diff --git a/hugr-py/src/hugr/__init__.py b/hugr-py/src/hugr/__init__.py
index 1b0b25aea..a1f36b133 100644
--- a/hugr-py/src/hugr/__init__.py
+++ b/hugr-py/src/hugr/__init__.py
@@ -2,10 +2,8 @@
 representation.
 """
 
-from .hugr import Hugr
-from .node_port import Direction, InPort, Node, OutPort, Wire
-from .ops import Op
-from .tys import Kind, Type
+from .hugr.base import Hugr
+from .hugr.node_port import Direction, InPort, Node, OutPort, Wire
 
 __all__ = [
     "Hugr",
@@ -13,9 +11,6 @@
     "OutPort",
     "InPort",
     "Direction",
-    "Op",
-    "Kind",
-    "Type",
     "Wire",
 ]
 
diff --git a/hugr-py/src/hugr/_serialization/ops.py b/hugr-py/src/hugr/_serialization/ops.py
index b2a899dc2..95b01beed 100644
--- a/hugr-py/src/hugr/_serialization/ops.py
+++ b/hugr-py/src/hugr/_serialization/ops.py
@@ -7,7 +7,9 @@
 
 from pydantic import ConfigDict, Field, RootModel
 
-from hugr.node_port import NodeIdx  # noqa: TCH001 # pydantic needs this alias in scope
+from hugr.hugr.node_port import (
+    NodeIdx,  # noqa: TCH001 # pydantic needs this alias in scope
+)
 from hugr.utils import deser_it
 
 from . import tys as stys
diff --git a/hugr-py/src/hugr/_serialization/serial_hugr.py b/hugr-py/src/hugr/_serialization/serial_hugr.py
index 8ccb58f85..264a84268 100644
--- a/hugr-py/src/hugr/_serialization/serial_hugr.py
+++ b/hugr-py/src/hugr/_serialization/serial_hugr.py
@@ -2,7 +2,7 @@
 
 from pydantic import ConfigDict, Field
 
-from hugr.node_port import NodeIdx, PortOffset
+from hugr.hugr.node_port import NodeIdx, PortOffset
 
 from .ops import OpType
 from .ops import classes as ops_classes
diff --git a/hugr-py/src/hugr/build/__init__.py b/hugr-py/src/hugr/build/__init__.py
new file mode 100644
index 000000000..def20437f
--- /dev/null
+++ b/hugr-py/src/hugr/build/__init__.py
@@ -0,0 +1,24 @@
+"""Classes for building HUGRs."""
+
+from .base import ParentBuilder
+from .cfg import Block, Cfg
+from .cond_loop import Case, Conditional, If, TailLoop
+from .dfg import DefinitionBuilder, DfBase, Dfg, Function
+from .function import Module
+from .tracked_dfg import TrackedDfg
+
+__all__ = [
+    "ParentBuilder",
+    "Cfg",
+    "Block",
+    "Case",
+    "If",
+    "Conditional",
+    "TailLoop",
+    "Function",
+    "Module",
+    "TrackedDfg",
+    "Dfg",
+    "DefinitionBuilder",
+    "DfBase",
+]
diff --git a/hugr-py/src/hugr/build/base.py b/hugr-py/src/hugr/build/base.py
new file mode 100644
index 000000000..65ec280bd
--- /dev/null
+++ b/hugr-py/src/hugr/build/base.py
@@ -0,0 +1,31 @@
+"""Base classes for HUGR builders."""
+
+from __future__ import annotations
+
+from typing import (
+    Protocol,
+    cast,
+)
+
+from hugr.hugr.base import Hugr, OpVar
+from hugr.hugr.node_port import (
+    Node,
+    ToNode,
+)
+
+
+class ParentBuilder(ToNode, Protocol[OpVar]):
+    """Abstract interface implemented by builders of nodes that contain child HUGRs."""
+
+    #: The child HUGR.
+    hugr: Hugr[OpVar]
+    # Unique parent node.
+    parent_node: Node
+
+    def to_node(self) -> Node:
+        return self.parent_node
+
+    @property
+    def parent_op(self) -> OpVar:
+        """The parent node's operation."""
+        return cast(OpVar, self.hugr[self.parent_node].op)
diff --git a/hugr-py/src/hugr/cfg.py b/hugr-py/src/hugr/build/cfg.py
similarity index 96%
rename from hugr-py/src/hugr/cfg.py
rename to hugr-py/src/hugr/build/cfg.py
index 9ee06d49a..f6b9c277d 100644
--- a/hugr-py/src/hugr/cfg.py
+++ b/hugr-py/src/hugr/build/cfg.py
@@ -9,14 +9,14 @@
 from typing_extensions import Self
 
