diff --git a/cateory-tree-sketch.json b/cateory-tree-sketch.json
new file mode 100644
index 0000000..24c7f3a
--- /dev/null
+++ b/cateory-tree-sketch.json
@@ -0,0 +1,27 @@
+{
+  "month": "2026-10",
+  "categories": [
+    {
+      "category": "super-category-1",
+      "total_amount": 100,
+      "total_budget": 120,
+      "sub_categories": [
+        {
+          "category": "category-1",
+          "total_amount": 60,
+          "total_budget": 50,
+          "sub_categories": [],
+          "transactions": []
+        },
+        {
+          "category": "category-2",
+          "total_amount": 40,
+          "total_budget": 60,
+          "sub_categories": [],
+          "transactions": []
+        }
+      ],
+      "transactions": []
+    }
+  ]
+}
diff --git a/src/models/category.py b/src/models/category.py
index 4c615a5..f11b709 100644
--- a/src/models/category.py
+++ b/src/models/category.py
@@ -18,7 +18,8 @@ from sqlalchemy import (
 from sqlalchemy.orm import Mapped, Session, mapped_column, relationship
 
 from src.db_utils import upsert
-from src.models.common import BaseModelDb, ExportBaseModel, hashify
+from src.models.common import Amount, AmountPos, BaseModelDb, ExportBaseModel, hashify
+from src.models.transactions import Transaction
 
 
 class UTCDateTime(TypeDecorator[datetime]):
@@ -75,6 +76,9 @@ class Category(ExportBaseModel):
     def __repr__(self):
         return f'Category.from_str("{self.__str__()}")'
 
+    def get_budget(self, date_valid: date | None) -> float:
+        ...
+
 
 class CategoryDb(BaseModelDb):
     __tablename__ = "category"
@@ -187,7 +191,7 @@ def upsert_category(
 
 class CategoryBudget(BaseModel):
     category: Category
-    amount_eur: Decimal = Field(gt=0, decimal_places=2)
+    amount_eur: AmountPos
     valid_from: date | None = Field(
         default=None, description="Start of validity. None means unbounded"
     )
@@ -228,3 +232,31 @@ class CategoryBudgetDb(BaseModelDb):
 
     # relationship
     category = relationship("CategoryDb", back_populates="budgets")
+
+
+class CategoryTree(BaseModel):
+    category: Category
+    total_amount: float
+    total_budget: float
+    sub_categories: list[ CategoryTree ] | None
+    transactions: list[Transaction] | None
+
+    def get_total_amount(self) -> float:
+        result = 0.0
+        if self.transactions:
+            sum(transaction.amount for transaction in self.transactions )
+        if self.sub_categories:
+            sum(sub_category.get_total_amount() for sub_category in self.sub_categories)
+        return result
+
+    def get_total_budget(self, date_valid: date) -> float:
+        result = 0.0
+        result += self.category.get_budget(date_valid)
+        if self.sub_categories:
+            result += sum(category.category.get_budget(date_valid) for category in self.sub_categories)
+        return result
+
+def build_category_tree(transactions: list[Transaction]) -> list[CategoryTree]:
+    result = []
+    for transaction in transactions:
+        pass
diff --git a/src/models/common.py b/src/models/common.py
index 3edd0c7..094ad5f 100644
--- a/src/models/common.py
+++ b/src/models/common.py
@@ -2,8 +2,9 @@ import hashlib
 from datetime import date
 from decimal import Decimal
 from enum import Enum
+from typing import Annotated
 
-from pydantic import BaseModel
+from pydantic import BaseModel, Field
 from sqlalchemy import Date, Numeric, String
 from sqlalchemy.orm import DeclarativeBase, Mapped, mapped_column
 
@@ -39,12 +40,15 @@ def date2str(value: date | None, default="--", date_format="%Y-%m-%d") -> str:
         return value.strftime(date_format)
     return default
 
+type AmountPos = Annotated[Decimal, Field(decimal_places=2, gt=0)]
+type Amount = Annotated[Decimal, Field(decimal_places=2)]
 
-def _parse_decimal_german(decimal_str: str) -> Decimal:
+
+def _parse_decimal_german(decimal_str: str) -> Amount:
     return Decimal(decimal_str.replace(".", "").replace(",", "."))
 
 
-def decimal2str(value: Decimal) -> str:
+def decimal2str(value: Amount) -> str:
     value = Decimal(f"{value:.2f}")
     int_part, _, frac = str(value).partition(".")
     groups: list[str] = []
@@ -60,6 +64,7 @@ type Csv2Dict = list[Csv2DictRow]
 type CsV2DictMulti = dict[str, Csv2Dict]
 
 
+
 class ExportBaseModel(BaseModel):
 
     @classmethod
diff --git a/src/models/transactions.py b/src/models/transactions.py
index 23fe48b..8ed9268 100644
--- a/src/models/transactions.py
+++ b/src/models/transactions.py
@@ -5,7 +5,7 @@ from pydantic import Field, computed_field
 from sqlalchemy import Date, ForeignKey, Index, Integer, String, UniqueConstraint
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 
-from src.models.common import ExportBaseModel, IngestdbBase, date2str, hashify
+from src.models.common import Amount, ExportBaseModel, IngestdbBase, date2str, hashify
 
 
 class Transaction(ExportBaseModel):
@@ -22,12 +22,13 @@ class Transaction(ExportBaseModel):
     account_iban: str
     account_currency: str
     date_valid: date
-    amount: Decimal = Field(description="Signed amount; negative = money out")
+    amount: Amount
     amount_currency: str
     date_book: date | None = None
     text: str | None = None
     other_name: str | None = None
     other_iban: str | None = None
+    # TODO: The Pydantic model should hold a category object, NOT the category id which is owned by the DB
     category_id: int | None = None
 
     @computed_field  # type: ignore[prop-decorator]