Step 1
Build a room of size 1×1 and build another room on top of that room (So you end up with 2 rooms on top of each other).

Step 2
Remove the bottom room, so that you end up with a floating room.

Step 3
Delete the ceiling and the walls of the floating room, so that you end up with a piece of floating floor. You can now change the height of the floating floor, make it as low as possible. Try to change the height until the floor is barely touching the ground (there should not be a slope).
Step 4
This floating floor will be your deck and you can build a pool underneath this using the pool tool. You can now drag the floating floor (the deck) to the size you want it.

Step 5: (Optional)
Follow these steps for a round deck shape.
Step 5.1
Go to Walls and Empty Rooms and select the “rounded deck” option. Place it on the existing deck.
Step 5.2
Place another one on the other side to form a half-circle, but make sure you place it one square into/onto the other quarter circle.
Step 5.3
Delete the 2 platform pieces outside of the circle using the sledgehammer tool.

Step 5.4
Delete the fences.

Step 5.5
To get a perfect round deck you can drag the platform one square smaller.
Step 6
Add a nice trim and floor to finish the look. Any stairs can be used to access the deck.

Tip #1:
You can use this method to build a large deck and add a “hole” in the middle. Make this hole any fun shape you want and a pool underneath to create a unique shaped pool.

Tip #2:
You can also use this method to build bridges. You can read more about building different types of bridges here.

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