diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7e1328e..759db4c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -12,6 +12,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - `PVSystem` enum inputs (`mounting`, `module_orientation`, `inverter_type`) accept case-insensitive strings,
   at construction and on assignment.
 - `PVSystem.with_defaults_applied()` returns a copy with all unset derived inputs resolved.
+- `PVSystem.terrain_azimuth` and `PVSystem.terrain_slope` set the terrain orientation of all layouts
+  (default `0.0`). They are ignored for flush-mount fixed-tilt systems.
 
 ### Changed
 
@@ -21,6 +23,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - `PVSystem.payload` is now an output-only snapshot (JSON string) and is no longer read back; it is omitted
   from `to_file()` output.
 - Building a payload no longer overwrites the plant's `dc_capacity_MW`, `ac_capacity_MW` or `gcr`.
+- `Layout.terrain_azimuth` is now validated to [-360, 360] and `Layout.terrain_slope` to [-90, 90].
 
 ### Fixed
 
diff --git a/docs/getting-started/workflow-2-pvplant-builder.md b/docs/getting-started/workflow-2-pvplant-builder.md
index 8a7932f..d25d72d 100644
--- a/docs/getting-started/workflow-2-pvplant-builder.md
+++ b/docs/getting-started/workflow-2-pvplant-builder.md
@@ -88,6 +88,10 @@ plant.azimuth = 180.0           # South-facing (0=North, 90=East, 180=South, 270
 plant.gcr = 0.4                 # Ground coverage ratio (spacing between rows)
 plant.flush_mount = False       # Flush-mounted or rack-mounted
 
+# Terrain (optional; ignored for flush-mounted fixed-tilt systems)
+plant.terrain_azimuth = 0.0     # Direction the ground slopes down, degrees clockwise from North
+plant.terrain_slope = 0.0       # Slope of the ground in degrees (0 = flat)
+
 # Module orientation
 plant.module_orientation = "Portrait"  # or "Landscape" (sf.OrientationType)
 plant.modules_across = 1        # Number of modules in height direction
diff --git a/solarfarmer/models/layout.py b/solarfarmer/models/layout.py
index 8846050..b7780d4 100644
--- a/solarfarmer/models/layout.py
+++ b/solarfarmer/models/layout.py
@@ -62,9 +62,10 @@ class Layout(SolarFarmerBaseModel):
     number_of_strings_in_isolated_row : int
         Number of strings with no near-shading neighbors
     terrain_azimuth : float
-        Terrain slope azimuth in degrees
+        Terrain slope azimuth in degrees, range [-360, 360]
     terrain_slope : float
-        Terrain slope angle in degrees
+        Terrain slope angle in degrees, range [-90, 90]. A negative slope is
+        equivalent to the opposite ``terrain_azimuth``. 90 is a vertical surface
     module_quality_factor : float or None
         Module quality factor override, negative implies loss, range [-0.4, 0.1]
     """
@@ -89,8 +90,8 @@ class Layout(SolarFarmerBaseModel):
     number_of_strings_in_right_row: int = 0
     number_of_strings_in_left_row: int = 0
     number_of_strings_in_isolated_row: int = 0
-    terrain_azimuth: float = 0.0
-    terrain_slope: float = 0.0
+    terrain_azimuth: float = Field(0.0, ge=-360, le=360)
+    terrain_slope: float = Field(0.0, ge=-90, le=90)
     module_quality_factor: float | None = Field(None, ge=-0.4, le=0.1)
 
