diff --git a/changelog.d/ni-business-rates-regional-district.changed.md b/changelog.d/ni-business-rates-regional-district.changed.md
new file mode 100644
index 0000000000..2bbb47a682
--- /dev/null
+++ b/changelog.d/ni-business-rates-regional-district.changed.md
@@ -0,0 +1 @@
+- Documented that the Northern Ireland business rates statistic counts the regional and district rates, while the OBR line that projects it appears to cover the regional rate only.
diff --git a/policyengine_uk/data/fiscal_partition.yaml b/policyengine_uk/data/fiscal_partition.yaml
index 5fb8ec17c4..30c01ae850 100644
--- a/policyengine_uk/data/fiscal_partition.yaml
+++ b/policyengine_uk/data/fiscal_partition.yaml
@@ -469,9 +469,13 @@ tax_heads:
       projected with OBR receipts (gov.hmrc.business_rates.statistics). In
       2026-27 the England, Scotland and Wales series total £37.26bn against
       £37.41bn on the OBR's Business rates line (receipts Table 3.8).
-      Northern Ireland's £0.78bn (LPS non-domestic rates paid) compares with
-      £0.40bn on the OBR's "Northern Ireland business rates" line, which the
-      OBR records under other taxes (Table 3.2). Measured 2026-10-11.
+      Northern Ireland's projected £0.78bn (LPS non-domestic rates paid,
+      regional and district rates together) compares with £0.40bn on the
+      OBR's "Northern Ireland business rates" line, which the OBR records
+      under other taxes (Table 3.2) and which appears to cover the regional
+      rate only. ONS records NI regional non-domestic rates (central
+      government, £327m in 2024-25) and district non-domestic rates (local
+      government, £378m) separately. Measured 2026-10-11.
   - id: sdlt_household
     label: Stamp duty land tax on the household's own purchases
     variable: expected_sdlt
diff --git a/policyengine_uk/parameters/gov/economic_assumptions/yoy_growth.yaml b/policyengine_uk/parameters/gov/economic_assumptions/yoy_growth.yaml
index 20b372e014..e69980e45a 100644
--- a/policyengine_uk/parameters/gov/economic_assumptions/yoy_growth.yaml
+++ b/policyengine_uk/parameters/gov/economic_assumptions/yoy_growth.yaml
@@ -808,7 +808,7 @@ obr:
           - title: OBR EFO March 2025 (detailed forecast tables, receipts, Table 3.9; 2023-24 outturn)
             href: https://obr.uk/efo/economic-and-fiscal-outlook-march-2025/
     northern_ireland:
-      description: Growth in Northern Ireland business rates receipts (the OBR "Northern Ireland business rates" line in other taxes).
+      description: Growth in Northern Ireland business rates receipts (the OBR "Northern Ireland business rates" line in other taxes, which appears to cover the regional rate only; also applied to district rates, for which we found no published forecast).
       values:
         # Outturn (2024-25 over 2023-24) from OBR EFO March 2026 over March 2025, receipts Table 3.2 "Northern Ireland business rates"
         2024-01-01: 0.032513
diff --git a/policyengine_uk/parameters/gov/hmrc/business_rates/statistics.yaml b/policyengine_uk/parameters/gov/hmrc/business_rates/statistics.yaml
index 0acc293323..150305dd49 100644
--- a/policyengine_uk/parameters/gov/hmrc/business_rates/statistics.yaml
+++ b/policyengine_uk/parameters/gov/hmrc/business_rates/statistics.yaml
@@ -9,8 +9,11 @@ metadata:
 #   the central rating lists (mostly extensive holdings of industries that
 #   span several authorities), whose rates are paid to the Secretary of State
 #   or the Welsh Government instead.
-# - Scotland: the contributable amount councils pay into the national pool.
-# - Northern Ireland: non-domestic rates paid to Land & Property Services.
+# - Scotland: the contributable amount, councils' nominal contribution to the
+#   national non-domestic rates pool.
+# - Northern Ireland: non-domestic rates paid to Land & Property Services,
+#   which levies the regional rate and each council's district rate as one
+#   bill.
 # business_rates allocates all of it by shareholding, a modelling convention
 # that applies to every ratepayer on the lists, including, on the Welsh
 # central list in 2024-25, Network Rail (£10.5m) and Dwr Cymru (£17.0m) of
@@ -26,7 +29,8 @@ metadata:
 # After its latest outturn year each series is projected by uprating with the
 # OBR's receipts forecast: England, Scotland and Wales with the OBR's
 # "Business rates" line, Northern Ireland with its "Northern Ireland business
-# rates" line (gov.economic_assumptions.yoy_growth.obr.business_rates). The
+# rates" line (gov.economic_assumptions.yoy_growth.obr.business_rates), which
+# appears to cover the regional rate only (see NORTHERN_IRELAND below). The
 # economic-assumption indices run to 2039, so every series is flat in cash
 # terms from 2039-40.
 revenue:
@@ -128,11 +132,33 @@ revenue:
       reference:
         - title: StatsWales, Non-domestic rates central list, "Total Rates Received", Wales (2019-20 to 2024-25)
           href: https://stats.gov.wales/en-GB/ddada8b1-60ac-4a9a-8a8e-4e04df23d2f8/start
+  # A Northern Ireland rate bill is the regional rate, made by the Department
+  # of Finance, plus the council's district rate, levied "as if they were
+  # items of a single rate" (Rates (Northern Ireland) Order 1977, art 9(1)).
+  # This series counts payments of both, as allocated to non-domestic
+  # ratepayer accounts by 31 March (mixed domestic and non-domestic
+  # properties included). Of the £1.59bn of rates that Land & Property
+  # Services paid into the NI Consolidated Fund in 2024-25, councils received
+  # £0.81bn as district rates (Public Income and Expenditure Account
+  # 2024-25, note 6).
+  # The OBR's "Northern Ireland business rates" line that projects this series
+  # is about half its level, £382m against £741m in 2024-25, and appears to
+  # cover the regional rate only, though the OBR's tables do not define it:
+  # - ONS records NI regional non-domestic rates (central government, £327m
+  #   in 2024-25) and district non-domestic rates (local government, £378m)
+  #   separately (Country and regional public sector finances, financial year
+  #   ending 2025).
+  # - Splitting each council's 2024-25 payments by its regional (29.02p) and
+  #   district (25.57p to 38.22p) poundages estimates the regional part at
+  #   £364m. This is an estimate, not a published split.
+  # We found no published forecast of NI district non-domestic rates, so the
+  # district part is assumed to grow with the OBR's regional line.
   NORTHERN_IRELAND:
     description: >-
-      Non-domestic rates paid against ratepayer accounts in the rating year,
-      Northern Ireland. The series starts in 2020-21 (a rates-holiday year),
-      and earlier years take that value.
+      Non-domestic rates paid against ratepayer accounts by the end of the
+      rating year, Northern Ireland, regional and district rates together.
+      The series starts in 2020-21 (a rates-holiday year), and 2015-16 to
+      2019-20 take that value.
     values:
       2020-01-01: 380_876_615
       2021-01-01: 446_688_825