jmc

The area of the land would be $s^2$, where $s$ is the length of the side. Since it has to be at least 400 square feet, we get $s^2\ge 400$. Therefore, we get that $s\le -20\text{ or }s\ge 20$. Since the dimensions can't be negative, the least value of $s$ would be $\boxed{20}$.