This means "It will be like that." How it will be like the time of Lot can be stated clearly as in the UDB: "People will be unprepared." (See: [[rc://en/ta/man/translate/figs-explicit]])
"when the Son of Man appears" or "when the Son of man comes"
## do not let him that is on the housetop go down ##
“Whoever is on the housetop must not go down” or “If anyone is on his housetop, he must not go down”
## on the housetop ##
Their housetops were flat and people could sit on them.
## his goods ##
"his possessions" or "his things"
## return back ##
The implied information that they were not to go back to the house to get anything and that they must flee quickly can be stated clearly as in the UDB.