Add LA-level council tax calibration targets (Band D + band distribution)

[vahid-ahmadi]

Summary

Adds 350 LA-level ons/council_tax_band_d/{code} targets (Band D amount per billing authority) and 2,541 ons/council_tax_band_count/{code}/{band} targets (dwellings per band A-H per LA) built from four public sources, covering all 296 English + 22 Welsh + 32 Scottish LAs in local_authorities_2021.csv. Follows the pattern of the regional land-value targets, one geography deeper.

Like #371 this is plumbing + data only — wiring these into datasets/local_areas/local_authorities/loss.py is a deliberate follow-up.

What this PR does

Four public sources, one canonical CSV

storage/la_council_tax.csv (360 rows, 31 KB) joins:

Column	Source	Coverage
band_d_amount (England)	MHCLG Council Tax levels set by local authorities in England 2026-27, Table 10, column 17	296/296 English LAs
band_d_amount (Wales)	Welsh Government Council Tax levels April 2026 to March 2027, Table 1 "Overall average band D"	22/22 Welsh LAs
band_d_amount (Scotland)	Scottish Government Council Tax Assumptions 2025, "CT by Band 2025-26" Band D column	32/32 Scottish LAs
band_A … band_H + total_dwellings	VOA Council Tax: Stock of Properties, 2025, CTSOP1.0 summary table	295 English + 22 Welsh LAs

Column 17 in DLUHC Table 10 is the right one: "Average (Band D 2 adult equivalent) council tax for area of the billing authority including both local and major precepts" — i.e. the full amount households pay, inclusive of county, police, fire and parish precepts.

Code joins reconciled

• Post-2023 South Yorkshire E-codes (E08000038 Barnsley, E08000039 Sheffield) are remapped to the pre-2023 codes used in the reference LA list (E08000016, E08000019).
• Scottish source name quirks normalised: Argyll & Bute → Argyll and Bute, Dumfries & Galloway → Dumfries and Galloway, Perth & Kinross → Perth and Kinross, Shetland Islands (double space) → Shetland Islands, Edinburgh, City of → City of Edinburgh.

New module

targets/sources/la_council_tax.py:

• get_targets() returns Target objects at geographic_level=LOCAL_AUTHORITY.
• Band D targets tagged with year 2026 (England, Wales) or 2025 (Scotland) based on the source's latest release.
• Per-country reference URLs so downstream consumers can audit each value to its publication.
• Missing values (NI has no council tax, Scottish band counts) are filtered out of values rather than emitted as NaN so calibrators cleanly skip them.

Documented coverage gaps

• Northern Ireland (10 LAs): NI uses domestic rates, not council tax. has_council_tax=False flag set; no targets emitted for NI.
• Scotland band counts (32 LAs): the VOA summary doesn't cover Scotland. Scottish Assessors publishes per-LA chargeable-dwellings separately; follow-up PR.
• City of London Band A: VOA suppresses this cell ([c]) for disclosure control; other bands populated.

Testing (the #371 lesson)

22 hermetic tests, all green locally:

• CSV structure (4): row count matches the reference LA list, expected columns present, four UK countries represented, every code matches the reference.
• Value plausibility (4): Band D amount in [£900, £3,500], total dwellings in [200, 800,000], explicit Isles of Scilly regression (total in [500, 5,000], guarding against the 2.49M outlier that slipped into Add LA-level household land value calibration targets #371), band totals sum to total dwellings within 20-property slack.
• Coverage expectations (4): every English / Welsh / Scottish LA has a Band D value; Northern Ireland is explicitly flagged as no-council-tax.
• Spot-checks (2): Wandsworth and Westminster are the two lowest-Band-D LAs (catches row-swap bugs); Scottish average Band D is £500+ below English average.
• Target-API invariants (8): get_targets() returns non-empty without network, Band D target count matches CSV, band count target count matches Σ non-null band columns, every target carries LOCAL_AUTHORITY geo level + geo_code, correct units and is_count=True on count targets, every target has ≥1 year of values, every band count ≤ 500k.

Sanity check — top 10 LAs by Band D amount (2026-27 where available)

LA	Country	Band D
Dorset	ENGLAND	£2,765
Lewes	ENGLAND	£2,756
Nottingham	ENGLAND	£2,755
Rutland	ENGLAND	£2,738
Wealden	ENGLAND	£2,728
Merthyr Tydfil	WALES	£2,594
Bridgend	WALES	£2,555
Hartlepool	ENGLAND	£2,560
Newport	WALES	£2,443
Blaenau Gwent	WALES	£2,452

Bottom of the table is dominated by the central-London LAs with the lowest precepts: Wandsworth (£1,028), Westminster (£1,050), City of London (£1,330).

