diff --git a/tests/unit_tests/core/test_config.py b/tests/unit_tests/core/test_config.py
new file mode 100644
index 0000000000..35245b82d3
--- /dev/null
+++ b/tests/unit_tests/core/test_config.py
@@ -0,0 +1,187 @@
+"""Unit tests for core config functionality including areas and devices."""
+
+from collections.abc import Callable
+from pathlib import Path
+from typing import Any
+
+import pytest
+
+from esphome import config, config_validation as cv
+from esphome.config import Config
+from esphome.const import CONF_AREA, CONF_AREAS, CONF_DEVICES
+from esphome.core import CORE
+from esphome.core.config import Area, validate_area_config
+
+FIXTURES_DIR = Path(__file__).parent.parent / "fixtures" / "core" / "config"
+
+
+@pytest.fixture
+def yaml_file(tmp_path: Path) -> Callable[[str], str]:
+    """Create a temporary YAML file for testing."""
+
+    def _yaml_file(content: str) -> str:
+        yaml_path = tmp_path / "test.yaml"
+        yaml_path.write_text(content)
+        return str(yaml_path)
+
+    return _yaml_file
+
+
+@pytest.fixture(autouse=True)
+def reset_core():
+    """Reset CORE after each test."""
+    yield
+    CORE.reset()
+
+
+def load_config_from_yaml(
+    yaml_file: Callable[[str], str], yaml_content: str
+) -> Config | None:
+    """Load configuration from YAML content."""
+    CORE.config_path = yaml_file(yaml_content)
+    return config.read_config({})
+
+
+def load_config_from_fixture(
+    yaml_file: Callable[[str], str], fixture_name: str
+) -> Config | None:
+    """Load configuration from a fixture file."""
+    fixture_path = FIXTURES_DIR / fixture_name
+    yaml_content = fixture_path.read_text()
+    return load_config_from_yaml(yaml_file, yaml_content)
+
+
+def test_validate_area_config_with_string() -> None:
+    """Test that string area config is converted to structured format."""
+    result: dict[str, Any] = validate_area_config("Living Room")
+
+    assert isinstance(result, dict)
+    assert "id" in result
+    assert "name" in result
+    assert result["name"] == "Living Room"
+    # ID should be based on slugified name
+    assert result["id"].id == "living_room"
+
+
+def test_validate_area_config_with_dict() -> None:
+    """Test that structured area config passes through unchanged."""
+    area_id = cv.declare_id(Area)("test_area")
+    input_config: dict[str, Any] = {
+        "id": area_id,
+        "name": "Test Area",
+    }
+
+    result: dict[str, Any] = validate_area_config(input_config)
+
+    assert result == input_config
+    assert result["id"] == area_id
+    assert result["name"] == "Test Area"
+
+
+def test_device_with_valid_area_id(yaml_file: Callable[[str], str]) -> None:
+    """Test that device with valid area_id works correctly."""
+    result = load_config_from_fixture(yaml_file, "valid_area_device.yaml")
+    assert result is not None
+
+    esphome_config = result["esphome"]
+
+    # Verify areas were parsed correctly
+    assert CONF_AREAS in esphome_config
+    areas = esphome_config[CONF_AREAS]
+    assert len(areas) == 1
+    assert areas[0]["id"].id == "bedroom_area"
+    assert areas[0]["name"] == "Bedroom"
+
+    # Verify devices were parsed correctly
+    assert CONF_DEVICES in esphome_config
+    devices = esphome_config[CONF_DEVICES]
+    assert len(devices) == 1
+    assert devices[0]["id"].id == "test_device"
+    assert devices[0]["name"] == "Test Device"
+    assert devices[0]["area_id"].id == "bedroom_area"
+
+
+def test_multiple_areas_and_devices(yaml_file: Callable[[str], str]) -> None:
+    """Test multiple areas and devices configuration."""
+    result = load_config_from_fixture(yaml_file, "multiple_areas_devices.yaml")
+    assert result is not None
+
+    esphome_config = result["esphome"]
+
+    # Verify main area
+    assert CONF_AREA in esphome_config
+    main_area = esphome_config[CONF_AREA]
+    assert main_area["id"].id == "main_area"
+    assert main_area["name"] == "Main Area"
+
+    # Verify additional areas
+    assert CONF_AREAS in esphome_config
+    areas = esphome_config[CONF_AREAS]
+    assert len(areas) == 2
+    area_ids = {area["id"].id for area in areas}
+    assert area_ids == {"area1", "area2"}
+
+    # Verify devices
+    assert CONF_DEVICES in esphome_config
+    devices = esphome_config[CONF_DEVICES]
+    assert len(devices) == 3
+
+    # Check device-area associations
+    device_area_map = {dev["id"].id: dev["area_id"].id for dev in devices}
+    assert device_area_map == {
+        "device1": "main_area",
+        "device2": "area1",
+        "device3": "area2",
+    }
+
+
+def test_legacy_string_area(
+    yaml_file: Callable[[str], str], caplog: pytest.LogCaptureFixture
+) -> None:
+    """Test legacy string area configuration with deprecation warning."""
+    result = load_config_from_fixture(yaml_file, "legacy_string_area.yaml")
+    assert result is not None
+
+    esphome_config = result["esphome"]
+
+    # Verify the string was converted to structured format
+    assert CONF_AREA in esphome_config
+    area = esphome_config[CONF_AREA]
+    assert isinstance(area, dict)
+    assert area["name"] == "Living Room"
+    assert area["id"].id == "living_room"
+
+    # Check for deprecation warning
+    assert "Using 'area' as a string is deprecated" in caplog.text
+
+
+def test_area_id_collision(
+    yaml_file: Callable[[str], str], capsys: pytest.CaptureFixture[str]
+) -> None:
+    """Test that duplicate area IDs are detected."""
+    result = load_config_from_fixture(yaml_file, "area_id_collision.yaml")
+    assert result is None
+
+    # Check for the specific error message in stdout
+    captured = capsys.readouterr()
+    assert "ID duplicate_id redefined! Check esphome->area->id." in captured.out
+
+
+def test_device_without_area(yaml_file: Callable[[str], str]) -> None:
+    """Test that devices without area_id work correctly."""
+    result = load_config_from_fixture(yaml_file, "device_without_area.yaml")
+    assert result is not None
+
+    esphome_config = result["esphome"]
+
+    # Verify device was parsed
+    assert CONF_DEVICES in esphome_config
+    devices = esphome_config[CONF_DEVICES]
+    assert len(devices) == 1
+
+    device = devices[0]
+    assert device["id"].id == "test_device"
+    assert device["name"] == "Test Device"
+
+    # Verify no area_id is present
+    assert "area_id" not in device