the company; growth-phase mechanics including EMI share options for employees + Patent Box on commercialised IP; scale-phase Corporation Tax + Marginal Relief management; exit via BADR (Business Asset Disposal Relief) + SSE (Substantial Shareholdings Exemption). This cohort guide pulls TaxKiln's relief deep-dives into a lifecycle narrative, founders typically navigate 8-12 of these reliefs across the 7-10 years from incorporation to exit. Most are missed without proactive planning.

Last reviewed: 23 May 2026

Guidance, not advice. We explain the rules, we don't assess your situation. Always seek financial or tax advice from your accountant, or contact HMRC. Read our editorial scope →

Founder lifecycle, reliefs by stage

Five stages from formation to exit. Each stage references the deep-dive relief page for the underlying mechanics. Most founders navigate 8-12 of these reliefs across the 7-10 year arc from incorporation to acquisition.

Stage 1: Pre-Incorporation

Before company formed + trading: gather receipts + invoices for setup costs (legal fees, professional advice, market research, website development, equipment, initial training). Pre-Trading Expenditure relief (s.57 revenue + s.12 capital) lets you claim these at Day 1 of trading, up to 7 years back. Ex-PAYE founders: year-one losses created by pre-trading expenditure can carry back via s.72 against 3 preceding PAYE years → cash refunds. Key reliefs at this stage: • Pre-Trading Expenditure, 7-year window, revenue + capital (/tax-reliefs/pre-trading-expenditure) • Trading Losses, s.72 early-years carry-back for ex-PAYE founders (/tax-reliefs/trading-losses) Common mistakes: missing the year-one claim window; treating capital as revenue under s.57; lost receipts.

Stage 2: Early-Stage (Year 1-3)

Company incorporated + trading. Typically loss-making or marginally profitable. Funding via founder cash, angel investors, SEIS/EIS rounds. Focus on capital structure + R&D tax position. Investor reliefs (attract funding): • SEIS, 50% income tax relief for investors up to £200k/year + 100% CGT exemption after 3 years (company SEIS lifetime cap £250k) (/tax-reliefs/seis) • EIS, 30% income tax relief up to £1m/year (£2m KIC) after SEIS allowance exhausted (/tax-reliefs/eis) Company reliefs (cash flow + tax position): • Employment Allowance, £10,500/year off employer Class 1 NI from April 2025 (/tax-reliefs/employment-allowance) • R&D Merged Scheme or ERIS (27p per £1 for loss-making R&D-intensive SMEs) (/tax-reliefs/rd-tax-credits-merged-scheme, /tax-reliefs/eris) • AIA / Full Expensing, capital allowances on equipment / IT / fitout (/tax-reliefs/annual-investment-allowance, /tax-reliefs/full-expensing) Founder-side reliefs: • Trading / Property Allowance for side income alongside Ltd Co • Class 2 NI Voluntary for any sole-trader-stage activity Strategic decisions: SEIS-first-then-EIS funding sequence; Patent applications to set up future Patent Box; founder remuneration structure (salary covering PA + dividends + employer pension contributions).

Stage 3: Growth (Year 3-7)

Revenue scaling. Hiring accelerating. Key talent retention via share-based compensation. R&D may continue or wind down. IP commercialising. Talent reliefs (attract + retain employees): • EMI, Enterprise Management Incentives. Up to £250k per employee. Substantial April 2026 expansion (gross assets £120m, headcount <500, company cap £6m). Key for tech start-ups + scale-ups. • Workplace Pension Employer Contributions, CT-deductible for founders + key staff (/tax-reliefs/workplace-pension-employer) Company tax management: • CT Marginal Relief, 26.5% effective marginal rate in £50k-£250k band; planning territory (/tax-reliefs/ct-marginal-relief) • R&D Merged Scheme, transitioning from ERIS as company moves to profitability • Patent Box, 10% effective CT on qualifying patent income; elect within 2 years (/tax-reliefs/patent-box) • Full Expensing, 100% on new main-rate plant (/tax-reliefs/full-expensing) Founder extraction: • Workplace Pension Employer, £60k AA + Carry Forward up to 3 years • EV FYA + EV Salary Sacrifice, 3% BIK rising to 7% by 2028/29 • Mobile Phone Exemption + Trivial Benefits + AMAP, stacked employer-benefit extraction • Director's Loan Account, careful s.455 + BIK management (/tax-reliefs/directors-loan-account)

Stage 4: Scale (Year 5-10)

