diff --git a/src/epyscan/__init__.py b/src/epyscan/__init__.py
index 2c03625..7284c8d 100644
--- a/src/epyscan/__init__.py
+++ b/src/epyscan/__init__.py
@@ -1,3 +1,4 @@
+import re
 from copy import deepcopy
 from pathlib import Path
 from typing import Union
@@ -7,8 +8,63 @@
 from scipy.stats import qmc
 
 
+def allocate_next_run_number(root: Path):
+    """
+    Return the next available run number in the given root directory.
+
+    This function searches all existing run directories and returns the
+    highest run number plus 1, or 0 if no runs exist. Only the lowest-level
+    run directories are considered (e.g., 'run_0', 'run_50').
+
+    Parameters
+    ----------
+    root : Path
+        The root directory to search for existing run directories.
+
+    Returns
+    -------
+    int
+        The next available run number, or 0 if none exist.
+    """
+    run_nums = [
+        int(path.name[4:])
+        for path in root.rglob("*")
+        if re.fullmatch(r"run_\d+", path.name)
+    ]
+
+    return max(run_nums, default=-1) + 1
+
+
 def rundir_hierarchy(root: Path, run_num: int) -> Path:
-    """Create nested directory structure for a run"""
+    """
+    Create the nested directory structure for a given run number.
+
+    Campaign runs are organised into a hierarchy to avoid putting too many
+    files in a single directory. The structure follows `rundir_hierarchy`:
+
+        ./run_0_1000000/run_0_10000/run_0_100/run_[0-99]
+
+    As the run numbers increase, higher-level directories are split further:
+
+        ./run_0_1000000/run_0_10000/run_100_200/run_[100-199]
+        ./run_0_1000000/run_10000_20000/run_10000_10100/run_[10000-10100]
+
+    The hierarchy ensures that only the lowest-level directories (e.g., 'run_0',
+    'run_50') contain individual simulation runs, while higher-level directories
+    manage grouping of runs in batches.
+
+    Parameters
+    ----------
+    root : Path
+        The root directory where the run hierarchy should be created.
+    run_num : int
+        The run number for which the directories will be created.
+
+    Returns
+    -------
+    Path
+        The full path to the newly created run directory.
+    """
 
     def level_dir(exponent: int) -> Path:
         level = 100**exponent
@@ -52,10 +108,12 @@ class Campaign:
         Base template deck as a Python dict (for example, as created by `epydeck`)
     root:
         Path to root run directory
+    append:
+        Flag to decide whether to append new runs to the end of an existing
+        campaign (if True) or overwrite it (if False)
 
     Examples
     --------
-
     >>> with open("template.deck") as f:
             template = epydeck.load(f)
     >>> campaign = Campaign(template, "grid_root")
@@ -63,8 +121,8 @@ class Campaign:
 
     """
 
-    def __init__(self, template: dict, root: Union[str, Path]):
-        self._counter = 0
+    def __init__(self, template: dict, root: Union[str, Path], append: bool = False):
+        self._counter = allocate_next_run_number(root) if append else 0
         self.template = template
         self.root = Path(root)
 
diff --git a/tests/test_epyscan.py b/tests/test_epyscan.py
index b226b07..bc0295d 100644
--- a/tests/test_epyscan.py
+++ b/tests/test_epyscan.py
@@ -1,3 +1,5 @@
+from pathlib import Path
+
 import epydeck
 import numpy as np
 
@@ -143,3 +145,66 @@ def test_campaign(tmp_path):
         actual_case_deck = epydeck.load(f)
 
     assert actual_case_deck == expected_case_deck
+
+
+def test_load_existing_campaign(tmp_path):
+    # Simulate a pre-existing campaign by making fake paths
+    base_path = tmp_path / "run_0_1000000/run_0_10000/run_0_100"
+
+    existing_campaign_paths = [
+        base_path / "run_0",
+        base_path / "run_1",
+        base_path / "run_2",
+        base_path / "run_3",
+        base_path / "run_4",
+    ]
+
+    for path in existing_campaign_paths:
+        Path(path).mkdir(parents=True, exist_ok=True)
+
+    template = {"block": {"var4": 1.23}, "other_block": {"var5": True}}
+    campaign = epyscan.Campaign(template, tmp_path, append=True)
+
+    new_path = campaign.setup_case({"block": {"var4": 1.24}})
+    assert new_path == (base_path / "run_5")
+    assert new_path.exists()
+
+
+def test_override_existing_campaign(tmp_path):
+    base_path = tmp_path / "run_0_1000000/run_0_10000/run_0_100"
+
+    existing_campaign_paths = [
+        base_path / "run_0",
+        base_path / "run_1",
+        base_path / "run_2",
+        base_path / "run_3",
+        base_path / "run_4",
+    ]
+
+    for path in existing_campaign_paths:
+        Path(path).mkdir(parents=True, exist_ok=True)
+
+    template = {"block": {"var4": 1.23}, "other_block": {"var5": True}}
+    campaign = epyscan.Campaign(template, tmp_path)
+
+    new_path = campaign.setup_case({"block": {"var4": 1.24}})
+    assert new_path == (base_path / "run_0")
+    assert new_path.exists()
+
+
+def test_retrieve_existing_run_count(tmp_path):
+    base_path = tmp_path / "run_0_1000000/run_0_10000/run_0_100"
+    (base_path / "run_0").mkdir(parents=True)
+    (base_path / "run_1").mkdir()
+    (base_path / "run_50").mkdir()
+    (base_path / "run_100").mkdir()
+
+    assert epyscan.allocate_next_run_number(tmp_path) == 101
+
+
+def test_retrieve_existing_run_count_edge_cases(tmp_path):
+    (tmp_path / "run_10").mkdir()
+    (tmp_path / "run_20_30").mkdir()
+    (tmp_path / "not_a_run").mkdir()
+
+    assert epyscan.allocate_next_run_number(tmp_path) == 11