As shown is the last photo preview there is a secret room in Kevin’s House that’s located under the front lawn.
Here's how you get into it;

1. Go into the closet under the basement stairs.
   
2. In the far back corner there is an axe lying on the ground
   
3. Pick up the axe and carry it out of the closet and behind the bar
   
4. There is a section of wall that should have slightly) messed up textures on it (this is on the same wall as the TV)
   
5. Using the HEAD of the axe, hit these wall sections causing them to break

And that's it! Picking up and carrying the axe is a little difficult because of gmods physics but I guess that’s part of the challenge