Learn German: Das Haus sieht schön aus. - The house looks beautiful.

The core verb here is aussehen, which means to look or to appear. It is a separable verb. In a standard main clause like this one, the prefix aus breaks off from the verb stem and gets pushed to the very end of the sentence.

If the verb is aussehen, why is it spelled sieht here instead of seht?

The base verb sehen (to see) is irregular. When used with he, she, or it (like das Haus), the stem vowel changes from e to ie, making it sieht. Separable verbs keep the exact same irregularities as their base verbs, so aussehen becomes sieht ... aus.

Could I just use sehen by itself to say the house looks beautiful?

No, because sehen means to physically see with your eyes. To say that something appears a certain way, you must use the separable verb aussehen. Without the aus at the end, this sentence would mean the house itself is using its eyes to see beautifully!

Why doesn't schön have an ending, like schönes, to match the neuter word das Haus?

Adjectives in German only get endings when they are placed directly in front of a noun (like in ein schönes Haus). Because schön comes after the verb here to describe the subject, it stays in its basic dictionary form and takes no extra letters.