Simply click the wooden stilt foundations and hover over your own foundation. There you'll find that the base of the house in the example picture bottom left of the interface has a click-able "Foundation" spot.
Taking advantage of this is not very difficult if you know where to look. Rather than leave Stilt Foundations for beach houses locked behind a purchase of Island Living, the developers have given it to base game owners as a free treat. Using Stilt Foundations (Free for All Players) You can find the foundation button down at the bottom of the house diagram used to navigate build mode. Here's a quick guide to using the new build mode features and dealing with problems that might come up. New stilt foundations are available to all players, and there's a new type of "floor" that can be used which is see-through.
Given the Island setting of Sulani, The Sims 4 Island Living naturally comes with a ton of new build mode items to complete that whole, "Tropical Paradise" look you might want. The Sims 4 Island Living New Build/Buy Mode Options Like Stilts and Clear Floors A house with stilt foundation is now possible for all players, but looks best out over the ocean.