Instructions
Enter build mode by clicking the "Build" button on your control panel. Select the pool tool by clicking on the picture of a pool in the left corner above the cutaway house.
Select the perfect location for your Sims pool, then click and drag to create it. Add lights, ladders and any other decor you desire from the pool section of the build menu.
Select the basement tool, which is on the upper right hand corner of the build menu. Hover the mouse near the side of the pool you want facing your basement, one square away from the actual pool wall. Click and drag to create your basement.
Hover the mouse over the leftmost point on the patch of grass between your pool and basement. Hold down the Ctrl button, then click and drag, selecting the entire patch. Release the mouse button to delete.
Release the Ctrl key and repeat, clicking and dragging until the patch of grass between the pool and basement is entirely green. Your basement is now directly next to the pool.