I remember I had a lot of trouble with this in geometry! This is a matter of ratios and division. You can grab a calculator and divide 19,880 by 14,000 (because the building increased, and you’re seeing how much bigger it is than the original). We can also do it by hand...
19880/14000
1988/1400 (divide by 10)
994/700 (divide by 2)
142/100 (divide by 7)
142% is your answer. The 100% implies you made another building, so the real answer is that the building size increased by 42% (since you already have the 14000 sqft building)
42%