Learn French: Le vieux bâtiment est laid.

In French, most adjectives go after the noun. However, a small group of common adjectives describing Beauty, Age, Number, Goodness, and Size (often remembered by the acronym BANGS) go before the noun. Since vieux describes age, it goes before bâtiment.

Could I use vieil instead of vieux here?

No. Although both mean old for masculine nouns, you must use vieux here because bâtiment starts with a consonant. You only use vieil before a masculine noun starting with a vowel or a mute H, like in un vieil ami.

Is laid the most common word for ugly?

laid is the standard, proper word for ugly. In spoken, everyday French, you will very often hear the colloquial word moche instead.

Why does bâtiment have a circumflex (little hat) over the first a?

A circumflex in French often shows where an S used to be in Old French. The old spelling was bastiment. Knowing this trick can sometimes help you guess the meaning or remember the spelling of French words!