What is the name of your state? OK
My brother-in-law has been living in our modular home for two yrs with no rental/lease agreement, just paying us monthly. While we were out of state on a job, he built an overhanging porch onto the front of the home. He is now mad, moving out and wants to take the materials the porch is made of with him. This will leave the home damaged where he connected the porch to the side and roof.
Is this legal?

Short and sweet:

If he leaves the place damaged, then you bill him for those damages... and sue him for them if he does not pay.

Take plenty of pictures.

Just to add onto this: Even if he decided to leave the addition on the place - and you did not want it there, you could still charge him to return the property to the way it was.