 from hugr import ops, tys, val
-
-from .dfg import DfBase
-from .exceptions import MismatchedExit, NoSiblingAncestor, NotInSameCfg
-from .hugr import Hugr, ParentBuilder
+from hugr.build.base import ParentBuilder
+from hugr.build.dfg import DfBase
+from hugr.exceptions import MismatchedExit, NoSiblingAncestor, NotInSameCfg
+from hugr.hugr import Hugr
 
 if TYPE_CHECKING:
-    from .node_port import Node, PortOffset, ToNode, Wire
-    from .tys import Type, TypeRow
+    from hugr.hugr.node_port import Node, PortOffset, ToNode, Wire
+    from hugr.tys import Type, TypeRow
 
 
 class Block(DfBase[ops.DataflowBlock]):
diff --git a/hugr-py/src/hugr/cond_loop.py b/hugr-py/src/hugr/build/cond_loop.py
similarity index 97%
rename from hugr-py/src/hugr/cond_loop.py
rename to hugr-py/src/hugr/build/cond_loop.py
index ca286fcc4..02b046251 100644
--- a/hugr-py/src/hugr/cond_loop.py
+++ b/hugr-py/src/hugr/build/cond_loop.py
@@ -11,14 +11,14 @@
 from typing_extensions import Self
 
 from hugr import ops
+from hugr.build.base import ParentBuilder
+from hugr.build.dfg import DfBase
+from hugr.hugr.base import Hugr
 from hugr.tys import Sum
 
-from .dfg import DfBase
-from .hugr import Hugr, ParentBuilder
-
 if TYPE_CHECKING:
-    from .node_port import Node, ToNode, Wire
-    from .tys import TypeRow
+    from hugr.hugr.node_port import Node, ToNode, Wire
+    from hugr.tys import TypeRow
 
 
 class Case(DfBase[ops.Case]):
diff --git a/hugr-py/src/hugr/dfg.py b/hugr-py/src/hugr/build/dfg.py
similarity index 99%
rename from hugr-py/src/hugr/dfg.py
rename to hugr-py/src/hugr/build/dfg.py
index a2b4dbb0e..8c0fb95c6 100644
--- a/hugr-py/src/hugr/dfg.py
+++ b/hugr-py/src/hugr/build/dfg.py
@@ -14,17 +14,18 @@
 from typing_extensions import Self
 
 from hugr import ops, tys, val
-
-from .exceptions import NoSiblingAncestor
-from .hugr import Hugr, ParentBuilder
+from hugr.build.base import ParentBuilder
+from hugr.exceptions import NoSiblingAncestor
+from hugr.hugr import Hugr
 
 if TYPE_CHECKING:
     from collections.abc import Iterable, Sequence
 
+    from hugr.hugr.node_port import Node, OutPort, PortOffset, ToNode, Wire
+    from hugr.tys import Type, TypeParam, TypeRow
+
     from .cfg import Cfg
     from .cond_loop import Conditional, If, TailLoop
-    from .node_port import Node, OutPort, PortOffset, ToNode, Wire
-    from .tys import Type, TypeParam, TypeRow
 
