We are making a bridge loan to be secured by vacant land. However, it is being pre-developed to build an industrial warehouse. Is flood insurance required?

If the land is in a SFHA, once the building is built and you have knowledge of it, that is a triggering event.

What if you don't take the building as collateral, just the land?

The mortgage will have a clause where it secures the loan with all existing and future improvements to the property.

Unless you somehow specifically excludes the building your mortgage will automatically give you a security interest in the building once complete.

Before you ever try something like this, you might want to visit with your foreclosure attorney and an appraiser. I am not sure how much the land would be worth when someone that does not either own or officially lease the property but owns on building on it.