Cheat sheet

ISA vs SIPP, one page

Both wrappers shelter investments from tax; they differ in when you pay tax and when you can touch the money. Most people eventually want both. 2026/27 figures.

Question	Stocks & Shares ISA	SIPP (personal pension)
Allowance	£20,000 per tax year (2026/27), across all your ISAs. From 6 April 2027 at most £12,000 of it can go into a cash ISA if you are under 65; the overall £20,000 stays.	£60,000 annual allowance (2026/27) including employer money, tapered for very high earners; up to three years' unused allowance can be carried forward.
Tax going in	None: you contribute from taxed income and that is the end of it.	Relief at your marginal rate: a £100 contribution costs a basic-rate taxpayer £80, a higher-rate taxpayer effectively £60 after reclaiming. This is the SIPP's superpower.
Tax coming out	Zero. No tax on gains, dividends or withdrawals, ever, and withdrawals do not touch your other allowances.	25% usually tax-free (within limits); the rest taxed as income when drawn. The bet is that your retirement tax rate is lower than today's.
When can you touch it	Any time, same week, no questions. That flexibility is why it is the default first wrapper.	Not until minimum pension age (57 from 2028). That lock is a feature for discipline and a bug for emergencies; never SIPP money you might need at 40.
Rule of thumb	Flexibility first: emergency fund, then ISA for goals this side of retirement.	Maximise any employer match first (free money), then SIPP shines for higher-rate taxpayers and for money you promise not to touch.

Not either/or: a common order is employer match, then ISA to comfort, then SIPP for the marginal-rate relief. Allowances current for 2026/27 and do change; confirmed against HL, AJ Bell and Fidelity, July 2026.

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Educational information, not financial advice. Figures current as of July 2026 where dated; allowances and rates change, so check the source before acting.