 OpVar = TypeVar("OpVar", bound=ops.Op)
 
diff --git a/hugr-py/src/hugr/function.py b/hugr-py/src/hugr/build/function.py
similarity index 89%
rename from hugr-py/src/hugr/function.py
rename to hugr-py/src/hugr/build/function.py
index 8e31b50b8..97ff17d9c 100644
--- a/hugr-py/src/hugr/function.py
+++ b/hugr-py/src/hugr/build/function.py
@@ -5,13 +5,13 @@
 from dataclasses import dataclass
 from typing import TYPE_CHECKING
 
-from . import ops
-from .dfg import DefinitionBuilder, Function
-from .hugr import Hugr
+from hugr import ops
+from hugr.build.dfg import DefinitionBuilder, Function
+from hugr.hugr import Hugr
 
 if TYPE_CHECKING:
-    from .node_port import Node
-    from .tys import PolyFuncType, TypeBound, TypeRow
+    from hugr.hugr.node_port import Node
+    from hugr.tys import PolyFuncType, TypeBound, TypeRow
 
 __all__ = ["Function", "Module"]
 
diff --git a/hugr-py/src/hugr/tracked_dfg.py b/hugr-py/src/hugr/build/tracked_dfg.py
similarity index 98%
rename from hugr-py/src/hugr/tracked_dfg.py
rename to hugr-py/src/hugr/build/tracked_dfg.py
index aa8d768bd..439d5680c 100644
--- a/hugr-py/src/hugr/tracked_dfg.py
+++ b/hugr-py/src/hugr/build/tracked_dfg.py
@@ -4,8 +4,8 @@
 from typing import Any
 
 from hugr import tys
-from hugr.dfg import Dfg
-from hugr.node_port import Node, Wire
+from hugr.build.dfg import Dfg
+from hugr.hugr.node_port import Node, Wire
 from hugr.ops import Command, ComWire
 
 
diff --git a/hugr-py/src/hugr/conftest.py b/hugr-py/src/hugr/conftest.py
index 2289f8128..daf1b82f1 100644
--- a/hugr-py/src/hugr/conftest.py
+++ b/hugr-py/src/hugr/conftest.py
@@ -2,7 +2,10 @@
 
 import pytest
 
-from hugr import dfg, hugr, node_port, ops, tys, val
+from hugr import ops, tys, val
+from hugr.build import dfg
+from hugr.hugr import base as hugr
+from hugr.hugr import node_port
 
 
 @pytest.fixture(autouse=True)
diff --git a/hugr-py/src/hugr/exceptions.py b/hugr-py/src/hugr/exceptions.py
index aa2767178..8568b3e99 100644
--- a/hugr-py/src/hugr/exceptions.py
+++ b/hugr-py/src/hugr/exceptions.py
@@ -2,7 +2,7 @@
 
 from dataclasses import dataclass
 
-from .node_port import NodeIdx
+from .hugr.node_port import NodeIdx
 
 
 @dataclass
diff --git a/hugr-py/src/hugr/hugr/__init__.py b/hugr-py/src/hugr/hugr/__init__.py
new file mode 100644
index 000000000..3cbc426d7
--- /dev/null
+++ b/hugr-py/src/hugr/hugr/__init__.py
@@ -0,0 +1,20 @@
+"""The main HUGR structure."""
+
+from .base import Hugr, NodeData
+from .node_port import (
+    Direction,
+    InPort,
+    Node,
+    OutPort,
+    Wire,
+)
+
+__all__ = [
+    "Hugr",
+    "NodeData",
+    "Direction",
+    "InPort",
+    "Wire",
+    "OutPort",
+    "Node",
+]
diff --git a/hugr-py/src/hugr/hugr.py b/hugr-py/src/hugr/hugr/base.py
similarity index 97%
rename from hugr-py/src/hugr/hugr.py
rename to hugr-py/src/hugr/hugr/base.py
index bb2926a4a..3a73a4a7e 100644
--- a/hugr-py/src/hugr/hugr.py
+++ b/hugr-py/src/hugr/hugr/base.py
@@ -9,7 +9,6 @@
     TYPE_CHECKING,
     Any,
     Generic,
-    Protocol,
     TypeVar,
     cast,
     overload,
@@ -17,7 +16,13 @@
 
