Homeowners often call after hearing noises in their loft. The call either goes "I have noises in my loft and I'm not sure what it is", or, "I have noises in my loft it must be squirrels"
Squirrels do often gain access to roof spaces and many people simply assume its a squirrel as they climb trees, so could easily climb a house, which of course they can. In fact much more often the animal causing the issue is something else. If you have squirrels in your garden then yes its a possibility but if you rarely see them nearby then it's likely to be something else.
Rats are good climbers as well but they are generally opportunists and usually find their way in through either a ground level entry point or a drain defect, from there they will climb up through either the wall cavity or by following pipework. I find rats are by far the most common cause of noises in lofts.
Mice are also often found in roof spaces, more often even than rats but due to their size they usually go unnoticed. A mouse only weighs around 20 grams so makes little in the way of noise when scurrying around your loft. They are generally only heard when they run across a plasterboard ceiling rather than through thick loft insulation.
The first step you should take is to try and identify the animal from its droppings, inspect the area and see if you can find any. I have written a separate blog post on how to identify droppings.