Utopía translates directly to "utopia," a word that evokes an ideal, often unattainable, perfect place or state.
In this song, Sabino uses utopía to describe the idea of living solely in his mind, creating a perfect world where his lost love still exists and loves him. He acknowledges that this mental escape might sound like an impossible dream to others, but for him, it's a necessary refuge from the pain of reality.
Have you ever wanted to escape reality so badly that you just create a whole new world in your head? That's exactly what Sabino is doing in "Una Casa"! He's saying goodbye to the real world, moving into his mind because he has "cosas aún pendientes" [things still pending] with someone he lost. He misses them so much and doesn't want this to be goodbye, so he's building a new life, a new reality, just for them.
He's the architect of this mental world, and he's inviting this person to share his "palacio" [palace]. He promises they'll find all his love in every room. It's a bittersweet fantasy: he was the one who had them, lost them, and couldn't stop himself from creating this whole new world where they love him back and try again. If he can't have them in real life, then at least in his mind, he'll build them "Una casa que sirva para que tú vivas / Aquí para siempre hoy" [A house for you to live in / Here forever today]. It's a pretty intense way to deal with heartbreak, don't you think?