 from hugr._serialization.ops import OpType as SerialOp
 from hugr._serialization.serial_hugr import SerialHugr
-from hugr.node_port import (
+from hugr.exceptions import ParentBeforeChild
+from hugr.ops import Call, Const, Custom, DataflowOp, Module, Op
+from hugr.tys import Kind, Type, ValueKind
+from hugr.utils import BiMap
+from hugr.val import Value
+
+from .node_port import (
     Direction,
     InPort,
     Node,
@@ -27,12 +32,6 @@
     ToNode,
     _SubPort,
 )
-from hugr.ops import Call, Const, Custom, DataflowOp, Module, Op
-from hugr.tys import Kind, Type, ValueKind
-from hugr.utils import BiMap
-from hugr.val import Value
-
-from .exceptions import ParentBeforeChild
 
 if TYPE_CHECKING:
     import graphviz as gv  # type: ignore[import-untyped]
@@ -69,23 +68,6 @@ def _to_serial(self, node: Node) -> SerialOp:
 OpVarCov = TypeVar("OpVarCov", bound=Op, covariant=True)
 
 
-class ParentBuilder(ToNode, Protocol[OpVar]):
-    """Abstract interface implemented by builders of nodes that contain child HUGRs."""
-
-    #: The child HUGR.
-    hugr: Hugr[OpVar]
-    # Unique parent node.
-    parent_node: Node
-
-    def to_node(self) -> Node:
-        return self.parent_node
-
-    @property
-    def parent_op(self) -> OpVar:
-        """The parent node's operation."""
-        return cast(OpVar, self.hugr[self.parent_node].op)
-
-
 @dataclass()
 class Hugr(Mapping[Node, NodeData], Generic[OpVarCov]):
     """The core HUGR datastructure.
diff --git a/hugr-py/src/hugr/node_port.py b/hugr-py/src/hugr/hugr/node_port.py
similarity index 100%
rename from hugr-py/src/hugr/node_port.py
rename to hugr-py/src/hugr/hugr/node_port.py
diff --git a/hugr-py/src/hugr/render.py b/hugr-py/src/hugr/hugr/render.py
similarity index 100%
rename from hugr-py/src/hugr/render.py
rename to hugr-py/src/hugr/hugr/render.py
diff --git a/hugr-py/src/hugr/ops.py b/hugr-py/src/hugr/ops.py
index 015385c00..65ad1623c 100644
--- a/hugr-py/src/hugr/ops.py
+++ b/hugr-py/src/hugr/ops.py
@@ -10,7 +10,7 @@
 
 import hugr._serialization.ops as sops
 from hugr import tys, val
-from hugr.node_port import Direction, InPort, Node, OutPort, PortOffset, Wire
+from hugr.hugr.node_port import Direction, InPort, Node, OutPort, PortOffset, Wire
 from hugr.utils import ser_it
 
 if TYPE_CHECKING:
diff --git a/hugr-py/tests/serialization/test_basic.py b/hugr-py/tests/serialization/test_basic.py
index 57d30af73..fd9dd099c 100644
--- a/hugr-py/tests/serialization/test_basic.py
+++ b/hugr-py/tests/serialization/test_basic.py
@@ -1,6 +1,6 @@
 from hugr import Hugr, tys
 from hugr._serialization.serial_hugr import SerialHugr, serialization_version
-from hugr.function import Module
+from hugr.build.function import Module
 