     @model_validator(mode="after")
diff --git a/solarfarmer/models/pvsystem/pvsystem.py b/solarfarmer/models/pvsystem/pvsystem.py
index 7b9826e..e603e3d 100644
--- a/solarfarmer/models/pvsystem/pvsystem.py
+++ b/solarfarmer/models/pvsystem/pvsystem.py
@@ -151,6 +151,21 @@ class PVSystem:
         Defines the symmetric rotation bounds: ``[-tracker_max_rotation_angle, +tracker_max_rotation_angle]``.
     azimuth : float
         Array azimuth in degrees (default 180, i.e., south-facing).
+    terrain_azimuth : float
+        Direction in which the terrain slopes down, in degrees clockwise from North,
+        range [-360, 360] (default 0.0). For example, an azimuth of 0 with a positive
+        ``terrain_slope`` is ground that descends towards the North (a north-facing slope).
+        For single-axis trackers, the angle between this direction and the tracker azimuth
+        (``azimuth``) determines how much of the slope tilts the tracker axis.
+        Ignored when ``flush_mount=True`` on fixed-tilt systems, where the terrain follows
+        the array plane (``azimuth`` and ``tilt``).
+    terrain_slope : float
+        Slope of the terrain in degrees, in the direction of ``terrain_azimuth``, range
+        [-90, 90] (default 0.0, i.e., flat ground). A negative slope is equivalent to the
+        opposite ``terrain_azimuth``.
+        For single-axis trackers, this is the slope that is resolved along the tracker axis
+        to give the axis tilt; the part across the axis is not applied.
+        Ignored when ``flush_mount=True`` on fixed-tilt systems.
     mounting : str
         Mounting configuration: 'Fixed' for fixed-tilt or 'Tracker' for single-axis trackers.
         Strings are case-insensitive (e.g., ``"tracker"``) and are stored as
@@ -315,6 +330,8 @@ class PVSystem:
     transformer_stages: int = 1
     plant_unavailability: float = 0.0
     grid_unavailability: float = 0.0
+    terrain_azimuth: float = TERRAIN_AZIMUTH
+    terrain_slope: float = TERRAIN_SLOPE
 
     # Effects and settings
     mounting_height: float | None = None  # None = derive from mounting at build time
@@ -831,6 +848,8 @@ def describe(self, verbose=False) -> None:
         else:
             print(f"Tilt: {self.tilt}°")
         print(f"Azimuth: {self.azimuth}°")
+        print(f"Terrain Azimuth: {self.terrain_azimuth}°")
+        print(f"Terrain Slope: {self.terrain_slope}°")
 
         # Mounting & Orientation
         print("\n--- MOUNTING & ORIENTATION ---")
@@ -1719,8 +1738,8 @@ def calculate_layout_objects(
             terrain_slope = plant.tilt
             number_strings_isolated = number_of_strings
         else:
-            terrain_azimuth = TERRAIN_AZIMUTH
-            terrain_slope = TERRAIN_SLOPE
+            terrain_azimuth = plant.terrain_azimuth
+            terrain_slope = plant.terrain_slope
             number_strings_isolated = STRINGS_IN_ISOLATED_ROW
 
         layout = Layout(
diff --git a/tests/test_construct_plant.py b/tests/test_construct_plant.py
index 09a85e7..8a710b7 100644
--- a/tests/test_construct_plant.py
+++ b/tests/test_construct_plant.py
@@ -2,6 +2,7 @@
 from pathlib import Path
 
 import pytest
+from pydantic import ValidationError
 
 from solarfarmer import PVSystem
 from solarfarmer.models import PVPlant
@@ -124,6 +125,80 @@ def total_inverters(pv_plant):
         assert total_inverters(small_plant) < total_inverters(large_plant)
 
