feat: implement LHA cap for private renters on HB and UC

LEGISLATIVE_REFERENCE.md

@@ -415,7 +415,22 @@ yet migrated (governed by `uc_migration.housing_benefit` rates).
 Source: SI 2006/213 reg.70 ([`uksi/2006/213/regulation/70`](https://www.legislation.gov.uk/uksi/2006/213/regulation/70))
 
 Maximum HB = 100% of eligible rent (subject to LHA caps for private renters). The model does
-not compute LHA caps; it uses reported rent directly.
+not compute LHA caps; it uses reported rent directly as eligible rent for all tenures.
+
+LHA cap is implemented in `src/variables/benefits.rs` via `lha_monthly_cap()`. When `params.lha`
+is present and enabled, eligible rent for private renters (TenureType::RentPrivately) is capped
+at the regional LHA rate for the household's bedroom entitlement, computed by
+`lha_bedroom_entitlement()` (implements UC Regs 2013 Sch.4 / HB Regs 2006 Sch.B1).
+
+The LHA rates are stored as region×category monthly amounts in `params.lha.rates_monthly`.
+Since the FRS suppresses BRMA identifiers for disclosure reasons, the model uses region-level
+30th percentile rates (derived from the VOA list of rents via policyengine-uk, uprated using the
+ONS Index of Private Housing Rental Prices). This understates within-region variation but
+captures the main regional gradient. The 2025/26 baseline uses rates frozen at the 2024/25
+reset level (April 2024 reset to 30th percentile, re-frozen April 2025).
+
+Reform scenarios can vary `params.lha.private_rent_index` (multiplicative uprating of all rates,
+e.g. 1.10 = 10% increase) without changing the underlying rate table.
 
 ### 7.2 Applicable Amount
 

changelog.d/added/lha-cap.md

@@ -0,0 +1 @@
+Implement Local Housing Allowance cap for private renters on HB and UC: bedroom entitlement logic (UC Regs 2013 Sch.4), region×category monthly rate table derived from VOA rent data uprated to 2024/25, and a `private_rent_index` scalar for modelling LHA unfreeze or rate changes.

parameters/2025_26.yaml

@@ -263,6 +263,33 @@ wealth_tax:
   threshold: 10000000.0
   rate: 0.01
 
+lha:
+  # Local Housing Allowance rates for 2025/26.
+  # LHA was re-frozen in April 2025 at the 2024/25 reset rates.
+  # 2024/25 rates: 30th percentile of private rents in each BRMA, reset from freeze.
+  # Source: VOA list of rents (2019–2020 data), uprated to April 2024 using ONS Index of
+  #   Private Housing Rental Prices (index: April 2020 = 109.0, April 2024 ≈ 125.0;
+  #   uprate factor = 1.1468). Aggregated to GOR (region) as FRS suppresses BRMA identifiers.
+  # Categories: [A=shared, B=1-bed, C=2-bed, D=3-bed, E=4+bed] — monthly amounts (£).
+  # Row order: NE, NW, Yorkshire, E.Midlands, W.Midlands, E.ofEngland, London,
+  #            S.East, S.West, Wales, Scotland, N.Ireland.
+  enabled: true
+  private_rent_index: 1.0  # 1.0 = frozen (no uprating). Reform: set >1.0 to unfreeze.
+  rates_monthly:
+    - [323.01,  428.86,  486.06,  566.12,  766.23]   # North East
+    - [332.95,  457.44,  566.12,  657.60,  860.11]   # North West
+    - [323.01,  451.72,  543.26,  600.41,  857.72]   # Yorkshire
+    - [372.71,  486.06,  600.41,  686.18,  914.92]   # East Midlands
+    - [390.10,  514.63,  629.03,  743.38,  972.12]   # West Midlands
+    - [360.23,  629.03,  771.95,  886.35, 1252.29]   # East of England
+    - [580.97, 1200.81, 1469.61, 1735.40, 2172.93]   # London
+    - [392.58,  737.66,  914.92, 1086.46, 1543.90]   # South East
+    - [390.10,  571.83,  737.66,  874.87, 1143.66]   # South West
+    - [299.41,  419.92,  514.63,  566.12,  686.18]   # Wales
+    - [323.01,  468.91,  543.26,  629.03,  886.35]   # Scotland
+    - [273.42,  361.97,  428.86,  514.63,  566.12]   # Northern Ireland
+
+
 labour_supply:
   # OBR labour supply elasticities (Slutsky decomposition).
   # Source: OBR (2023) "Costing a cut in National Insurance contributions:

