property.council-tax-band Unreviewed

pub fn council_tax_band(band_d_charge: &Money, band: &str, nation: &str, on_date: &str) -> CouncilTaxBandCharge

/// The charge and the ratio that produced it.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct CouncilTaxBandCharge {
    pub band: String,
    /// the band's proportion
    pub ratio_numerator: i64,
    /// Band D's proportion: 9 (England, Wales) or 360 (Scotland)
    pub ratio_denominator: i64,
    /// Band D × numerator ÷ denominator, rounded half-up to the penny
    pub charge: Money,
}

fune!(property.council-tax-band@^1);  // then call council_tax_band(…)

use super::funejson::Value;  ← the fune runtime: the JSON value the test vectors use; fune build keeps it only where a signature takes one
use super::math_round_div::round_div;  ← from math.round-div ^1.0.0 · built alongside by fune
use super::money_amount::{money, money_from_value, money_to_value, Money};  ← from money.amount ^1.0.0 · built alongside by fune
use super::property_council_tax_band_data::{BandRatio, BAND_RATIOS, BAND_RATIOS_HISTORY};  ← this capability’s own data, compiled from data/band-ratios.json into the same file by fune build

// Keeps Band D × the largest proportion exact as a JavaScript number; the same limit here.
const MAX_MINOR: i64 = 1_000_000_000_000;

fn is_iso_date(value: &str) -> bool {
    let bytes = value.as_bytes();
    bytes.len() == 10
        && bytes.iter().enumerate().all(|(i, b)| if i == 4 || i == 7 { *b == b'-' } else { b.is_ascii_digit() })
}

/// The council tax for a band: Band D × the band's statutory proportion ÷ Band
/// D's, rounded half-up to the penny once. The proportions are dated rows, so
/// a Scottish band E before April 2017 uses 440/360 and after it 473/360.
///
/// # Panics
/// Panics on a malformed date, an unknown nation, a Band D charge not in GBP
/// or out of range, or a band the nation did not have on that date.
pub fn council_tax_band(band_d_charge: &Money, band: &str, nation: &str, on_date: &str) -> CouncilTaxBandCharge {
    if !is_iso_date(on_date) {
        panic!("onDate must be an ISO date (YYYY-MM-DD), received \"{}\"", on_date);
    }
    if !["england", "wales", "scotland"].contains(&nation) {
        panic!("unknown nation \"{}\": use england, wales or scotland", nation);
    }
    if band_d_charge.currency != "GBP" {
        panic!("bandDCharge must be in GBP, received {}", band_d_charge.currency);
    }
    if band_d_charge.minor < 0 || band_d_charge.minor > MAX_MINOR {
        panic!("bandDCharge must be between 0 and {} minor units, received {}", MAX_MINOR, band_d_charge.minor);
    }
    let mut found: Option<&'static BandRatio> = None;
    let mut earliest: Option<&'static str> = None;
    for row in BAND_RATIOS {
        if row.nation != nation {
            continue;
        }
        if earliest.map_or(true, |e| row.valid_from < e) {
            earliest = Some(row.valid_from);
        }
        if row.band != band || on_date < row.valid_from {
            continue;
        }
        if let Some(to) = row.valid_to {
            if on_date > to {
                continue;
            }
        }
        found = Some(row);
    }
    let row = match found {
        Some(row) => row,
        None => {
            if BAND_RATIOS_HISTORY != "full" {
                if let Some(e) = earliest {
                    if on_date < e {
                        panic!(
                            "no council tax band {} for {} on {}: this build was installed with history={}, so it only carries rules from {}",
                            band, nation, on_date, BAND_RATIOS_HISTORY, e
                        );
                    }
                }
            }
            panic!("no council tax band {} for {} on {}", band, nation, on_date)
        }
    };
    CouncilTaxBandCharge {
        band: band.to_string(),
        ratio_numerator: row.numerator,
        ratio_denominator: row.denominator,
        charge: money(round_div(band_d_charge.minor * row.numerator, row.denominator, "half-up"), "GBP"),
    }
}

pub fn council_tax_band_charge_to_value(result: &CouncilTaxBandCharge) -> Value {
    Value::obj(vec![
        ("band", Value::str(&result.band)),
        ("ratioNumerator", Value::Int(result.ratio_numerator)),
        ("ratioDenominator", Value::Int(result.ratio_denominator)),
        ("charge", money_to_value(&result.charge)),
    ])
}

pub fn fune_vector(args: &[Value]) -> Value {
    council_tax_band_charge_to_value(&council_tax_band(
        &money_from_value(&args[0]),
        args[1].as_str(),
        args[2].as_str(),
        args[3].as_str(),
    ))
}

Install

fune build

With that line in your source, in a Rust project (language rust in fune.project), fune build resolves it and its 2 dependencies, pins them in fune.lock, downloads only the Rust package of each, and builds the code above into your project’s .fune/build, one readable file per capability with a header linking back here. A crate’s build.rs runs it before every compile. Or pin a range in fune.project and build in one step:

fune add property.council-tax-band

The manifest, vectors and README with only the Rust implementation. Install it without the registry with fune add ./property.council-tax-band-1.0.2-rust.fune, or fetch it from a terminal with fune pull property.council-tax-band@1.0.2:rust.

