A house mouse can pass through an opening about a quarter of an inch across. The limiting factor is the skull, not the body, and a mouse skull is smaller than most people picture. If you can slide a pencil into a gap, a mouse can use it.
That changes how you look at a house. The question stops being whether there is a hole and becomes where the holes are, because on a Long Island house built before 1980 there are always several.
The Eight We Find Most Often
The garage door corners. The bottom seal almost never closes tight at the two bottom corners. This is the single most common entry point we find, and most people have never looked at it.
Utility penetrations. Where the cable line, the AC line set, the gas line, or the outdoor spigot passes through the wall. The installer drilled an oversized hole and did not always seal it.
The dryer vent. The flap gets stuck open with lint or breaks off entirely.
The sill plate gap. Where the wooden frame sits on the concrete foundation. On older houses this joint can be surprisingly open, and it is hidden behind siding.
Crawlspace vents. Screens rust out and nobody looks under there for years.
Under the front steps or a stoop. Settling opens a gap between the concrete and the house.
The roofline and soffit returns. Mice climb well. Rough siding, downspouts, and overhanging branches all get used.
Basement window frames. Especially wood frames that have rotted at the corners.
Why November
Mice are outdoors most of the year and do not need you. What changes in late fall is that overnight temperatures drop and food gets scarce, and the warm structure with the crumbs in it becomes worth the trip. This is why calls spike in November on Long Island, and why a pest program that shuts down in October leaves you covered for exactly the wrong nine months.
How to Tell What You Have
Droppings. Mouse droppings are small, dark, and pointed at the ends, roughly the size of a grain of rice. They concentrate along walls and inside cabinets under the sink.
Gnaw marks. Fresh gnawing on cardboard, food packaging, or the corner of a door. Mice gnaw constantly because their incisors never stop growing.
Rub marks. Mice travel the same routes and leave a faint greasy smudge along baseboards and through openings.
Sound. Scratching in a wall or ceiling between roughly ten at night and two in the morning.
The Part Most Companies Do Not Say Out Loud
Trapping removes the mice that are inside. It does not close the opening they used. As long as that opening is there, new mice keep replacing the ones you catch, and you can pay somebody monthly to keep emptying traps forever without the problem ever ending.
That is a treadmill, and it is a profitable one for a pest control company. We would rather tell you the truth: sealing is the only thing that ends a rodent problem permanently. It is carpentry rather than pest control, so we quote it separately, and we will point out every entry point we find at no charge whether or not you hire us to close them.
What You Can Do This Weekend
Stand inside the garage with the door closed and look at the two bottom corners for daylight.
Walk the foundation and put a hand near every pipe and wire that enters the house.
Check the dryer vent flap actually closes.
Cut back any branch touching the roof.
Steel wool packed into a gap is a fine temporary fix. It is not permanent, and mice will eventually work around it, but it buys time.
How We Handle It
Mechanical snap traps in the interior runs, checked, cleared and reset at every visit where we have interior access. We do not place rodenticide bait inside your home, for reasons worth their own article. And at every visit we note the entry points we find and physically show you where they are.