src/engine/entities.rs

@@ -560,6 +560,26 @@ impl Region {
         }
     }
 
+    /// LHA region index for rate table lookup (0–11, matching rates_monthly row order).
+    /// Order: NE=0, NW=1, Yorks=2, EM=3, WM=4, EofE=5, London=6, SE=7, SW=8,
+    ///        Wales=9, Scotland=10, NI=11.
+    pub fn to_lha_region_idx(&self) -> usize {
+        match self {
+            Region::NorthEast => 0,
+            Region::NorthWest => 1,
+            Region::Yorkshire => 2,
+            Region::EastMidlands => 3,
+            Region::WestMidlands => 4,
+            Region::EastOfEngland => 5,
+            Region::London => 6,
+            Region::SouthEast => 7,
+            Region::SouthWest => 8,
+            Region::Wales => 9,
+            Region::Scotland => 10,
+            Region::NorthernIreland => 11,
+        }
+    }
+
     pub fn name(&self) -> &'static str {
         match self {
             Region::NorthEast => "North East",

src/parameters/mod.rs

@@ -58,6 +58,11 @@ pub struct Parameters {
     /// Annual wealth tax (hypothetical — disabled by default).
     #[serde(default)]
     pub wealth_tax: Option<WealthTaxParams>,
+    /// Local Housing Allowance cap parameters.
+    /// When present, caps eligible rent for private renters at the regional LHA rate
+    /// for their bedroom entitlement category. Authority: HB Regs 2006 reg.13D.
+    #[serde(default)]
+    pub lha: Option<LhaParams>,
     /// OBR labour supply response elasticities.
     /// When enabled, the Slutsky-decomposition elasticities from OBR (2023) are applied
     /// to estimate intensive-margin labour supply responses to tax-benefit reforms.
@@ -564,6 +569,68 @@ impl Default for LabourSupplyParams {
     }
 }
 
+/// Local Housing Allowance (LHA) parameters.
+///
+/// LHA caps the eligible rent for private renters on HB/UC at the 30th percentile of
+/// local rents in each Broad Rental Market Area (BRMA), by bedroom entitlement category
+/// (A = shared, B = 1-bed, C = 2-bed, D = 3-bed, E = 4+-bed).
+///
+/// Since the FRS suppresses BRMA identifiers for disclosure control, we use region-level
+/// 30th percentile rates derived from the VOA rent data (same underlying source as BRMA
+/// rates, aggregated to GOR). This understates within-region variation but captures the
+/// main regional gradient.
+///
+/// Rate history:
+///   - Frozen: April 2020 – March 2024 (SI 2019/1303)
+///   - Reset to 30th percentile: April 2024 (gov.uk announcement, 28 Nov 2023)
+///   - Re-frozen: April 2025 (OBR EFO March 2025 assumption)
+///
+/// Uprating: `private_rent_index` uprates the baseline rates for reform scenarios;
+/// e.g. setting `private_rent_index: 1.10` models a 10% LHA increase.
+/// For the baseline (frozen), this field is 1.0.
+///
+/// Source: VOA list of rents (via policyengine-uk), uprated using ONS Index of Private
+/// Housing Rental Prices (base: April 2020 = 100, April 2024 ≈ 114.7).
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct LhaParams {
+    /// Whether LHA cap is active. If false, eligible rent = actual rent (pre-reform default).
+    #[serde(default = "default_true_lha")]
+    pub enabled: bool,
+    /// Multiplier applied to all rates for reform scenarios (e.g. 1.1 = 10% increase).
+    /// For baseline frozen scenario this is 1.0.
+    #[serde(default = "default_one")]
+    pub private_rent_index: f64,
+    /// LHA rates by region (11 regions matching Region enum) then by bedroom category
+    /// (index 0=shared/A, 1=1-bed/B, 2=2-bed/C, 3=3-bed/D, 4=4+bed/E).
+    /// Values are monthly amounts (£).
+    /// Order: NorthEast, NorthWest, Yorkshire, EastMidlands, WestMidlands,
+    ///        EastOfEngland, London, SouthEast, SouthWest, Wales, Scotland, NorthernIreland.
+    pub rates_monthly: Vec<[f64; 5]>,
+}
+
+fn default_true_lha() -> bool { true }
+fn default_one() -> f64 { 1.0 }
+
+impl LhaParams {
+    /// Return the monthly LHA cap (£) for a given region and bedroom entitlement.
+    ///
+    /// `region_idx` maps to Region::to_rf_code() (0=NE, 1=NW, 2=Yorks, 3=EM, 4=WM,
+    /// 5=EofE, 6=London, 7=SE, 8=SW, 9=Wales/NI, 10=Scotland, 11=NI).
+    /// `bedrooms` is the LHA bedroom entitlement (1–4+), or 0 for shared accommodation.
+    /// Returns `None` if rates_monthly is empty or region_idx out of range.
+    pub fn monthly_cap(&self, region_idx: usize, bedrooms: u32) -> Option<f64> {
+        let row = self.rates_monthly.get(region_idx)?;
+        let col = match bedrooms {
+            0 => 0, // shared accommodation (Category A)
+            1 => 1,
+            2 => 2,
+            3 => 3,
+            _ => 4, // 4+ bedrooms → Category E
+        };
+        Some(row[col] * self.private_rent_index)
+    }
+}
+
 /// Annual wealth tax parameters (hypothetical — disabled by default).
 ///
 /// No current UK wealth tax exists. These parameters support modelling