Substantial revenue + profits. Possibly multi-product or multi-market. Founder considering exit + succession planning. Tax position management: • CT Marginal Relief, relevant if profits cross £250k upper threshold (back to flat 25% main rate) • Patent Box, full election by this stage if not earlier; nexus fraction documented from inception • SSE, Substantial Shareholdings Exemption planning for subsidiary disposals (/tax-reliefs/sse) • Director's Loan Account, careful management across multiple subsidiaries if group structure Founder remuneration: • Maximum pension contributions via Carry Forward, substantial wealth accumulation in tax-protected wrapper • Spouse-shareholder + spouse-director arrangements for income-shifting (mindful of settlements provisions s.624 ITTOIA 2005) • EV Salary Sacrifice + Workplace Pension + Mobile Phone + AMAP, stacked extraction routes Pre-exit planning (24+ months before sale): • Confirm BADR eligibility, 5%+ shareholding + officer/employee + qualifying trading status (or EMI shares avoid 5% requirement) • Trading-company status check for SSE (group + target both trading) • Document R&D nexus fraction for ongoing Patent Box • Family + estate planning (EIS + Business Relief positioning for IHT)

Stage 5: Exit (Year 7-10+)

Sale of business, to trade buyer, PE house, MBO, EOT, or IPO. Founder-side exit reliefs: • BADR at 14% (2025/26) on first £1m lifetime gains; rises to 18% from 6 April 2026. Anti-forestalling rules apply for contracts straddling rate change. (/tax-reliefs/business-asset-disposal-relief) • Investors' Relief, external investor analogue at same 14%/18% rates; £1m lifetime cap (reduced from £10m on 30 October 2024) (/tax-reliefs/investors-relief) • EMI shares disposed via exit qualify for BADR with NO 5% shareholding requirement (key advantage) • EIS shares held 3+ years: gain CGT-EXEMPT (provided income tax relief retained) • SEIS shares held 3+ years: gain CGT-EXEMPT • SSE, corporate disposals of subsidiary shares Personal post-exit planning: • LSA + LSDBA, Lump Sum Allowance + LSDBA for pension lump sum extraction in retirement • Gift Aid, substantial charitable giving with full higher-rate / additional-rate relief • IHT planning: 7-year gifting + spouse exemption + April 2026 BR/APR £2.5m combined cap Successor-company planning (where founder reinvests): • SEIS / EIS reinvestment relief for gains rolled into new ventures • Pension contribution acceleration in exit year (Carry Forward to absorb gain)

Worked example

Olufemi + Adaeze, a Manchester-based Co-founders of Manchester biotech Ltd Co from incorporation 2020 to acquisition 2027, in Lifecycle 2020-2027:

Olufemi + Adaeze incorporated their biotech Ltd Co in January 2020 to commercialise novel diagnostic assay research. £100k founder cash + 12 months of pre-incorporation R&D effort. Sold to US trade buyer March 2027 for £18m. Throughout the journey navigated SEIS + EIS + ERIS + Patent Box + EMI + BADR.

Year 0 (2019-2020 pre-incorporation): Pre-Trading Expenditure £35k revenue + £15k capital. Year-one trading loss carried back via s.72 to recover Olufemi's prior PAYE tax (~£8k refund).

  Year 1-3 (2020-2023): Raised £200k SEIS (4 angel investors) + £600k EIS (8 investors). Company R&D spend £400k/year, loss-making. ERIS claims: £108k cash credit/year across years 1-3 = ~£325k cumulative ERIS credit. Employment Allowance £5k-£10.5k/year on growing payroll. AIA on £150k of lab equipment. Patent applications filed in years 1-3 (3 patents).

  Year 4-6 (2023-2026): First commercial revenue 2024. Patent granted Q3 2025. Company moves to profitability. Patent Box elected for 2025/26: £180k qualifying patent income → 60% additional deduction → £27k CT saving year 1, building substantially. EMI grants to 6 key employees (£200k options each = £1.2m total under the £3m company cap pre-April 2026). R&D Merged Scheme transitioning from ERIS. Workplace pension contributions for founders + key staff.

  Year 7 (2026-2027): Acquisition by US buyer March 2027 for £18m. Allocation: Olufemi + Adaeze each hold 35% = £6.3m each. 6 EMI employees holding 20% combined = £3.6m. Other shareholders (SEIS + EIS investors) 10% = £1.8m.

  Founder BADR position (Olufemi + Adaeze):
  Each has £6.3m gain. BADR at 18% (post-April 2026 rate) on first £1m lifetime each. Above £1m: standard 24% CGT.
  • BADR portion: £1m × 18% = £180k each
  • Standard portion: £5.3m × 24% = £1.272m each
  • Total CGT per founder: £1.452m on £6.3m gain (23% effective)

  EMI employees: each holds £600k of shares. 5+ year hold, exercised at grant value. CGT only on disposal.
  • BADR at 18% on first £1m: £600k × 18% = £108k per employee (18% effective), vs ~36% on equivalent salary extraction over 7 years.

  SEIS investors: £200k initial → £1.8m exit value × proportional share. SEIS shares held 7 years → CGT EXEMPT. Substantial wealth creation entirely tax-free.