The whole function, every language, is one file too: property.council-tax-band-1.0.2.fune, 31,407 bytes, sha256 5f8b90e78d630afbffb126b21c594e6bb7a5a0bf3fff236c747fe911a51d6094. It installs into a project of any language.

// fune: before property.council-tax-band

// fune: after property.council-tax-band

// fune: replace math.round-div in property.council-tax-band
// fune: replace money.amount in property.council-tax-band

// fune: step property.council-tax-band after <n|label>

More from the author

`councilTaxBand(money(200000, "GBP"), "A", "england", "2026-04-01")` is £1,333.33: six ninths of £2,000.

## The ratios, as dated data

`data/band-ratios.json` holds each nation's proportions with the dates they apply from:

| nation | from | A | B | C | D | E | F | G | H | I | |---|---|---|---|---|---|---|---|---|---|---| | England | 1 April 1993 | 6 | 7 | 8 | 9 | 11 | 13 | 15 | 18 | | | Wales | 1 April 1993 | 6 | 7 | 8 | 9 | 11 | 13 | 15 | 18 | | | Wales | 1 April 2005 | 6 | 7 | 8 | 9 | 11 | 13 | 15 | 18 | 21 | | Scotland | 1 April 1993 | 240 | 280 | 320 | 360 | 440 | 520 | 600 | 720 | | | Scotland | 1 April 2017 | 240 | 280 | 320 | 360 | 473 | 585 | 705 | 882 | |

England and Wales are ninths of Band D; Scotland is written in 360ths. Band I exists only in Wales, from the 2005 revaluation; asking for it elsewhere, or for a band before it existed, is an error. Scotland's 2017 change raised bands E to H only.

Pass any date inside the financial year (1 April to 31 March) being charged.

## Rounding

The exact fraction is rounded half-up to the penny, once. Authorities differ in how they round instalments; that is not this function's job.

## What it does not do

Discounts (the 25% single person discount), exemptions, disabled band reduction, premiums on empty and second homes, and splitting the charge into instalments. Northern Ireland has domestic rates, not council tax.

## Sources

- Local Government Finance Act 1992, section 5(1) (England: 6:7:8:9:11:13:15:18) and 5(1A) (Wales, for financial years beginning on or after 1 April 2005: 6:7:8:9:11:13:15:18:21): https://www.legislation.gov.uk/ukpga/1992/14/section/5 - Local Government Finance Act 1992, section 74(1) (Scotland: 240:280:320:360:473:585:705:882): https://www.legislation.gov.uk/ukpga/1992/14/section/74 - The Council Tax (Substitution of Proportion) (Scotland) Order 2016 (SSI 2016/368), for financial years beginning on or after 1 April 2017, the first change to the Scottish proportion since 1992: https://www.legislation.gov.uk/ssi/2016/368/made . The pre-2017 Scottish row (240:280:320:360:440:520:600:720) is the original 1992 proportion expressed in 360ths, identical to England's 6:7:8:9:11:13:15:18.

1.0.1 fixes Python accepting a trailing newline or non-ASCII digits in onDate; adds tests.

## Before you rely on this

**Not professional advice.** This capability calculates property figures from published rules. It is a software component for developers, not legal or financial advice. Rules change and every rate here has an effective date. Check that the dates cover your case. Verify results against the official sources listed above, and have a conveyancer or tax adviser review how you use it, before anyone relies on the output. Provided "as is" under its licence, without warranty.

**Unreviewed.** This capability's implementations agree in every language and pass its published test vectors, which were worked out from the official sources cited. But no qualified conveyancer or tax adviser has yet checked those vectors, or confirmed that the capability covers the cases it claims. Treat it as a draft. Do not use it for real people, money or decisions without your own expert review. Once a qualified reviewer signs off, this notice is replaced with their name, qualification and the date. Each new version needs fresh sign-off.

## Notices

Contains public sector information licensed under the Open Government Licence v3.0 (https://www.nationalarchives.gov.uk/doc/open-government-licence/version/3/).

Legislation: Crown copyright and database right.

1.0.2 marks it unreviewed and adds its attribution notices (NOTICE). The code and the tests are unchanged.

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