 
+class TestTerrainProperties:
+    """terrain_azimuth / terrain_slope flow into the layouts, defaulting to 0.0."""
+
+    @staticmethod
+    def _layouts(plant):
+        pv_plant, _ = design_plant(plant.with_defaults_applied())
+        layouts = [
+            layout for t in pv_plant.transformers for inv in t.inverters for layout in inv.layouts
+        ]
+        assert layouts
+        return layouts
+
+    def test_defaults_are_zero(self, plant):
+        assert plant.terrain_azimuth == 0.0
+        assert plant.terrain_slope == 0.0
+
+    @pytest.mark.parametrize(
+        ("mounting", "flush_mount"),
+        [("Fixed", False), ("Tracker", False), ("Tracker", True)],
+    )
+    def test_user_values_applied_to_all_layouts(self, plant, mounting, flush_mount):
+        plant.mounting = mounting
+        plant.flush_mount = flush_mount
+        plant.terrain_azimuth = 90.0
+        plant.terrain_slope = 5.0
+        layouts = self._layouts(plant)
+        assert all(lo.terrain_azimuth == pytest.approx(90.0) for lo in layouts)
+        assert all(lo.terrain_slope == pytest.approx(5.0) for lo in layouts)
+
+    @pytest.mark.parametrize(("azimuth", "slope"), [(-360.0, -90.0), (360.0, 90.0), (-90.0, -5.0)])
+    def test_values_within_bounds_are_accepted(self, plant, azimuth, slope):
+        plant.terrain_azimuth = azimuth
+        plant.terrain_slope = slope
+        layouts = self._layouts(plant)
+        assert all(lo.terrain_azimuth == pytest.approx(azimuth) for lo in layouts)
+        assert all(lo.terrain_slope == pytest.approx(slope) for lo in layouts)
+
+    @pytest.mark.parametrize(
+        ("field", "value"),
+        [
+            ("terrain_azimuth", -360.1),
+            ("terrain_azimuth", 360.1),
+            ("terrain_slope", -90.1),
+            ("terrain_slope", 90.1),
+        ],
+    )
+    def test_values_outside_bounds_are_rejected(self, plant, field, value):
+        setattr(plant, field, value)
+        with pytest.raises(ValidationError, match=field):
+            design_plant(plant.with_defaults_applied())
+
+    def test_vertical_flush_mount_is_allowed(self, plant):
+        plant.flush_mount = True
+        plant.tilt = 90.0
+        assert all(lo.terrain_slope == pytest.approx(90.0) for lo in self._layouts(plant))
+
+    def test_flush_mount_ignores_terrain_properties(self, plant):
+        plant.flush_mount = True
+        plant.tilt = 15.0
+        plant.terrain_azimuth = 90.0
+        plant.terrain_slope = 5.0
+        layouts = self._layouts(plant)
+        assert all(lo.terrain_azimuth == pytest.approx(plant.azimuth) for lo in layouts)
+        assert all(lo.terrain_slope == pytest.approx(15.0) for lo in layouts)
+
+    def test_values_survive_file_round_trip(self, plant, tmp_path):
+        plant.terrain_azimuth = 45.0
+        plant.terrain_slope = 3.0
+        plant.to_file(tmp_path / "plant.json")
+        loaded = PVSystem.from_file(tmp_path / "plant.json")
+        assert loaded.terrain_azimuth == 45.0
+        assert loaded.terrain_slope == 3.0
+
+
 class TestGenerateMountingSpecs:
     """Regression tests for generate_mounting_specs()."""
 
diff --git a/tests/test_models/test_validation.py b/tests/test_models/test_validation.py
index 5edd1b9..5fd276e 100644
--- a/tests/test_models/test_validation.py
+++ b/tests/test_models/test_validation.py
@@ -127,6 +127,48 @@ def test_azimuth_out_of_range(self) -> None:
                 string_length=1,
             )
 
+    @pytest.mark.parametrize(
+        ("field", "value"),
+        [
+            ("terrain_azimuth", -360.1),
+            ("terrain_azimuth", 360.1),
+            ("terrain_slope", -90.1),
+            ("terrain_slope", 90.1),
+        ],
+    )
+    def test_terrain_out_of_range(self, field: str, value: float) -> None:
+        with pytest.raises(ValidationError, match=field):
+            Layout(
+                layout_count=1,
+                module_specification_id="m",
+                mounting_type_id="mt",
+                is_trackers=False,
+                azimuth=180,
+                pitch=5,
+                total_number_of_strings=1,
+                string_length=1,
+                **{field: value},
+            )
+
+    @pytest.mark.parametrize(
+        ("azimuth", "slope"), [(-360.0, -90.0), (360.0, 90.0), (-90.0, -5.0), (0.0, 0.0)]
+    )
+    def test_terrain_limits_are_inclusive(self, azimuth: float, slope: float) -> None:
+        layout = Layout(
+            layout_count=1,
+            module_specification_id="m",
+            mounting_type_id="mt",
+            is_trackers=False,
+            azimuth=180,
+            pitch=5,
+            total_number_of_strings=1,
+            string_length=1,
+            terrain_azimuth=azimuth,
+            terrain_slope=slope,
+        )
+        assert layout.terrain_azimuth == azimuth
+        assert layout.terrain_slope == slope
+
     def test_dc_ohmic_resistance_and_loss_both_set_raises(self) -> None:
         with pytest.raises(ValidationError, match="dc_ohmic_connector_resistance"):
             Layout(