If your Self Assessment bill is over £1,000 (and less than 80% of your tax is collected at source), HMRC asks you to pay next year's tax in advance: two payments on account, each 50% of this year's bill, due 31 January and 31 July. Any shortfall is settled as a balancing payment the following January.
The system catches people out twice: the first year you qualify, you effectively pay 150% of a year's tax in one go; and if your income falls, you must actively apply to reduce the payments or lend HMRC the difference until the refund. The Payments on Account Calculator maps your exact instalments.
Your first Self Assessment bill is £4,000. On 31 January you pay that £4,000 plus a £2,000 first payment on account; on 31 July another £2,000 follows. That is £8,000 leaving your account within six months of a £4,000 bill, entirely legal and entirely predictable, but a brutal surprise if nobody warned you.