 
 def test_empty():
diff --git a/hugr-py/tests/test_cfg.py b/hugr-py/tests/test_cfg.py
index 7bf24ab2b..14753e2f4 100644
--- a/hugr-py/tests/test_cfg.py
+++ b/hugr-py/tests/test_cfg.py
@@ -1,6 +1,5 @@
 from hugr import ops, tys, val
-from hugr.cfg import Cfg
-from hugr.dfg import Dfg
+from hugr.build import Cfg, Dfg
 from hugr.std.int import INT_T, DivMod, IntVal
 
 from .conftest import validate
diff --git a/hugr-py/tests/test_cond_loop.py b/hugr-py/tests/test_cond_loop.py
index 5e6a2e46c..b49ea69c6 100644
--- a/hugr-py/tests/test_cond_loop.py
+++ b/hugr-py/tests/test_cond_loop.py
@@ -1,8 +1,8 @@
 import pytest
 
 from hugr import ops, tys, val
-from hugr.cond_loop import Conditional, ConditionalError, TailLoop
-from hugr.dfg import Dfg
+from hugr.build.cond_loop import Conditional, ConditionalError, TailLoop
+from hugr.build.dfg import Dfg
 from hugr.ext import Package
 from hugr.std.int import INT_T, IntVal
 
diff --git a/hugr-py/tests/test_custom.py b/hugr-py/tests/test_custom.py
index 7d91066c4..73ef5eea3 100644
--- a/hugr-py/tests/test_custom.py
+++ b/hugr-py/tests/test_custom.py
@@ -3,9 +3,8 @@
 import pytest
 
 from hugr import ext, ops, tys
-from hugr.dfg import Dfg
-from hugr.hugr import Hugr
-from hugr.node_port import Node
+from hugr.build.dfg import Dfg
+from hugr.hugr import Hugr, Node
 from hugr.ops import AsExtOp, Custom, ExtOp
 from hugr.std.float import EXTENSION as FLOAT_EXT
 from hugr.std.float import FLOAT_T
diff --git a/hugr-py/tests/test_hugr_build.py b/hugr-py/tests/test_hugr_build.py
index ae5b060ff..3f7145a87 100644
--- a/hugr-py/tests/test_hugr_build.py
+++ b/hugr-py/tests/test_hugr_build.py
@@ -5,10 +5,10 @@
 import hugr.ops as ops
 import hugr.tys as tys
 import hugr.val as val
-from hugr.dfg import Dfg, _ancestral_sibling
-from hugr.function import Module
+from hugr.build.dfg import Dfg, _ancestral_sibling
+from hugr.build.function import Module
 from hugr.hugr import Hugr
-from hugr.node_port import Node, _SubPort
+from hugr.hugr.node_port import Node, _SubPort
 from hugr.ops import NoConcreteFunc
 from hugr.std.int import INT_T, DivMod, IntVal
 from hugr.std.logic import Not
diff --git a/hugr-py/tests/test_nodes.py b/hugr-py/tests/test_nodes.py
index 7fc1d30db..16c3b259a 100644
--- a/hugr-py/tests/test_nodes.py
+++ b/hugr-py/tests/test_nodes.py
@@ -1,6 +1,6 @@
 import pytest
 
-from hugr.node_port import Node, OutPort
+from hugr.hugr import Node, OutPort
 
 
 def test_index():
diff --git a/hugr-py/tests/test_tracked_dfg.py b/hugr-py/tests/test_tracked_dfg.py
index b558bf1c9..cac9ec0ee 100644
--- a/hugr-py/tests/test_tracked_dfg.py
+++ b/hugr-py/tests/test_tracked_dfg.py
@@ -1,10 +1,10 @@
 import pytest
 
 from hugr import tys
+from hugr.build.tracked_dfg import TrackedDfg
 from hugr.ext import Package
 from hugr.std.float import FLOAT_T, FloatVal
 from hugr.std.logic import Not
-from hugr.tracked_dfg import TrackedDfg
 
 from .conftest import CX, QUANTUM_EXT, H, Measure, Rz, validate