diff --git a/virtualizarr/writers/icechunk.py b/virtualizarr/writers/icechunk.py
index 6dadbc0..6ca97da 100644
--- a/virtualizarr/writers/icechunk.py
+++ b/virtualizarr/writers/icechunk.py
@@ -1,4 +1,4 @@
-from typing import TYPE_CHECKING, cast
+from typing import TYPE_CHECKING, Optional, cast
 
 import numpy as np
 from xarray import Dataset
@@ -24,7 +24,9 @@
 }
 
 
-def dataset_to_icechunk(ds: Dataset, store: "IcechunkStore") -> None:
+def dataset_to_icechunk(
+    ds: Dataset, store: "IcechunkStore", append_dim: Optional[str] = None
+) -> None:
     """
     Write an xarray dataset whose variables wrap ManifestArrays to an Icechunk store.
 
@@ -51,7 +53,10 @@ def dataset_to_icechunk(ds: Dataset, store: "IcechunkStore") -> None:
 
     # TODO only supports writing to the root group currently
     # TODO pass zarr_format kwarg?
-    root_group = Group.from_store(store=store)
+    if store.mode.str == "a":
+        root_group = Group.open(store=store, zarr_format=3)
+    else:
+        root_group = Group.from_store(store=store)
 
     # TODO this is Frozen, the API for setting attributes must be something else
     # root_group.attrs = ds.attrs
@@ -63,6 +68,7 @@ def dataset_to_icechunk(ds: Dataset, store: "IcechunkStore") -> None:
         ds.attrs,
         store=store,
         group=root_group,
+        append_dim=append_dim,
     )
 
 
@@ -71,6 +77,7 @@ def write_variables_to_icechunk_group(
     attrs,
     store,
     group,
+    append_dim: Optional[str] = None,
 ):
     virtual_variables = {
         name: var
@@ -96,6 +103,7 @@ def write_variables_to_icechunk_group(
             group=group,
             name=name,
             var=var,
+            append_dim=append_dim,
         )
 
 
@@ -104,6 +112,7 @@ def write_variable_to_icechunk(
     group: "Group",
     name: str,
     var: Variable,
+    append_dim: Optional[str] = None,
 ) -> None:
     """Write a single (possibly virtual) variable into an icechunk store"""
     if isinstance(var.data, ManifestArray):
@@ -112,6 +121,7 @@ def write_variable_to_icechunk(
             group=group,
             name=name,
             var=var,
+            append_dim=append_dim,
         )
     else:
         raise ValueError(
@@ -124,15 +134,37 @@ def write_virtual_variable_to_icechunk(
     group: "Group",
     name: str,
     var: Variable,
+    append_dim: Optional[str] = None,
 ) -> None:
     """Write a single virtual variable into an icechunk store"""
     ma = cast(ManifestArray, var.data)
     zarray = ma.zarray
+    shape = zarray.shape
+    mode = store.mode.str
+
+    # Aimee: resize the array if it already exists
+    # TODO: assert chunking and encoding is the same
+    existing_keys = tuple(group.array_keys())
+    append_axis, existing_num_chunks = None, None
+    if name in existing_keys and mode == "a":
+        # resize
+        dims = var.dims
+        if append_dim in dims:
+            append_axis = dims.index(append_dim)
+            existing_array = group[name]
+            existing_size = existing_array.shape[append_axis]
+            existing_num_chunks = int(
+                existing_size / existing_array.chunks[append_axis]
+            )
+            new_shape = list(existing_array.shape)
+            new_shape[append_axis] += var.shape[append_axis]
+            shape = tuple(new_shape)
+            existing_array.resize(new_shape)
 
     # creates array if it doesn't already exist
     arr = group.require_array(
         name=name,
-        shape=zarray.shape,
+        shape=shape,
         chunk_shape=zarray.chunks,
         dtype=encode_dtype(zarray.dtype),
         codecs=zarray._v3_codec_pipeline(),
@@ -142,6 +174,7 @@ def write_virtual_variable_to_icechunk(
     )
 
     # TODO it would be nice if we could assign directly to the .attrs property
+    # Aimee: assert that new attributes are the same as existing attributes
     for k, v in var.attrs.items():
         arr.attrs[k] = encode_zarr_attr_value(v)
     arr.attrs["_ARRAY_DIMENSIONS"] = encode_zarr_attr_value(var.dims)
@@ -156,6 +189,8 @@ def write_virtual_variable_to_icechunk(
         group=group,
         arr_name=name,
         manifest=ma.manifest,
+        append_axis=append_axis,
+        existing_num_chunks=existing_num_chunks,
     )
 
 
@@ -164,6 +199,8 @@ def write_manifest_virtual_refs(
     group: "Group",
     arr_name: str,
     manifest: ChunkManifest,
+    append_axis: Optional[int] = None,
+    existing_num_chunks: Optional[int] = None,
 ) -> None:
     """Write all the virtual references for one array manifest at once."""
 
@@ -181,8 +218,14 @@ def write_manifest_virtual_refs(
         ],
         op_flags=[["readonly"]] * 3,  # type: ignore
     )
+
     for path, offset, length in it:
         index = it.multi_index
+        if append_axis is not None:
+            list_index = list(index)
+            # Offset by the number of existing chunks on the append axis
+            list_index[append_axis] += existing_num_chunks
+            index = tuple(list_index)
         chunk_key = "/".join(str(i) for i in index)
 
         # set each reference individually