# He will require

Here "He" refers to the priest.

# all the inside walls of the house to be scraped

"that the owner scrapes all the inside walls"

# the contaminated material that is scraped off

"the contaminated material that they scraped off"

# the unclean place

A place that is unfit for people to occupy or to be used for God's purposes is spoken of as if it were physically unclean.