Welcome To...
How and when do I build a barricade?
The designers settled this between 2019 and 2022; the 2021 correction overturns the earlier answer.
Only with the Fence action, in the house where you write the number, just like a pool. The Fence action lets you build either a fence or a barricade, never both. You cannot place a barricade under an empty or unnumbered house or in front of a house already infected with a Z, because you must write a number to take the Fence action. An earlier 2020 designer reply that you could barricade an infested house was corrected in 2021.