When trying to build a Pool next to Foundation in The Sims 4, the Pool will automatically shift to a level your foundation is.
Here’s how to build a Pool next to Foundation without changing the Pool’s level:
First you need to place a Square Pool tile from the Pool menu. Make sure you place it at least 1 tile away from the Foundation. Then, push and pull it using the arrows to the foundation and adjust it’s width with arrows as well.
If you’d like to connect your existing Pool with sides of foundation, just add a Square Pool, push & pull it to the foundation and connect it with the existing Pool. This allows you to place one giant Pool around your whole lot!
With this level of customization you can also delete a small part of the Pool next to the Foundation by pressing CTRL + dragging the left mouse click. You can decorate your Pool by adding a ladder and a fence to make the entrance to this type of Pool more ”official”.