UK: impute ISA subscriptions per adult by type, calibrated to HMRC's ISA tables

Read the existing was_lisa stage and was_wealth outputs, then inspect how Chronicle datasets are added, especially hmrc/cgt_size_of_gain_2026. Check whether the round-7 WAS tabs are available in the private mirror and compare any balance changes with HMRC Tables 9.7 and 9.8. Done means producing documented per-adult subscription inputs by ISA type, with calibrated counts and amounts, and resolving the donor, calibration, split, and timing decisions.

Summary

The UK build carries ISA balances but no ISA subscriptions:

• cash and stocks-and-shares balances per household (was_wealth);
• Lifetime ISA balances per person (was_lisa).

PolicyEngine/policyengine-uk#2710 adds per-person cash_isa_subscriptions and stocks_and_shares_isa_subscriptions inputs and the ISA limits. Without data the inputs stay at zero, so every ISA-limit reform scores £0 on Microcosm. This issue imputes yearly subscriptions per adult by ISA type, calibrated to HMRC's ISA statistics.

PolicyEngine/uk-isa-allowance#1 answers the current questions from HMRC's tables directly. This is what modelling them needs.

Why there's no donor

No survey we hold records how much people pay into ISAs:

• WAS round 8, person tab:
  • whether the person holds ISAs (HASISAR8) and which types (FISA1R8–FISA4R8);
  • the value of each type (DVCISAvR8, DVIISAvR8);
  • whether they took money out of a cash ISA in the last two years (FCISAWdR8).
  • No amounts paid in.
• FRS 2024-25: ISA interest, under account code 21 with no type field. No payments.
• LCFS 2024-25: the diary item "ISA" is pooled into code 20.5.1.1.1 ("Savings, investments (excl AVCs)") with bank savings, shares and premium bonds. It covers two weeks. isaamt is ISA interest.
• WAS panel (possible donor, unchecked): round-8 records carry round-7 case and person ids (CASER7, personr7).
  • A person's change in balance by type from round 7 to round 8 is a net-flow proxy: about two years of payments, plus growth, minus withdrawals.
  • Whether the round-7 tabs are in the private mirror's was_2006_22.zip hasn't been checked.

Targets: HMRC's ISA tables, through Chronicle first

HMRC, ISA tables, September 2026 (2023-24 unless noted). These go into Chronicle first, as a new hmrc/isa_statistics_2026 dataset following hmrc/cgt_size_of_gain_2026:

• Table 9.7: individuals by income (11 bands), size of subscription and ISA type (cash only, stocks and shares only, both).
  • The size has 11 bands, with an edge at £12,000 and a top band of exactly £20,000.
  • The table also counts holders who made no further subscription.
• Table 9.8: by age (21 bands), sex and type. No amounts.
• Table 9.9: by region and type.
• Table 9.4: accounts and amounts subscribed by type, 2008-09 to 2024-25. These give growth to 2024-25.
• Tables 9.10–9.12: holders by market value and by income, age or region. A check on the balances the build already imputes.
• HMRC's impact note (17 September 2026): 22% of cash ISA subscribers under 65 paid in more than £12,000 in 2022-23. This is the only published fact that crosses age with amount.

Notes for the Chronicle dataset:

• The cells are counts of people, although the headings say "(thousands)".
• Innovative Finance ISAs are excluded.
• The notes don't say how Lifetime ISAs are classed.
• In the "both" block the band is the combined amount across types.

Proposed stage

A person-grain stage after was_lisa:

1. Per-person balances by type. Carry DVCISAvR8 and DVIISAvR8 from the WAS person tab the way was_lisa carries DVFLISAvR8, with the household totals as sums.
   • They are predictors here, and #2710's per-person balances need them.
   • It changes was_wealth's outputs, so it needs an A/B against main.
2. Who subscribes, and to which types. A subscriber class for each adult (cash only, stocks and shares only, both, or a holder who didn't subscribe), conditional on age, income and the balances. Calibrated to the margins of Tables 9.7 and 9.8.
3. How much. A subscription band per subscriber from Table 9.7's distribution given income and type, then an amount within the band.
   • The £20,000 band is exact.
   • Within a cell, rank by balance so larger holders tend to pay in more; this needs deciding.
4. The split for subscribers to both types. No table splits their combined amount. Options:
   • split by the person's balance shares;
   • split by Table 9.4's subscribed amounts by type.
5. Age. Table 9.7 has no age. Hold the impact note's 22% under-65 share as a check or a target.
6. Year. Grow the 2023-24 counts to the build year with Table 9.4's accounts by type.

Export the person cash_isa_subscriptions and stocks_and_shares_isa_subscriptions that #2710 defines, and the per-person balances if (1) lands.

Decisions owed

• Donor: a draw from the tables only, or the WAS round-7 to round-8 panel as a donor for who pays in large amounts.
• Calibration: bind the Table 9.7 cells as calibration targets, or hold them in stage-time gates.
• Split: the rule for subscribers to both types.
• Timing: whether per-person balances ship in this issue or later.

Disclosure

• Receipts and diagnostics follow the unit-record guard: no per-record survey values, no extremes, and no cell on one or two records.
• The HMRC targets are published statistics.

Related

• PolicyEngine/policyengine-uk#2710 (engine inputs and limits)
• PolicyEngine/uk-isa-allowance#1
• #1003 (Lifetime ISA holdings)
• #1360 (versioned ISA income export)
• #1095 (UK tracker)