  EIS investors: £600k initial → £1.8m × proportional. EIS shares held 7 years → CGT EXEMPT. IHT Business Relief on shares held at death (subject to April 2026 £2.5m combined cap).

  Combined relief value across the lifecycle:
  • Pre-incorporation s.72 refunds: £8k
  • ERIS cumulative credits: £325k
  • Patent Box CT savings: ~£40k/year × 3 years pre-exit = £120k
  • AIA + Full Expensing relief: ~£50k
  • Employment Allowance: ~£30k cumulative
  • BADR/EMI/EIS/SEIS at exit: ~£3-4m of tax saved vs equivalent income/dividend extraction

  Strategic conclusion: lifecycle reliefs combine to deliver ~£3-5m of relief value over a 7-year scale-and-exit for a moderately successful tech start-up. Most require proactive planning + documentation from inception (Patent Box nexus tracking, EMI eligibility maintenance, SEIS/EIS investor compliance).

Frequently asked questions

What happens if I miss the Self Assessment deadline?+

The Self Assessment deadline is 31 January (online filing) for the previous tax year. Miss it and HMRC apply an automatic £100 penalty. Beyond that: £10 per day from 3 months late (capped at £900), 5% of tax due at 6 months late, and another 5% at 12 months late, under Schedule 55 of the Taxes Management Act 1970. If you have a genuine reason (serious illness, bereavement, technical issue with HMRC's systems) you can appeal with evidence; HMRC accepts reasonable excuse appeals in most genuine cases.

Do I need an accountant or can I file Self Assessment myself?+

Legally you can file Self Assessment yourself via gov.uk for free, most simple sole-trader returns (single income source, basic expenses) are realistic to self-file. An accountant adds real value when: your trading profit is above £40,000 (extraction-strategy decisions matter), you have multiple income streams (PAYE + self-employment + property + dividends), you've crossed the £90,000 VAT threshold, you're considering incorporation, or you have an HMRC enquiry. Expect to pay £400-£1,500/year for a typical sole-trader accountant; the cost is itself a deductible expense.

How do payments on account work?+

When your Self Assessment tax bill exceeds £1,000 for the first time, HMRC requires payments on account toward NEXT year's tax. Half the current bill is due 31 January (alongside the current bill); the other half is due 31 July. So your first January after crossing the threshold can hit with a double-bill: last year's balance + first payment on account. Adjust via Form SA303 if you expect next year's income to drop substantially. Payments on account don't apply if more than 80% of your tax is collected via PAYE.

I'm pre-incorporation, can I claim setup costs back when I do incorporate?+

Yes, under pre-trading expenditure rules (ITTOIA 2005 s.57 for revenue + CAA 2001 s.12 for capital). 7-year window before trading commences. Costs treated as incurred on Day 1 of trading. For ex-PAYE founders launching businesses, year-one losses created by pre-trading expenditure can be carried back via s.72 early-years carry-back against 3 preceding PAYE years, generating cash refunds of tax already paid. Often £5,000-£15,000 of recoverable tax for substantial pre-launch investment.

Should I raise via SEIS or EIS first?+

SEIS first if the company qualifies (early-stage, <3 years trading, gross assets <£350k, <25 employees, lifetime SEIS raise <£250k). SEIS gives investors 50% income tax relief + 100% CGT exemption after 3 years + 50% permanent CGT exemption on reinvested gains. After SEIS allowance exhausted, transition to EIS (30% income tax relief, CGT deferral, IHT Business Relief). Most start-ups follow SEIS-first-then-EIS pattern.

What's the most-overlooked relief for tech start-ups?+

ERIS (Enhanced R&D Intensive Support), for loss-making SMEs with R&D ≥30% of total expenditure. 27p per £1 of qualifying R&D vs 16.2p under the standard Merged Scheme. Pre-revenue biotech, deep-tech, AI research start-ups burn cash on R&D before generating commercial revenue, ERIS gives substantial cash credit (14.5% of enhanced loss surrendered) during the pre-revenue phase. Many founders default to the Merged Scheme without checking ERIS eligibility. For genuine R&D-intensive SMEs, ERIS often delivers £10,000-£50,000+/year more relief than Merged Scheme.

When do I think about EMI share options?+

From the point you're recruiting key employees + can't pay competitive market salaries. EMI lets you offer up to £250,000 of share options per employee (£3m company-wide cap pre-April 2026; £6m from April 2026) with extraordinary tax treatment: no income tax at grant or exercise if granted at market value + held appropriate period; CGT only on disposal of shares; BADR-eligible at 14%/18% with NO 5% shareholding requirement (the key EMI advantage). April 2026 EMI expansion: gross assets test rises £30m → £120m (4x); employee headcount <250 → <500; company-wide cap £3m → £6m.

Last reviewed: 23 May 2026