Improve rent uprating (#1227)

* Improve rent uprating

* Add safety check

* Add links to docs

Author: nikhilwoodruff

changelog_entry.yaml

@@ -0,0 +1,4 @@
+- bump: minor
+  changes:
+    fixed:
+    - Uprating for rent split by private and social rented sectors.

docs/book/assumptions/growthfactors.md

@@ -1,6 +1,6 @@
 # Economic assumptions
 
-We project economic variables using year-over-year growth rates stored in `parameters/gov/economic_assumptions/yoy_growth.yaml`. We generate index values from these rates to update household variables. We source all values from the OBR's Economic and Fiscal Outlook (March 2025) unless we specify otherwise.
+We project economic variables using year-over-year growth rates stored in `parameters/gov/economic_assumptions/yoy_growth.yaml`. We generate index values from these rates to update household variables. We source all values from the OBR's Economic and Fiscal Outlook ([March 2025](https://obr.uk/efo/economic-and-fiscal-outlook-march-2025/)) unless we specify otherwise.
 
 ## Consumer price index
 
@@ -95,7 +95,7 @@ We apply this to: `self_employment_income`
 
 ## Population
 
-We use ONS population projections.
+We use ONS population [projections](https://www.ons.gov.uk/peoplepopulationandcommunity/populationandmigration/populationprojections/bulletins/nationalpopulationprojections/2022based).
 
 | Fiscal year | 2022 | 2023 | 2024 | 2025 | 2026 | 2027 | 2028 | 2029 |
 |-------------|------|------|------|------|------|------|------|------|

policyengine_uk/parameters/gov/economic_assumptions/yoy_growth.yaml

@@ -254,21 +254,20 @@ obr:
         - title: OBR EFO March 2025
           href: https://obr.uk/efo/economic-and-fiscal-outlook-march-2025/
 
-  rent:
-    description: Rent year-on-year growth.
+  social_rent:
+    description: Rent year-on-year growth (CPI+1%, one year lagged).
     values:
-      2021-01-01: 0.018
-      2022-01-01: 0.04
-      2023-01-01: 0.063
-      2024-01-01: 0.074
-      2025-01-01: 0.057
-      2026-01-01: 0.036
-      2027-01-01: 0.027
-      2028-01-01: 0.023
-      2029-01-01: 0.024
+      2022-01-01: 0.050
+      2023-01-01: 0.110
+      2024-01-01: 0.067
+      2025-01-01: 0.033
+      2026-01-01: 0.042
+      2027-01-01: 0.029
+      2028-01-01: 0.030
+      2029-01-01: 0.030
     metadata:
       unit: /1
-      label: rent growth
+      label: social rent growth
       reference:
         - title: OBR EFO March 2025
           href: https://obr.uk/efo/economic-and-fiscal-outlook-march-2025/

policyengine_uk/variables/household/consumption/benunit_rent.py

@@ -5,8 +5,8 @@ class benunit_rent(Variable):
     value_type = float
     entity = BenUnit
     label = "Rent"
-    documentation = "Gross rent that members of this family are liable for"
+    documentation = "Gross rent that members of this family are liable for (social housing only)"
     definition_period = YEAR
     unit = GBP
-
+    uprating = "gov.economic_assumptions.indices.obr.social_rent"
     adds = ["personal_rent"]

policyengine_uk/variables/input/consumption/property/council_tax.py

@@ -15,6 +15,9 @@ def formula(household, period, parameters):
             # We don't have growth rates for council tax by nation before this.
             return 0
 
+        if household.simulation.dataset is None:
+            return 0
+
         data_year = household.simulation.dataset.time_period
 
         original_ct = household("council_tax", data_year)

policyengine_uk/variables/input/rent.py

@@ -11,4 +11,43 @@ class rent(Variable):
     value_type = float
     unit = GBP
     quantity_type = FLOW
-    uprating = "gov.economic_assumptions.indices.obr.rent"
+
+    def formula(household, period, parameters):
+        if period.start.year < 2023:
+            # We don't have growth rates for rent before this.
+            return 0
+
+        if household.simulation.dataset is None:
+            return 0
+
+        data_year = household.simulation.dataset.time_period
+        original_rent = household("rent", data_year)
+        tenure_type = household("tenure_type", period).decode_to_str()
+
+        is_social_rent = (tenure_type == "RENT_FROM_COUNCIL") | (
+            tenure_type == "RENT_FROM_HA"
+        )
+
+        is_private_rent = tenure_type == "RENT_PRIVATELY"
+
+        obr = parameters.gov.economic_assumptions.indices.obr
+
+        private_rent_uprating = obr.lagged_average_earnings(
+            period
+        ) / obr.lagged_average_earnings(data_year)
+        social_rent_uprating = obr.social_rent(period) / obr.social_rent(
+            data_year
+        )
+
+        return select(
+            [
+                is_social_rent,
+                is_private_rent,
+                True,
+            ],
+            [
+                original_rent * social_rent_uprating,
+                original_rent * private_rent_uprating,
+                original_rent,
+            ],
+        )