diff --git a/homeassistant/components/knx/__init__.py b/homeassistant/components/knx/__init__.py
index 99b461dda1b..709a82b31fd 100644
--- a/homeassistant/components/knx/__init__.py
+++ b/homeassistant/components/knx/__init__.py
@@ -333,7 +333,7 @@ class KNXModule:
 
     async def start(self) -> None:
         """Start XKNX object. Connect to tunneling or Routing device."""
-        await self.project.load_project()
+        await self.project.load_project(self.xknx)
         await self.config_store.load_data()
         await self.telegrams.load_history()
         await self.xknx.start()
diff --git a/homeassistant/components/knx/project.py b/homeassistant/components/knx/project.py
index 13e71dbbe38..3b3309dfc7d 100644
--- a/homeassistant/components/knx/project.py
+++ b/homeassistant/components/knx/project.py
@@ -6,6 +6,7 @@ from dataclasses import dataclass
 import logging
 from typing import Final
 
+from xknx import XKNX
 from xknx.dpt import DPTBase
 from xknxproject import XKNXProj
 from xknxproject.models import (
@@ -80,15 +81,23 @@ class KNXProject:
         self.group_addresses = {}
         self.info = None
 
-    async def load_project(self, data: KNXProjectModel | None = None) -> None:
+    async def load_project(
+        self, xknx: XKNX, data: KNXProjectModel | None = None
+    ) -> None:
         """Load project data from storage."""
         if project := data or await self._store.async_load():
             self.devices = project["devices"]
             self.info = project["info"]
+            xknx.group_address_dpt.clear()
+            xknx_ga_dict = {}
 
             for ga_model in project["group_addresses"].values():
                 ga_info = _create_group_address_info(ga_model)
                 self.group_addresses[ga_info.address] = ga_info
+                if (dpt_model := ga_model.get("dpt")) is not None:
+                    xknx_ga_dict[ga_model["address"]] = dpt_model
+
+            xknx.group_address_dpt.set(xknx_ga_dict)  # type: ignore[arg-type]
 
             _LOGGER.debug(
                 "Loaded KNX project data with %s group addresses from storage",
@@ -96,7 +105,9 @@ class KNXProject:
             )
             self.loaded = True
 
-    async def process_project_file(self, file_id: str, password: str) -> None:
+    async def process_project_file(
+        self, xknx: XKNX, file_id: str, password: str
+    ) -> None:
         """Process an uploaded project file."""
 
         def _parse_project() -> KNXProjectModel:
@@ -110,7 +121,7 @@ class KNXProject:
 
         project = await self.hass.async_add_executor_job(_parse_project)
         await self._store.async_save(project)
-        await self.load_project(data=project)
+        await self.load_project(xknx, data=project)
 
     async def remove_project_file(self) -> None:
         """Remove project file from storage."""
diff --git a/homeassistant/components/knx/telegrams.py b/homeassistant/components/knx/telegrams.py
index 2ad46326b8e..a96d841a07d 100644
--- a/homeassistant/components/knx/telegrams.py
+++ b/homeassistant/components/knx/telegrams.py
@@ -35,7 +35,7 @@ class DecodedTelegramPayload(TypedDict):
     dpt_sub: int | None
     dpt_name: str | None
     unit: str | None
-    value: str | int | float | bool | None
+    value: bool | str | int | float | dict[str, str | int | float | bool] | None
 
 
 class TelegramDict(DecodedTelegramPayload):
@@ -106,7 +106,7 @@ class Telegrams:
         payload_data: int | tuple[int, ...] | None = None
         src_name = ""
         transcoder = None
-        decoded_payload: DecodedTelegramPayload | None = None
+        value = None
 
         if (
             ga_info := self.project.group_addresses.get(
@@ -114,7 +114,6 @@ class Telegrams:
             )
         ) is not None:
             dst_name = ga_info.name
-            transcoder = ga_info.transcoder
 
         if (
             device := self.project.devices.get(f"{telegram.source_address}")
@@ -123,45 +122,49 @@ class Telegrams:
 