Out of scope for this PR (follow-ups)

• Wiring these targets into datasets/local_areas/local_authorities/loss.py so the LA reweighting actually calibrates on them.
• Scottish Assessors per-LA chargeable-dwellings to fill the Scotland band-count gap.
• Council Tax Support caseload per LA (DWP StatXplore) — caseload-style, different target shape.
• Single Person Discount rate per LA (CIPFA Council Tax Base) — same.

Related

• #371 — Add LA-level household land value calibration targets — the regional methodology this PR parallels; also the origin of the disclosure-control bound-check test lesson.
• targets/sources/voa_council_tax.py already provides band targets at the REGION level; this PR adds the LA level alongside it without modifying the regional path.

[vahid-ahmadi]

Review — items to address before merge

Blocking

1. Welsh Band I is dropped. Wales has had 9 council tax bands (A–I) since its 2005 revaluation. _BAND_COUNT_COLUMNS in la_council_tax.py only maps A–H, and the CSV has no band_I column, so every Welsh Band I dwelling is silently missing from the emitted targets. If a calibrator consumes these to set the band distribution, Welsh Band I is implicitly targeted to zero.

Fix: add band_I column to la_council_tax.csv (populated for Welsh rows, NaN elsewhere), add "I": "band_I" to _BAND_COUNT_COLUMNS, update the module docstring, and add a test asserting non-null Band I for the larger Welsh LAs (Cardiff, Swansea, Vale of Glamorgan, Monmouthshire).

2. total_dwellings is Σ(A..H) for Welsh rows, not VOA "All properties". Verified from the CSV:

• Cardiff (W06000015): 5320 + 20420 + 34920 + 37370 + 31540 + 22250 + 10590 + 2850 = 165,260 — matches CSV exactly.
• Monmouthshire (W06000021): 520 + 3450 + 7140 + 9460 + 7660 + 8100 + 5520 + 1790 = 43,640 — matches CSV exactly.

Because both sides of test_band_counts_sum_to_total are derived from the same A–H sum, the test passes trivially and does not validate fidelity to the published total. Fix: re-source total_dwellings from the VOA "All properties" column. Once done, the test becomes a real fidelity check with the existing 20-property slack.

Coherence

3. variable field diverges from voa_council_tax.py. The existing regional source uses variable="council_tax_band" with no breakdown_variable; this PR uses variable="council_tax_band_count" with breakdown_variable=f"council_tax_band_{band}". Downstream loss.py wiring will then need to know two names for the same conceptual quantity. Prefer aligning — either update voa_council_tax.py to the new scheme or match its convention here.

4. CSV column asymmetry. band_A, band_B, band_C, band_D_count, band_E, band_F, band_G, band_H — the _count suffix only on D to avoid clashing with band_d_amount. Cleaner to use a count_band_X prefix scheme, or rename band_d_amount → amount_band_d so the count columns stay symmetric. This also simplifies _BAND_COUNT_COLUMNS to a plain prefix map.

Nits

5. _load_table() re-reads the CSV on every get_targets() call. @lru_cache matches the pattern in voa_council_tax.py and is free.

6. has_council_tax column is derivable from country == "NORTHERN_IRELAND". Arguably self-documenting, so leave if preferred.

7. Module docstring opens with "band A–H" — update once Band I is added.

PR description

With Band I added, the "2,541 band-count targets" figure will rise by the number of non-null Welsh Band I cells — worth regenerating the description from the final CSV before merging. My earlier arithmetic of 318 × 8 = 2,544 minus City of London band A = 2,543 gave 2 more suppressions than the description claimed; Band I coverage may resolve part of that gap too.

[vahid-ahmadi]

Pushed d2d2acf addressing the review items:

Blocking fixes

• Welsh Band I now sourced from VOA CTSOP1.0 for all 22 Welsh LAs (Cardiff 1480, Monmouthshire 670, Vale of Glamorgan 1060, etc.); emitted as voa/council_tax/{code}/I targets. New tests: test_welsh_las_have_band_i, test_english_las_have_no_band_i, test_cardiff_band_i_matches_published_figure.
• total_dwellings now sourced from VOA "All properties" for every English + Welsh row rather than derived from Σ(A..H). test_band_counts_sum_to_total is now a real fidelity check (max discrepancy 10, VOA rounding).

Coherence fixes

• Band-count targets renamed to voa/council_tax/{code}/{band} with variable="council_tax_band" (aligned with the existing regional voa_council_tax.py). breakdown_variable dropped to match. council_tax_band is the actual PolicyEngine-UK enum variable and has Band I in its possible values.
• CSV count columns renamed to the symmetric count_band_A..count_band_I scheme (old band_D_count outlier removed).

Nits

• _load_table() now @lru_cache-ed.
• Module docstring clarified: "A–H in England/Scotland, A–I in Wales".

Final target counts: 350 Band D amount + 2,563 band counts (up from 2,541: +22 Welsh Band I, +2 resolved [c] suppressions). 25/25 tests pass locally via uv run pytest.