Weep holes, dryer vents, roofline junctions and three more. A weekend with mesh and sealant closes most of them.
A house mouse fits through a gap the width of a pencil; a young rat needs about the size of a thumb. In practice that means the six places we check first are the same in almost every property: weep holes in brickwork, unscreened dryer and extractor vents, the junction where the roofline meets the fascia, gaps around incoming service pipes, garage door seals, and the void behind a poorly fitted airbrick.
Stainless wire wool packed into a gap and sealed over holds far better than expanding foam on its own, which rodents chew straight through. Exclusion is the only permanent fix — baiting without sealing just creates a vacancy for the next one.