         if isinstance(telegram.payload, (GroupValueWrite, GroupValueResponse)):
             payload_data = telegram.payload.value.value
-            if transcoder is not None:
-                decoded_payload = decode_telegram_payload(
-                    payload=telegram.payload.value, transcoder=transcoder
-                )
+
+        if telegram.decoded_data is not None:
+            transcoder = telegram.decoded_data.transcoder
+            value = _serializable_decoded_data(telegram.decoded_data.value)
 
         return TelegramDict(
             destination=f"{telegram.destination_address}",
             destination_name=dst_name,
             direction=telegram.direction.value,
-            dpt_main=decoded_payload["dpt_main"]
-            if decoded_payload is not None
-            else None,
-            dpt_sub=decoded_payload["dpt_sub"] if decoded_payload is not None else None,
-            dpt_name=decoded_payload["dpt_name"]
-            if decoded_payload is not None
-            else None,
+            dpt_main=transcoder.dpt_main_number if transcoder is not None else None,
+            dpt_sub=transcoder.dpt_sub_number if transcoder is not None else None,
+            dpt_name=transcoder.value_type if transcoder is not None else None,
             payload=payload_data,
             source=f"{telegram.source_address}",
             source_name=src_name,
             telegramtype=telegram.payload.__class__.__name__,
             timestamp=dt_util.now().isoformat(),
-            unit=decoded_payload["unit"] if decoded_payload is not None else None,
-            value=decoded_payload["value"] if decoded_payload is not None else None,
+            unit=transcoder.unit if transcoder is not None else None,
+            value=value,
         )
 
 
+def _serializable_decoded_data(
+    value: bool | float | str | DPTComplexData | DPTEnumData,
+) -> bool | str | int | float | dict[str, str | int | float | bool]:
+    """Return a serializable representation of decoded data."""
+    if isinstance(value, DPTComplexData):
+        return value.as_dict()
+    if isinstance(value, DPTEnumData):
+        return value.name.lower()
+    return value
+
+
 def decode_telegram_payload(
     payload: DPTArray | DPTBinary, transcoder: type[DPTBase]
 ) -> DecodedTelegramPayload:
-    """Decode the payload of a KNX telegram."""
+    """Decode the payload of a KNX telegram with custom transcoder."""
     try:
         value = transcoder.from_knx(payload)
     except XKNXException:
         value = "Error decoding value"
 
-    if isinstance(value, DPTComplexData):
-        value = value.as_dict()
-    elif isinstance(value, DPTEnumData):
-        value = value.name.lower()
+    value = _serializable_decoded_data(value)
 
     return DecodedTelegramPayload(
         dpt_main=transcoder.dpt_main_number,
diff --git a/homeassistant/components/knx/websocket.py b/homeassistant/components/knx/websocket.py
index 97758dc87c9..4af3012741a 100644
--- a/homeassistant/components/knx/websocket.py
+++ b/homeassistant/components/knx/websocket.py
@@ -154,6 +154,7 @@ async def ws_project_file_process(
     knx: KNXModule = hass.data[DOMAIN]
     try:
         await knx.project.process_project_file(
+            xknx=knx.xknx,
             file_id=msg["file_id"],
             password=msg["password"],
         )
diff --git a/tests/components/knx/test_websocket.py b/tests/components/knx/test_websocket.py
index eb22bac85bc..309ea111709 100644
--- a/tests/components/knx/test_websocket.py
+++ b/tests/components/knx/test_websocket.py
@@ -346,7 +346,7 @@ async def test_knx_subscribe_telegrams_command_project(
     assert res["event"]["destination"] == "0/1/1"
     assert res["event"]["destination_name"] == "percent"
     assert res["event"]["payload"] == 1
-    assert res["event"]["value"] == "Error decoding value"
+    assert res["event"]["value"] is None
     assert res["event"]["telegramtype"] == "GroupValueWrite"
     assert res["event"]["source"] == "1.1.6"
     assert (