Merge pull request #1768 from PolicyEngine/codex/fix-class4-maximum-steps-20260606

Fix remaining Class 4 NI annual maximum steps

Author: MaxGhenis

changelog.d/class4-maximum-steps.md

@@ -0,0 +1 @@
+- Apply the Class 4 National Insurance annual maximum's Step Six and Step Nine calculations from Regulation 100.

policyengine_uk/tests/policy/baseline/gov/hmrc/national_insurance/class_4/ni_class_4_maximum.yaml

@@ -5,4 +5,4 @@
     self_employment_income: 100_000
     employment_income: 100_000
   output:
-    ni_class_4_maximum: 48_661
+    ni_class_4_maximum: 3_083.22

policyengine_uk/tests/test_ni_class_4_maximum.py

@@ -3,7 +3,7 @@
 from policyengine_uk import Simulation
 
 
-def test_class_4_annual_maximum_uses_current_main_rate_factor():
+def test_class_4_annual_maximum_applies_case_3_steps():
     year = 2026
     monthly_primary_class_1 = {f"{year}-{month:02d}": 500 for month in range(1, 13)}
     sim = Simulation(
@@ -21,6 +21,29 @@ def test_class_4_annual_maximum_uses_current_main_rate_factor():
     )
 
     assert sim.calculate("ni_class_4_maximum", year)[0] == pytest.approx(
-        46_484,
+        1_748.60,
         abs=0.01,
     )
+    assert sim.calculate("ni_class_4", year)[0] == pytest.approx(1_748.60, abs=0.01)
+
+
+def test_class_4_annual_maximum_does_not_bind_without_class_1():
+    year = 2026
+    sim = Simulation(
+        situation={
+            "people": {
+                "person": {
+                    "age": {year: 40},
+                    "self_employment_income": {year: 60_000},
+                }
+            },
+            "benunits": {"benunit": {"members": ["person"]}},
+            "households": {"household": {"members": ["person"]}},
+        }
+    )
+
+    assert sim.calculate("ni_class_4_maximum", year)[0] == pytest.approx(
+        2_456.60,
+        abs=0.01,
+    )
+    assert sim.calculate("ni_class_4", year)[0] == pytest.approx(2_456.60, abs=0.01)

policyengine_uk/variables/gov/hmrc/national_insurance/class_4/ni_class_4_maximum.py

@@ -20,22 +20,23 @@ def formula(person, period, parameters):
         step_3 = step_2 + 53 * ni.class_2.flat_rate
         class_2_contributions = person("ni_class_2", period)
         primary_class_1_contributions = person("ni_class_1_employee_primary", period)
-        step_4 = step_3 - class_2_contributions - primary_class_1_contributions
+        step_4_raw = step_3 - class_2_contributions - primary_class_1_contributions
+        step_4 = max_(step_4_raw, 0)
         class_4_main_contributions = person("ni_class_4_main", period)
         other_aggregate_contributions = (
             primary_class_1_contributions
             + class_2_contributions
             + class_4_main_contributions
         )
-        case_1 = (step_4 >= 0) & (step_4 > other_aggregate_contributions)
-        case_2 = (step_4 >= 0) & (step_4 <= other_aggregate_contributions)
-        case_3 = step_4 < 0
+        case_1 = (step_4_raw >= 0) & (step_4_raw > other_aggregate_contributions)
+        case_2 = (step_4_raw >= 0) & (step_4_raw <= other_aggregate_contributions)
+        case_3 = step_4_raw < 0
         step_5 = step_4 / main_rate
         profits = person("self_employment_income", period)
-        step_6 = lpl - min_(upl, profits)
+        step_6 = min_(upl, profits) - lpl
         step_7 = max_(0, step_6 - step_5)
         step_8 = step_7 * add_rate
-        step_9 = max_(0, profits - upl)
+        step_9 = max_(0, profits - upl) * add_rate
 
         return select(
             [