[juaristi22]

Fixes #2232.

Document the ordinary Housing Benefit child allowances that the model currently omits,
including the GB/NI two-child restriction dates, child-specific transitional protection,
CTC ordinary-child-element assessments and historical exceptions. Explain why current
family composition or cash CTC receipt cannot recover the required claim and assessment
history. Link the assumptions page from the Housing Benefit parameter README.

The page preserves the decision to defer C1 implementation, including claim-history or
exception inputs and an unrestricted-period-only implementation. It distinguishes the
legal removal dates from annual modelling conventions and keeps disability/family-premium
work separate. No formulas, parameter values, eligibility rules, inputs or tests change.

Validation: the actual HTML documentation build passed, including the
new Assumptions page, its navigation entry and source links. All 2,969 other tracked files
remain byte-for-byte unchanged. Required formatting, lint and diff checks passed.
No numerical model tests or population estimate were run for this prose-only change.

axiom: n/a: historical documentation only; eligibility, inputs, parameter values and calculations are unchanged.

[vahid-ahmadi]

Looks right, fine to approve.

@juaristi22 this is a prose-only change and the legal content checks out against the primary sources. Two small citation nits, neither blocking.

1. nit: docs/book/assumptions/housing-benefit-child-allowances.md:72-74. The Child Tax Credit comparison is cited only to working-age HB regulation 22(3)-(5) (SI 2006/213) and SR 2017/79 reg 6. The pension-age equivalent sat in regulation 22(5A)-(5D) of SI 2006/214 until SI 2024/611 reg 6 omitted it. Since the table covers the pension-age end date, a pointer to that paragraph would make the page complete.
2. nit: docs/book/assumptions/housing-benefit-child-allowances.md:62 cites "DWP Circular A5/2017, paragraphs 15-16 and example 3" with no link, while the earlier references to the same circular are linked. Linking it again would help readers.

What I checked

• Read the full diff, PR body and issue #2232.
• Confirmed on main that housing_benefit_applicable_amount adds only the adult personal allowance and benefits_premiums, so the page's description of the omission is accurate.
• Checked the dates and provisions on legislation.gov.uk: SI 2024/611 (in force 8 June 2024, reg 6 removes the two-child cap from pension-age HB reg 22), SR 2024/119 (8 June 2024, reg 5), SI 2026/316 (6 April 2026, regs 2 and 4, including omission of SI 2017/376 reg 9), SR 2026/68 (6 April 2026, regs 2 and 4), and SR 2017/79 (reg 1(2) 11 May 2017, reg 6 CTC comparison, reg 8 protection keyed to 10 May 2017). All match the table and text.
• Hand-checked the twins example against the CTC multiple-birth exception: two existing children plus twins gives three ordinary elements. Correct.
• The README's relative link resolves to the new page. The page is picked up by the existing docs/book/assumptions/*.md pattern in myst.yml.
• No code, parameters or tests change, so there was nothing to run. I did not build the docs locally; hosted CI shows the documentation build passing.
• Overlaps: no other open PR touches these files, including the docs pages in #2386.

🤖 Generated with Claude Code

[vahid-ahmadi]

Approving, per my review above. Nits are optional.