The ratio of 21:63 can be written as a simple ratio of 1:3. It is important you have labels on your numbers.
Here is the ratio rewritten with labels:
21 - old patio: 63 - new patio
1 - old patio: 3 - new patio
This can be switched around to say 3:1 as the ratio of the area of the new to the area of the old.
Each of the number would be square units because they are measuring area.